PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)NAMEperlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an
expression. They fall into two major categories: list
operators and named unary operators. These differ in
their precedence relationship with a following comma.
(See the precedence table in the perlop manpage.) List
operators take more than one argument, while unary opera
tors can never take more than one argument. Thus, a comma
terminates the argument of a unary operator, but merely
separates the arguments of a list operator. A unary oper
ator generally provides a scalar context to its argument,
while a list operator may provide either scalar or list
contexts for its arguments. If it does both, the scalar
arguments will be first, and the list argument will fol
low. (Note that there can ever be only one such list
argument.) For instance, splice() has three scalar argu
ments followed by a list, whereas gethostbyname() has four
scalar arguments.
In the syntax descriptions that follow, list operators
that expect a list (and provide list context for the ele
ments of the list) are shown with LIST as an argument.
Such a list may consist of any combination of scalar argu
ments or list values; the list values will be included in
the list as if each individual element were interpolated
at that point in the list, forming a longer single-dimen
sional list value. Elements of the LIST should be sepa
rated by commas.
Any function in the list below may be used either with or
without parentheses around its arguments. (The syntax
descriptions omit the parentheses.) If you use the paren
theses, the simple (but occasionally surprising) rule is
this: It looks like a function, therefore it is a func
tion, and precedence doesn't matter. Otherwise it's a
list operator or unary operator, and precedence does mat
ter. And whitespace between the function and left paren
thesis doesn't count--so you need to be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about
this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore
work as neither unary nor list operators. These include
such functions as "time" and "endpwent". For example,
"time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list
context, nonabortive failure is generally indicated in a
scalar context by returning the undefined value, and in a
list context by returning the null list.
Remember the following important rule: There is no rule
that relates the behavior of an expression in list context
to its behavior in scalar context, or vice versa. It
might do two totally different things. Each operator and
function decides which sort of value it would be most
appropriate to return in scalar context. Some operators
return the length of the list that would have been
returned in list context. Some operators return the first
value in the list. Some operators return the last value
in the list. Some operators return a count of successful
operations. In general, they do what you want, unless you
want consistency.
An named array in scalar context is quite different from
what would at first glance appear to be a list in scalar
context. You can't get a list like "(1,2,3)" into being
in scalar context, because the compiler knows the context
at compile time. It would generate the scalar comma oper
ator there, not the list construction version of the
comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for
system calls of the same name (like chown(2), fork(2),
closedir(2), etc.) all return true when they succeed and
"undef" otherwise, as is usually mentioned in the descrip
tions below. This is different from the C interfaces,
which return "-1" on failure. Exceptions to this rule are
"wait", "waitpid", and "syscall". System calls also set
the special "$!" variable on failure. Other functions do
not, except accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look like
functions, like some keywords and named operators)
arranged by category. Some functions appear in more than
one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc",
"lcfirst", "length", "oct", "ord", "pack",
"q/STRING/", "qq/STRING/", "reverse", "rindex",
"sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study",
"qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log",
"oct", "rand", "sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw/STRING/", "reverse",
"sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen",
"die", "eof", "fileno", "flock", "format", "getc",
"print", "printf", "read", "readdir", "rewinddir",
"seek", "seekdir", "select", "syscall", "sysread",
"sysseek", "syswrite", "tell", "telldir", "truncate",
"warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite",
"unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl",
"glob", "ioctl", "link", "lstat", "mkdir", "open",
"opendir", "readlink", "rename", "rmdir", "stat",
"symlink", "umask", "unlink", "utime"
Keywords related to the control flow of your perl program
"caller", "continue", "die", "do", "dump", "eval",
"exit", "goto", "last", "next", "redo", "return",
"sub", "wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package",
"use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my",
"our", "reset", "scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "get
priority", "kill", "pipe", "qx/STRING/", "setpgrp",
"setpriority", "sleep", "system", "times", "wait",
"waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref",
"tie", "tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsock
name", "getsockopt", "listen", "recv", "send", "set
sockopt", "shutdown", "socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl",
"semget", "semop", "shmctl", "shmget", "shmread",
"shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent",
"getgrent", "getgrgid", "getgrnam", "getlogin", "getp
went", "getpwnam", "getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "geth
ostbyname", "gethostent", "getnetbyaddr", "getnetby
name", "getnetent", "getprotobyname", "getprotobynum
ber", "getprotoent", "getservbyname", "getservbyport",
"getservent", "sethostent", "setnetent", "setpro
toent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline",
"glob", "import", "lc", "lcfirst", "map", "my", "no",
"our", "prototype", "qx", "qw", "readline", "read
pipe", "ref", "sub*", "sysopen", "tie", "tied", "uc",
"ucfirst", "untie", "use"
* - "sub" was a keyword in perl4, but in perl5 it is
an operator, which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common
Unix system calls. In non-Unix environments, the func
tionality of some Unix system calls may not be available,
or details of the available functionality may differ
slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt",
"dbmclose", "dbmopen", "dump", "endgrent", "endhostent",
"endnetent", "endprotoent", "endpwent", "endservent",
"exec", "fcntl", "flock", "fork", "getgrent", "getgrgid",
"gethostent", "getlogin", "getnetbyaddr", "getnetbyname",
"getnetent", "getppid", "getprgp", "getpriority", "getpro
tobynumber", "getprotoent", "getpwent", "getpwnam", "getp
wuid", "getservbyport", "getservent", "getsockopt",
"glob", "ioctl", "kill", "link", "lstat", "msgctl",
"msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink",
"rename", "select", "semctl", "semget", "semop", "set
grent", "sethostent", "setnetent", "setpgrp", "setprior
ity", "setprotoent", "setpwent", "setservent", "setsock
opt", "shmctl", "shmget", "shmread", "shmwrite", "socket",
"socketpair", "stat", "symlink", "syscall", "sysopen",
"system", "times", "truncate", "umask", "unlink", "utime",
"wait", "waitpid"
For more information about the portability of these func
tions, see the perlport manpage and other available plat
form-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed
below. This unary operator takes one argument,
either a filename or a filehandle, and tests the
associated file to see if something is true about
it. If the argument is omitted, tests "$_",
except for "-t", which tests STDIN. Unless other
wise documented, it returns "1" for true and "''"
for false, or the undefined value if the file
doesn't exist. Despite the funny names, prece
dence is the same as any other named unary opera
tor, and the argument may be parenthesized like
any other unary operator. The operator may be any
of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file.
-B File is a "binary" file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission opera
tors "-r", "-R", "-w", "-W", "-x", and "-X" is by
default based solely on the mode of the file and
the uids and gids of the user. There may be other
reasons you can't actually read, write, or execute
the file. Such reasons may be for example network
filesystem access controls, ACLs (access control
lists), read-only filesystems, and unrecognized
executable formats.
Also note that, for the superuser on the local
filesystems, the "-r", "-R", "-w", and "-W" tests
always return 1, and "-x" and "-X" return 1 if any
execute bit is set in the mode. Scripts run by
the superuser may thus need to do a stat() to
determine the actual mode of the file, or tem
porarily set their effective uid to something
else.
If you are using ACLs, there is a pragma called
"filetest" that may produce more accurate results
than the bare stat() mode bits. When under the
"use filetest 'access'" the above-mentioned
filetests will test whether the permission can
(not) be granted using the access() family of sys
tem calls. Also note that the "-x" and "-X" may
under this pragma return true even if there are no
execute permission bits set (nor any extra execute
permission ACLs). This strangeness is due to the
underlying system calls' definitions. Read the
documentation for the "filetest" pragma for more
information.
Note that "-s/a/b/" does not do a negated substi
tution. Saying "-exp($foo)" still works as
expected, however--only single letters following a
minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The
first block or so of the file is examined for odd
characters such as strange control codes or char
acters with the high bit set. If too many strange
characters (>30%) are found, it's a "-B" file,
otherwise it's a "-T" file. Also, any file con
taining null in the first block is considered a
binary file. If "-T" or "-B" is used on a file
handle, the current stdio buffer is examined
rather than the first block. Both "-T" and "-B"
return true on a null file, or a file at EOF when
testing a filehandle. Because you have to read a
file to do the "-T" test, on most occasions you
want to use a "-f" against the file first, as in
"next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or
"lstat" operators) are given the special filehan
dle consisting of a solitary underline, then the
stat structure of the previous file test (or stat
operator) is used, saving a system call. (This
doesn't work with "-t", and you need to remember
that lstat() and "-l" will leave values in the
stat structure for the symbolic link, not the real
file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If
VALUE is omitted, uses "$_".
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the
accept(2) system call does. Returns the packed
address if it succeeded, false otherwise. See the
example in the Sockets: Client/Server Communica
tion entry in the perlipc manpage.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor, as determined by the value of
$^F. See the section on "$^F" in the perlvar man
page.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this pro
cess after the specified number of seconds have
elapsed. If SECONDS is not specified, the value
stored in "$_" is used. (On some machines, unfor
tunately, the elapsed time may be up to one second
less than you specified because of how seconds are
counted.) Only one timer may be counting at once.
Each call disables the previous timer, and an
argument of "0" may be supplied to cancel the pre
vious timer without starting a new one. The
returned value is the amount of time remaining on
the previous timer.
For delays of finer granularity than one second,
you may use Perl's four-argument version of
select() leaving the first three arguments unde
fined, or you might be able to use the "syscall"
interface to access setitimer(2) if your system
supports it. The Time::HiRes module from CPAN may
also prove useful.
It is usually a mistake to intermix "alarm" and
"sleep" calls. ("sleep" may be internally imple
mented in your system with "alarm")
If you want to use "alarm" to time out a system
call you need to use an "eval"/"die" pair. You
can't rely on the alarm causing the system call to
fail with "$!" set to "EINTR" because Perl sets up
signal handlers to restart system calls on some
systems. Using "eval"/"die" always works, modulo
the caveats given in the Signals entry in the per
lipc manpage.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
atan2 Y,X
Returns the arctangent of Y/X in the range -pi to
pi.
For the tangent operation, you may use the
"Math::Trig::tan" function, or use the familiar
relation:
sub tan { sin($_[0]) / cos($_[0]) }
bind SOCKET,NAME
Binds a network address to a socket, just as the
bind system call does. Returns true if it suc
ceeded, false otherwise. NAME should be a packed
address of the appropriate type for the socket.
See the examples in the Sockets: Client/Server
Communication entry in the perlipc manpage.
binmode FILEHANDLE, DISCIPLINE
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in
"binary" or "text" mode on systems where the run-
time libraries distinguish between binary and text
files. If FILEHANDLE is an expression, the value
is taken as the name of the filehandle. DISCI
PLINE can be either of "":raw"" for binary mode or
"":crlf"" for "text" mode. If the DISCIPLINE is
omitted, it defaults to "":raw"".
binmode() should be called after open() but before
any I/O is done on the filehandle.
On many systems binmode() currently has no effect,
but in future, it will be extended to support
user-defined input and output disciplines. On
some systems binmode() is necessary when you're
not working with a text file. For the sake of
portability it is a good idea to always use it
when appropriate, and to never use it when it
isn't appropriate.
In other words: Regardless of platform, use bin_
mode() on binary files, and do not use binmode()
on text files.
The "open" pragma can be used to establish default
disciplines. See the open manpage.
The operating system, device drivers, C libraries,
and Perl run-time system all work together to let
the programmer treat a single character ("\n") as
the line terminator, irrespective of the external
representation. On many operating systems, the
native text file representation matches the inter
nal representation, but on some platforms the
external representation of "\n" is made up of more
than one character.
Mac OS and all variants of Unix use a single char
acter to end each line in the external representa
tion of text (even though that single character is
not necessarily the same across these platforms).
Consequently binmode() has no effect on these
operating systems. In other systems like VMS, MS-
DOS and the various flavors of MS-Windows your
program sees a "\n" as a simple "\cJ", but what's
stored in text files are the two characters
"\cM\cJ". That means that, if you don't use bin_
mode() on these systems, "\cM\cJ" sequences on
disk will be converted to "\n" on input, and any
"\n" in your program will be converted back to
"\cM\cJ" on output. This is what you want for
text files, but it can be disastrous for binary
files.
Another consequence of using binmode() (on some
systems) is that special end-of-file markers will
be seen as part of the data stream. For systems
from the Microsoft family this means that if your
binary data contains "\cZ", the I/O subsystem will
regard it as the end of the file, unless you use
binmode().
binmode() is not only important for readline() and
print() operations, but also when using read(),
seek(), sysread(), syswrite() and tell() (see the
perlport manpage for more details). See the "$/"
and "$\" variables in the perlvar manpage for how
to manually set your input and output line-termi
nation sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF
that it is now an object in the CLASSNAME package.
If CLASSNAME is omitted, the current package is
used. Because a "bless" is often the last thing
in a constructor, it returns the reference for
convenience. Always use the two-argument version
if the function doing the blessing might be inher
ited by a derived class. See the perltoot manpage
and the perlobj manpage for more about the bless
ing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs
that are mixed case. Namespaces with all lower
case names are considered reserved for Perl prag
mata. Builtin types have all uppercase names, so
to prevent confusion, you may wish to avoid such
package names as well. Make sure that CLASSNAME
is a true value.
See the Perl Modules entry in the perlmod manpage.
caller EXPR
caller Returns the context of the current subroutine
call. In scalar context, returns the caller's
package name if there is a caller, that is, if
we're in a subroutine or "eval" or "require", and
the undefined value otherwise. In list context,
returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that
the debugger uses to print a stack trace. The
value of EXPR indicates how many call frames to go
back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is
not a subroutine call, but an "eval". In such a
case additional elements $evaltext and
"$is_require" are set: "$is_require" is true if
the frame is created by a "require" or "use"
statement, $evaltext contains the text of the
"eval EXPR" statement. In particular, for an
"eval BLOCK" statement, $filename is "(eval)", but
$evaltext is undefined. (Note also that each
"use" statement creates a "require" frame inside
an "eval EXPR") frame. "$hasargs" is true if a
new instance of "@_" was set up for the frame.
"$hints" and "$bitmask" contain pragmatic hints
that the caller was compiled with. The "$hints"
and "$bitmask" values are subject to change
between versions of Perl, and are not meant for
external use.
Furthermore, when called from within the DB pack
age, caller returns more detailed information: it
sets the list variable "@DB::args" to be the argu
ments with which the subroutine was invoked.
Be aware that the optimizer might have optimized
call frames away before "caller" had a chance to
get the information. That means that "caller(N)"
might not return information about the call frame
you expect it do, for "N > 1". In particular,
"@DB::args" might have information from the previ
ous time "caller" was called.
chdir EXPR
Changes the working directory to EXPR, if possi
ble. If EXPR is omitted, changes to the directory
specified by "$ENV{HOME}", if set; if not, changes
to the directory specified by "$ENV{LOGDIR}". If
neither is set, "chdir" does nothing. It returns
true upon success, false otherwise. See the exam
ple under "die".
chmod LIST
Changes the permissions of a list of files. The
first element of the list must be the numerical
mode, which should probably be an octal number,
and which definitely should not a string of octal
digits: "0644" is okay, "'0644'" is not. Returns
the number of files successfully changed. See
also the oct entry elsewhere in this document, if
all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
You can also import the symbolic "S_I*" constants
from the Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp LIST
chomp This safer version of the chop entry elsewhere in
this document removes any trailing string that
corresponds to the current value of "$/" (also
known as $INPUT_RECORD_SEPARATOR in the "English"
module). It returns the total number of charac
ters removed from all its arguments. It's often
used to remove the newline from the end of an
input record when you're worried that the final
record may be missing its newline. When in para
graph mode ("$/ = """), it removes all trailing
newlines from the string. When in slurp mode ("$/
= undef") or fixed-length record mode ("$/" is a
reference to an integer or the like, see the perl
var manpage) chomp() won't remove anything. If
VARIABLE is omitted, it chomps "$_". Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's val
ues, but not its keys.
You can actually chomp anything that's an lvalue,
including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and
the total number of characters removed is
returned.
chop VARIABLE
chop LIST
chop Chops off the last character of a string and
returns the character chopped. It is much more
efficient than "s/.$//s" because it neither scans
nor copies the string. If VARIABLE is omitted,
chops "$_". If VARIABLE is a hash, it chops the
hash's values, but not its keys.
You can actually chop anything that's an lvalue,
including an assignment.
If you chop a list, each element is chopped. Only
the value of the last "chop" is returned.
Note that "chop" returns the last character. To
return all but the last character, use "sub
str($string, 0, -1)".
chown LIST
Changes the owner (and group) of a list of files.
The first two elements of the list must be the
numeric uid and gid, in that order. A value of -1
in either position is interpreted by most systems
to leave that value unchanged. Returns the number
of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in
the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the
ownership of the file unless you're the superuser,
although you should be able to change the group to
any of your secondary groups. On insecure sys
tems, these restrictions may be relaxed, but this
is not a portable assumption. On POSIX systems,
you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER
in the character set. For example, "chr(65)" is
""A"" in either ASCII or Unicode, and chr(0x263a)
is a Unicode smiley face (but only within the
scope of a "use utf8"). For the reverse, use the
ord entry elsewhere in this document. See the
utf8 manpage for more about Unicode.
If NUMBER is omitted, uses "$_".
chroot FILENAME
chroot This function works like the system call by the
same name: it makes the named directory the new
root directory for all further pathnames that
begin with a "/" by your process and all its chil
dren. (It doesn't change your current working
directory, which is unaffected.) For security
reasons, this call is restricted to the superuser.
If FILENAME is omitted, does a "chroot" to "$_".
close FILEHANDLE
close Closes the file or pipe associated with the file
handle, returning true only if stdio successfully
flushes buffers and closes the system file
descriptor. Closes the currently selected file
handle if the argument is omitted.
You don't have to close FILEHANDLE if you are
immediately going to do another "open" on it,
because "open" will close it for you. (See
"open".) However, an explicit "close" on an input
file resets the line counter ("$."), while the
implicit close done by "open" does not.
If the file handle came from a piped open "close"
will additionally return false if one of the other
system calls involved fails or if the program
exits with non-zero status. (If the only problem
was that the program exited non-zero "$!" will be
set to "0".) Closing a pipe also waits for the
process executing on the pipe to complete, in case
you want to look at the output of the pipe after
wards, and implicitly puts the exit status value
of that command into "$?".
Prematurely closing the read end of a pipe (i.e.
before the process writing to it at the other end
has closed it) will result in a SIGPIPE being
delivered to the writer. If the other end can't
handle that, be sure to read all the data before
closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be
used as an indirect filehandle, usually the real
filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns
the success of that system call.
DIRHANDLE may be an expression whose value can be
used as an indirect dirhandle, usually the real
dirhandle name.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as
the connect system call does. Returns true if it
succeeded, false otherwise. NAME should be a
packed address of the appropriate type for the
socket. See the examples in the Sockets:
Client/Server Communication entry in the perlipc
manpage.
continue BLOCK
Actually a flow control statement rather than a
function. If there is a "continue" BLOCK attached
to a BLOCK (typically in a "while" or "foreach"),
it is always executed just before the conditional
is about to be evaluated again, just like the
third part of a "for" loop in C. Thus it can be
used to increment a loop variable, even when the
loop has been continued via the "next" statement
(which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a
"continue" block. "last" and "redo" will behave
as if they had been executed within the main
block. So will "next", but since it will execute
a "continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically
equivalent to using an empty one, logically
enough. In that case, "next" goes directly back
to check the condition at the top of the loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians).
If EXPR is omitted, takes cosine of "$_".
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this rela
tion:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) func
tion in the C library (assuming that you actually
have a version there that has not been extirpated
as a potential munition). This can prove useful
for checking the password file for lousy pass
words, amongst other things. Only the guys wear
ing white hats should do this.
Note that "crypt" is intended to be a one-way
function, much like breaking eggs to make an
omelette. There is no (known) corresponding
decrypt function. As a result, this function
isn't all that useful for cryptography. (For
that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you
should use the encrypted text as the salt (like
"crypt($plain, $crypted) eq $crypted"). This
allows your code to work with the standard "crypt"
and with more exotic implementations. When choos
ing a new salt create a random two character
string whose characters come from the set
"[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9,
'A'..'Z', 'a'..'z')[rand 64, rand 64]").
Here's an example that makes sure that whoever
runs this program knows their own password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever
asks you for it is unwise.
The the crypt manpage function is unsuitable for
encrypting large quantities of data, not least of
all because you can't get the information back.
Look at the by-module/Crypt and by-module/PGP
directories on your favorite CPAN mirror for a
slew of potentially useful modules.
dbmclose HASH
[This function has been largely superseded by the
"untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the
"tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or
Berkeley DB file to a hash. HASH is the name of
the hash. (Unlike normal "open", the first argu
ment is not a filehandle, even though it looks
like one). DBNAME is the name of the database
(without the .dir or .pag extension if any). If
the database does not exist, it is created with
protection specified by MASK (as modified by the
"umask"). If your system supports only the older
DBM functions, you may perform only one "dbmopen"
in your program. In older versions of Perl, if
your system had neither DBM nor ndbm, calling
"dbmopen" produced a fatal error; it now falls
back to sdbm(3).
If you don't have write access to the DBM file,
you can only read hash variables, not set them.
If you want to test whether you can write, either
use file tests or try setting a dummy hash entry
inside an "eval", which will trap the error.
Note that functions such as "keys" and "values"
may return huge lists when used on large DBM
files. You may prefer to use the "each" function
to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more gen
eral description of the pros and cons of the vari
ous dbm approaches, as well as the DB_File manpage
for a particularly rich implementation.
You can control which DBM library you use by load
ing that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a
value other than the undefined value "undef". If
EXPR is not present, "$_" will be checked.
Many operations return "undef" to indicate fail
ure, end of file, system error, uninitialized
variable, and other exceptional conditions. This
function allows you to distinguish "undef" from
other values. (A simple Boolean test will not
distinguish among "undef", zero, the empty string,
and ""0"", which are all equally false.) Note
that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condi
tion: "pop" returns "undef" when its argument is
an empty array, or when the element to return hap
pens to be "undef".
You may also use "defined(&func)" to check whether
subroutine "&func" has ever been defined. The
return value is unaffected by any forward declara
tions of "&foo". Note that a subroutine which is
not defined may still be callable: its package may
have an "AUTOLOAD" method that makes it spring
into existence the first time that it is called --
see the perlsub manpage.
Use of "defined" on aggregates (hashes and arrays)
is deprecated. It used to report whether memory
for that aggregate has ever been allocated. This
behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether
the value is defined, not whether the key exists
in the hash. Use the exists entry elsewhere in
this document for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and
then are surprised to discover that the number "0"
and """" (the zero-length string) are, in fact,
defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and "$1" is defined,
despite the fact that it matched "nothing". But
it didn't really match nothing--rather, it matched
something that happened to be zero characters
long. This is all very above-board and honest.
When a function returns an undefined value, it's
an admission that it couldn't give you an honest
answer. So you should use "defined" only when
you're questioning the integrity of what you're
trying to do. At other times, a simple comparison
to "0" or """" is what you want.
See also the undef, exists, and ref entries else
where in this document.
delete EXPR
Given an expression that specifies a hash element,
array element, hash slice, or array slice, deletes
the specified element(s) from the hash or array.
In the case of an array, if the array elements
happen to be at the end, the size of the array
will shrink to the highest element that tests true
for exists() (or 0 if no such element exists).
Returns each element so deleted or the undefined
value if there was no such element. Deleting from
"$ENV{}" modifies the environment. Deleting from
a hash tied to a DBM file deletes the entry from
the DBM file. Deleting from a "tie"d hash or
array may not necessarily return anything.
Deleting an array element effectively returns that
position of the array to its initial, uninitial
ized state. Subsequently testing for the same
element with exists() will return false. Note
that deleting array elements in the middle of an
array will not shift the index of the ones after
them down--use splice() for that. See the exists
entry elsewhere in this document.
The following (inefficiently) deletes all the val
ues of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning
the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated
as long as the final operation is a hash element,
array element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to
"STDERR" and exits with the current value of "$!"
(errno). If "$!" is "0", exits with the value of
"($? >> 8)" (backtick `command` status). If "($?
>> 8)" is "0", exits with "255". Inside an
"eval()," the error message is stuffed into "$@"
and the "eval" is terminated with the undefined
value. This makes "die" the way to raise an
exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline,
the current script line number and input line num
ber (if any) are also printed, and a newline is
supplied. Note that the "input line number" (also
known as "chunk") is subject to whatever notion of
"line" happens to be currently in effect, and is
also available as the special variable "$.". See
the section on "$/" in the perlvar manpage and the
section on "$." in the perlvar manpage.
Hint: sometimes appending "", stopped"" to your
message will cause it to make better sense when
the string ""at foo line 123"" is appended. Sup
pose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and "$@" already contains a value
(typically from a previous eval) that value is
reused after appending ""\t...propagated"". This
is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If "$@" is empty then the string ""Died"" is used.
die() can also be called with a reference argu
ment. If this happens to be trapped within an
eval(), $@ contains the reference. This behavior
permits a more elaborate exception handling imple
mentation using objects that maintain arbitrary
state about the nature of the exception. Such a
scheme is sometimes preferable to matching partic
ular string values of $@ using regular expres
sions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception
messages before displaying them, you may want to
overload stringification operations on such custom
exception objects. See the overload manpage for
details about that.
You can arrange for a callback to be run just
before the "die" does its deed, by setting the
"$SIG{__DIE__}" hook. The associated handler will
be called with the error text and can change the
error message, if it sees fit, by calling "die"
again. See the section on "$SIG{expr}" in the
perlvar manpage for details on setting "%SIG"
entries, and the section on "eval BLOCK" for some
examples. Although this feature was meant to be
run only right before your program was to exit,
this is not currently the case--the
"$SIG{__DIE__}" hook is currently called even
inside eval()ed blocks/strings! If one wants the
hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see the section
on "$^S" in the perlvar manpage). Because this
promotes strange action at a distance, this coun
terintuitive behavior may be fixed in a future
release.
do BLOCK
Not really a function. Returns the value of the
last command in the sequence of commands indicated
by BLOCK. When modified by a loop modifier, exe
cutes the BLOCK once before testing the loop con
dition. (On other statements the loop modifiers
test the conditional first.)
"do BLOCK" does not count as a loop, so the loop
control statements "next", "last", or "redo" can
not be used to leave or restart the block. See
the perlsyn manpage for alternative strategies.
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See the
perlsub manpage.
do EXPR Uses the value of EXPR as a filename and executes
the contents of the file as a Perl script. Its
primary use is to include subroutines from a Perl
subroutine library.
do 'stat.pl';
is just like
scalar eval `cat stat.pl`;
except that it's more efficient and concise, keeps
track of the current filename for error messages,
searches the @INC libraries, and updates "%INC" if
the file is found. See the Predefined Names entry
in the perlvar manpage for these variables. It
also differs in that code evaluated with "do FILE
NAME" cannot see lexicals in the enclosing scope;
"eval STRING" does. It's the same, however, in
that it does reparse the file every time you call
it, so you probably don't want to do this inside a
loop.
If "do" cannot read the file, it returns undef and
sets "$!" to the error. If "do" can read the file
but cannot compile it, it returns undef and sets
an error message in "$@". If the file is suc
cessfully compiled, "do" returns the value of the
last expression evaluated.
Note that inclusion of library modules is better
done with the "use" and "require" operators, which
also do automatic error checking and raise an
exception if there's a problem.
You might like to use "do" to read in a program
configuration file. Manual error checking can be
done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See
also the -u command-line switch in the perlrun
manpage, which does the same thing. Primarily
this is so that you can use the undump program
(not supplied) to turn your core dump into an exe
cutable binary after having initialized all your
variables at the beginning of the program. When
the new binary is executed it will begin by exe
cuting a "goto LABEL" (with all the restrictions
that "goto" suffers). Think of it as a goto with
an intervening core dump and reincarnation. If
"LABEL" is omitted, restarts the program from the
top.
WARNING: Any files opened at the time of the dump
will not be open any more when the program is
reincarnated, with possible resulting confusion on
the part of Perl.
This function is now largely obsolete, partly
because it's very hard to convert a core file into
an executable, and because the real compiler back
ends for generating portable bytecode and compil
able C code have superseded it.
If you're looking to use the dump manpage to speed
up your program, consider generating bytecode or
native C code as described in the perlcc manpage.
If you're just trying to accelerate a CGI script,
consider using the "mod_perl" extension to Apache,
or the CPAN module, Fast::CGI. You might also
consider autoloading or selfloading, which at
least make your program appear to run faster.
each HASH
When called in list context, returns a 2-element
list consisting of the key and value for the next
element of a hash, so that you can iterate over
it. When called in scalar context, returns only
the key for the next element in the hash.
Entries are returned in an apparently random
order. The actual random order is subject to
change in future versions of perl, but it is guar
anteed to be in the same order as either the
"keys" or "values" function would produce on the
same (unmodified) hash.
When the hash is entirely read, a null array is
returned in list context (which when assigned pro
duces a false ("0") value), and "undef" in scalar
context. The next call to "each" after that will
start iterating again. There is a single iterator
for each hash, shared by all "each", "keys", and
"values" function calls in the program; it can be
reset by reading all the elements from the hash,
or by evaluating "keys HASH" or "values HASH". If
you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or
duplicated, so don't. Exception: It is always
safe to delete the item most recently returned by
"each()", which means that the following code will
work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the
printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will
return end of file, or if FILEHANDLE is not open.
FILEHANDLE may be an expression whose value gives
the real filehandle. (Note that this function
actually reads a character and then "ungetc"s it,
so isn't very useful in an interactive context.)
Do not read from a terminal file (or call
"eof(FILEHANDLE)" on it) after end-of-file is
reached. File types such as terminals may lose
the end-of-file condition if you do.
An "eof" without an argument uses the last file
read. Using "eof()" with empty parentheses is
very different. It refers to the pseudo file
formed from the files listed on the command line
and accessed via the "<>" operator. Since "<>"
isn't explicitly opened, as a normal filehandle
is, an "eof()" before "<>" has been used will
cause "@ARGV" to be examined to determine if input
is available.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can
be used to detect the end of each file, "eof()"
will only detect the end of the last file. Exam
ples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of current file
print "--------------\n";
close(ARGV); # close or last; is needed if we
# are reading from the terminal
}
print;
}
Practical hint: you almost never need to use "eof"
in Perl, because the input operators typically
return "undef" when they run out of data, or if
there was an error.
eval EXPR
eval BLOCK
In the first form, the return value of EXPR is
parsed and executed as if it were a little Perl
program. The value of the expression (which is
itself determined within scalar context) is first
parsed, and if there weren't any errors, executed
in the lexical context of the current Perl pro
gram, so that any variable settings or subroutine
and format definitions remain afterwards. Note
that the value is parsed every time the eval exe
cutes. If EXPR is omitted, evaluates "$_". This
form is typically used to delay parsing and subse
quent execution of the text of EXPR until run
time.
In the second form, the code within the BLOCK is
parsed only once--at the same time the code sur
rounding the eval itself was parsed--and executed
within the context of the current Perl program.
This form is typically used to trap exceptions
more efficiently than the first (see below), while
also providing the benefit of checking the code
within BLOCK at compile time.
The final semicolon, if any, may be omitted from
the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of
the last expression evaluated inside the mini-pro
gram; a return statement may be also used, just as
with subroutines. The expression providing the
return value is evaluated in void, scalar, or list
context, depending on the context of the eval
itself. See the wantarray entry elsewhere in this
document for more on how the evaluation context
can be determined.
If there is a syntax error or runtime error, or a
"die" statement is executed, an undefined value is
returned by "eval", and "$@" is set to the error
message. If there was no error, "$@" is guaran
teed to be a null string. Beware that using
"eval" neither silences perl from printing warn
ings to STDERR, nor does it stuff the text of
warning messages into "$@". To do either of
those, you have to use the "$SIG{__WARN__}" facil
ity. See the warn entry elsewhere in this docu
ment and the perlvar manpage.
Note that, because "eval" traps otherwise-fatal
errors, it is useful for determining whether a
particular feature (such as "socket" or "symlink")
is implemented. It is also Perl's exception trap
ping mechanism, where the die operator is used to
raise exceptions.
If the code to be executed doesn't vary, you may
use the eval-BLOCK form to trap run-time errors
without incurring the penalty of recompiling each
time. The error, if any, is still returned in
"$@". Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of
"__DIE__" hooks, when using the "eval{}" form as
an exception trap in libraries, you may wish not
to trigger any "__DIE__" hooks that user code may
have installed. You can use the "local
$SIG{__DIE__}" construct for this purpose, as
shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that
"__DIE__" hooks can call "die" again, which has
the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this
counterintuitive behavior may be fixed in a future
release.
With an "eval", you should be especially careful
to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run
the code contained in the variable $x. (Although
case 2 has misleading double quotes making the
reader wonder what else might be happening (noth
ing is).) Cases 3 and 4 likewise behave in the
same way: they run the code "'$x'", which does
nothing but return the value of $x. (Case 4 is
preferred for purely visual reasons, but it also
has the advantage of compiling at compile-time
instead of at run-time.) Case 5 is a place where
normally you would like to use double quotes,
except that in this particular situation, you can
just use symbolic references instead, as in case
6.
"eval BLOCK" does not count as a loop, so the loop
control statements "next", "last", or "redo" can
not be used to leave or restart the block.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and
never returns-- use "system" instead of "exec" if
you want it to return. It fails and returns false
only if the command does not exist and it is exe
cuted directly instead of via your system's com
mand shell (see below).
Since it's a common mistake to use "exec" instead
of "system", Perl warns you if there is a follow
ing statement which isn't "die", "warn", or "exit"
(if "-w" is set - but you always do that). If
you really want to follow an "exec" with some
other statement, you can use one of these styles
to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if
LIST is an array with more than one value, calls
execvp(3) with the arguments in LIST. If there is
only one scalar argument or an array with one ele
ment in it, the argument is checked for shell
metacharacters, and if there are any, the entire
argument is passed to the system's command shell
for parsing (this is "/bin/sh -c" on Unix plat
forms, but varies on other platforms). If there
are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp",
which is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first
argument, but want to lie to the program you are
executing about its own name, you can specify the
program you actually want to run as an "indirect
object" (without a comma) in front of the LIST.
(This always forces interpretation of the LIST as
a multivalued list, even if there is only a single
scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system
shell, results will be subject to its quirks and
capabilities. See the section on "`STRING`" in
the perlop manpage for details.
Using an indirect object with "exec" or "system"
is also more secure. This usage (which also works
fine with system()) forces interpretation of the
arguments as a multivalued list, even if the list
had just one argument. That way you're safe from
the shell expanding wildcards or splitting up
words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect
object, ran the echo program, passing it ""sur
prise"" an argument. The second version
didn't--it tried to run a program literally called
"echo surprise", didn't find it, and set "$?" to a
non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before the exec, but
this may not be supported on some platforms (see
the perlport manpage). To be safe, you may need
to set "$|" ($AUTOFLUSH in English) or call the
"autoflush()" method of "IO::Handle" on any open
handles in order to avoid lost output.
Note that "exec" will not call your "END" blocks,
nor will it call any "DESTROY" methods in your
objects.
exists EXPR
Given an expression that specifies a hash element
or array element, returns true if the specified
element in the hash or array has ever been ini
tialized, even if the corresponding value is unde
fined. The element is not autovivified if it
doesn't exist.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's
defined, and defined if it exists, but the reverse
doesn't necessarily hold true.
Given an expression that specifies the name of a
subroutine, returns true if the specified subrou
tine has ever been declared, even if it is unde
fined. Mentioning a subroutine name for exists or
defined does not count as declaring it. Note that
a subroutine which does not exist may still be
callable: its package may have an "AUTOLOAD"
method that makes it spring into existence the
first time that it is called -- see the perlsub
manpage.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated
as long as the final operation is a hash or array
key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not
spring into existence just because its existence
was tested, any intervening ones will. Thus
"$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring
into existence due to the existence test for the
$key element above. This happens anywhere the
arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not
at first--or even second--glance appear to be an
lvalue context may be fixed in a future release.
See the Pseudo-hashes: Using an array as a hash
entry in the perlref manpage for specifics on how
exists() acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine
name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
Evaluates EXPR and exits immediately with that
value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with
"0" status. The only universally recognized val
ues for EXPR are "0" for success and "1" for
error; other values are subject to interpretation
depending on the environment in which the Perl
program is running. For example, exiting 69
(EX_UNAVAILABLE) from a sendmail incoming-mail
filter will cause the mailer to return the item
undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's
any chance that someone might want to trap what
ever error happened. Use "die" instead, which can
be trapped by an "eval".
The exit() function does not always exit immedi
ately. It calls any defined "END" routines first,
but these "END" routines may not themselves abort
the exit. Likewise any object destructors that
need to be called are called before the real exit.
If this is a problem, you can call
"POSIX:_exit($status)" to avoid END and destructor
processing. See the perlmod manpage for details.
exp EXPR
exp Returns e (the natural logarithm base) to the
power of EXPR. If EXPR is omitted, gives
"exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably
have to say
use Fcntl;
first to get the correct constant definitions.
Argument processing and value return works just
like "ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the
return from "fnctl". Like "ioctl", it maps a "0"
return from the system call into ""0 but true"" in
Perl. This string is true in boolean context and
"0" in numeric context. It is also exempt from
the normal -w warnings on improper numeric conver
sions.
Note that "fcntl" will produce a fatal error if
used on a machine that doesn't implement fcntl(2).
See the Fcntl module or your fcntl(2) manpage to
learn what functions are available on your system.
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or
undefined if the filehandle is not open. This is
mainly useful for constructing bitmaps for
"select" and low-level POSIX tty-handling opera
tions. If FILEHANDLE is an expression, the value
is taken as an indirect filehandle, generally its
name.
You can use this to find out whether two handles
refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHAN
DLE. Returns true for success, false on failure.
Produces a fatal error if used on a machine that
doesn't implement flock(2), fcntl(2) locking, or
lockf(3). "flock" is Perl's portable file locking
interface, although it locks only entire files,
not records.
Two potentially non-obvious but traditional
"flock" semantics are that it waits indefinitely
until the lock is granted, and that its locks
merely advisory. Such discretionary locks are
more flexible, but offer fewer guarantees. This
means that files locked with "flock" may be modi
fied by programs that do not also use "flock".
See the perlport manpage, your port's specific
documentation, or your system-specific local man
pages for details. It's best to assume tradi
tional behavior if you're writing portable pro
grams. (But if you're not, you should as always
feel perfectly free to write for your own system's
idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns
shouldn't get in the way of your getting your job
done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN,
possibly combined with LOCK_NB. These constants
are traditionally valued 1, 2, 8 and 4, but you
can use the symbolic names if you import them from
the Fcntl module, either individually, or as a
group using the ':flock' tag. LOCK_SH requests a
shared lock, LOCK_EX requests an exclusive lock,
and LOCK_UN releases a previously requested lock.
If LOCK_NB is bitwise-or'ed with LOCK_SH or
LOCK_EX then "flock" will return immediately
rather than blocking waiting for the lock (check
the return status to see if you got it).
To avoid the possibility of miscoordination, Perl
now flushes FILEHANDLE before locking or unlocking
it.
Note that the emulation built with lockf(3)
doesn't provide shared locks, and it requires that
FILEHANDLE be open with write intent. These are
the semantics that lockf(3) implements. Most if
not all systems implement lockf(3) in terms of
fcntl(2) locking, though, so the differing seman
tics shouldn't bite too many people.
Note also that some versions of "flock" cannot
lock things over the network; you would need to
use the more system-specific "fcntl" for that. If
you like you can force Perl to ignore your
system's flock(2) function, and so provide its own
fcntl(2)-based emulation, by passing the switch
"-Ud_flock" to the Configure program when you con
figure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are
inherited across fork() calls, whereas those that
must resort to the more capricious fcntl() func
tion lose the locks, making it harder to write
servers.
See also the DB_File manpage for other flock()
examples.
fork Does a fork(2) system call to create a new process
running the same program at the same point. It
returns the child pid to the parent process, "0"
to the child process, or "undef" if the fork is
unsuccessful. File descriptors (and sometimes
locks on those descriptors) are shared, while
everything else is copied. On most systems sup
porting fork(), great care has gone into making it
extremely efficient (for example, using copy-on-
write technology on data pages), making it the
dominant paradigm for multitasking over the last
few decades.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before forking the
child process, but this may not be supported on
some platforms (see the perlport manpage). To be
safe, you may need to set "$|" ($AUTOFLUSH in
English) or call the "autoflush()" method of
"IO::Handle" on any open handles in order to avoid
duplicate output.
If you "fork" without ever waiting on your chil
dren, you will accumulate zombies. On some sys
tems, you can avoid this by setting "$SIG{CHLD}"
to ""IGNORE"". See also the perlipc manpage for
more examples of forking and reaping moribund
children.
Note that if your forked child inherits system
file descriptors like STDIN and STDOUT that are
actually connected by a pipe or socket, even if
you exit, then the remote server (such as, say, a
CGI script or a backgrounded job launched from a
remote shell) won't think you're done. You should
reopen those to /dev/null if it's any issue.
format Declare a picture format for use by the "write"
function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and
examples.
formline PICTURE,LIST
This is an internal function used by "format"s,
though you may call it, too. It formats (see the
perlform manpage) a list of values according to
the contents of PICTURE, placing the output into
the format output accumulator, "$^A" (or "$ACCUMU
LATOR" in English). Eventually, when a "write" is
done, the contents of "$^A" are written to some
filehandle, but you could also read "$^A" yourself
and then set "$^A" back to """". Note that a for
mat typically does one "formline" per line of
form, but the "formline" function itself doesn't
care how many newlines are embedded in the PIC
TURE. This means that the "~" and "~~" tokens
will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines
to implement a single record format, just like the
format compiler.
Be careful if you put double quotes around the
picture, because an "@" character may be taken to
mean the beginning of an array name. "formline"
always returns true. See the perlform manpage for
other examples.
getc FILEHANDLE
getc Returns the next character from the input file
attached to FILEHANDLE, or the undefined value at
end of file, or if there was an error. If FILE
HANDLE is omitted, reads from STDIN. This is not
particularly efficient. However, it cannot be
used by itself to fetch single characters without
waiting for the user to hit enter. For that, try
something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set
is left as an exercise to the reader.
The "POSIX::getattr" function can do this more
portably on systems purporting POSIX compliance.
See also the "Term::ReadKey" module from your
nearest CPAN site; details on CPAN can be found on
the CPAN entry in the perlmodlib manpage.
getlogin
Implements the C library function of the same
name, which on most systems returns the current
login from /etc/utmp, if any. If null, use "getp
wuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it
is not as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end
of the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the speci
fied PID. Use a PID of "0" to get the current
process group for the current process. Will raise
an exception if used on a machine that doesn't
implement getpgrp(2). If PID is omitted, returns
process group of current process. Note that the
POSIX version of "getpgrp" does not accept a PID
argument, so only "PID==0" is truly portable.
getppid Returns the process id of the parent process.
getpriority WHICH,WHO
Returns the current priority for a process, a pro
cess group, or a user. (See getpriority(2).)
Will raise a fatal exception if used on a machine
that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their
counterparts in the system library. In list con
text, the return values from the various get rou
tines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it
usually contains the real name of the user (as
opposed to the login name) and other information
pertaining to the user. Beware, however, that in
many system users are able to change this informa
tion and therefore it cannot be trusted and there
fore the $gcos is tainted (see the perlsec man
page). The $passwd and $shell, user's encrypted
password and login shell, are also tainted,
because of the same reason.
In scalar context, you get the name, unless the
function was a lookup by name, in which case you
get the other thing, whatever it is. (If the
entry doesn't exist you get the undefined value.)
For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num;
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and
$expire are special cases in the sense that in
many systems they are unsupported. If the $quota
is unsupported, it is an empty scalar. If it is
supported, it usually encodes the disk quota. If
the $comment field is unsupported, it is an empty
scalar. If it is supported it usually encodes
some administrative comment about the user. In
some systems the $quota field may be $change or
$age, fields that have to do with password aging.
In some systems the $comment field may be $class.
The $expire field, if present, encodes the expira
tion period of the account or the password. For
the availability and the exact meaning of these
fields in your system, please consult your getpw_
nam(3) documentation and your pwd.h file. You can
also find out from within Perl what your $quota
and $comment fields mean and whether you have the
$expire field by using the "Config" module and the
values "d_pwquota", "d_pwage", "d_pwchange",
"d_pwcomment", and "d_pwexpire". Shadow password
files are only supported if your vendor has imple
mented them in the intuitive fashion that calling
the regular C library routines gets the shadow
versions if you're running under privilege or if
there exists the shadow(3) functions as found in
System V ( this includes Solaris and Linux.)
Those systems which implement a proprietary shadow
password facility are unlikely to be supported.
The $members value returned by getgr*() is a space
separated list of the login names of the members
of the group.
For the gethost*() functions, if the "h_errno"
variable is supported in C, it will be returned to
you via "$?" if the function call fails. The
"@addrs" value returned by a successful call is a
list of the raw addresses returned by the corre
sponding system library call. In the Internet
domain, each address is four bytes long and you
can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of
the return list contains which return value, by-
name interfaces are provided in standard modules:
"File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These over
ride the normal built-ins, supplying versions that
return objects with the appropriate names for each
field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method
calls (uid), they aren't, because a "File::stat"
object is different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of
the SOCKET connection, in case you don't know the
address because you have several different IPs
that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undef if
there is an error.
glob EXPR
glob Returns the value of EXPR with filename expansions
such as the standard Unix shell /bin/csh would do.
This is the internal function implementing the
"<*.c>" operator, but you can use it directly. If
EXPR is omitted, "$_" is used. The "<*.c>" opera
tor is discussed in more detail in the I/O Opera
tors entry in the perlop manpage.
Beginning with v5.6.0, this operator is imple
mented using the standard "File::Glob" extension.
See the File::Glob manpage for details.
gmtime EXPR
Converts a time as returned by the time function
to a 8-element list with the time localized for
the standard Greenwich time zone. Typically used
as follows:
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
All list elements are numeric, and come straight
out of the C `struct tm'. $sec, $min, and $hour
are the seconds, minutes, and hours of the speci
fied time. $mday is the day of the month, and
$mon is the month itself, in the range "0..11"
with 0 indicating January and 11 indicating Decem
ber. $year is the number of years since 1900.
That is, $year is "123" in year 2023. $wday is
the day of the week, with 0 indicating Sunday and
3 indicating Wednesday. $yday is the day of the
year, in the range "0..364" (or "0..365" in leap
years.)
Note that the $year element is not simply the last
two digits of the year. If you assume it is, then
you create non-Y2K-compliant programs--and you
wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is
simply:
$year += 1900;
And to get the last two digits of the year (e.g.,
'01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current
time ("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3)
value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
Also see the "timegm" function provided by the
"Time::Local" module, and the strftime(3) function
available via the POSIX module.
This scalar value is not locale dependent (see the
perllocale manpage), but is instead a Perl
builtin. Also see the "Time::Local" module, and
the strftime(3) and mktime(3) functions available
via the POSIX module. To get somewhat similar but
locale dependent date strings, set up your locale
environment variables appropriately (please see
the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the "%a" and "%b" escapes, which repre
sent the short forms of the day of the week and
the month of the year, may not necessarily be
three characters wide in all locales.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled
with LABEL and resumes execution there. It may
not be used to go into any construct that requires
initialization, such as a subroutine or a "fore
ach" loop. It also can't be used to go into a
construct that is optimized away, or to get out of
a block or subroutine given to "sort". It can be
used to go almost anywhere else within the dynamic
scope, including out of subroutines, but it's usu
ally better to use some other construct such as
"last" or "die". The author of Perl has never
felt the need to use this form of "goto" (in Perl,
that is--C is another matter).
The "goto-EXPR" form expects a label name, whose
scope will be resolved dynamically. This allows
for computed "goto"s per FORTRAN, but isn't neces
sarily recommended if you're optimizing for main
tainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the
other forms of "goto". In fact, it isn't a goto
in the normal sense at all, and doesn't have the
stigma associated with other gotos. Instead, it
substitutes a call to the named subroutine for the
currently running subroutine. This is used by
"AUTOLOAD" subroutines that wish to load another
subroutine and then pretend that the other subrou
tine had been called in the first place (except
that any modifications to "@_" in the current sub
routine are propagated to the other subroutine.)
After the "goto", not even "caller" will be able
to tell that this routine was called first.
NAME needn't be the name of a subroutine; it can
be a scalar variable containing a code reference,
or a block which evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as,
grep(1) and its relatives. In particular, it is
not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of
LIST (locally setting "$_" to each element) and
returns the list value consisting of those ele
ments for which the expression evaluated to true.
In scalar context, returns the number of times the
expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that "$_" is an alias to the list value, so
it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause
bizarre results if the elements of LIST are not
variables. Similarly, grep returns aliases into
the original list, much as a for loop's index
variable aliases the list elements. That is, mod
ifying an element of a list returned by grep (for
example, in a "foreach", "map" or another "grep")
actually modifies the element in the original
list. This is usually something to be avoided
when writing clear code.
See also the map entry elsewhere in this document
for a list composed of the results of the BLOCK or
EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the
corresponding value. (To convert strings that
might start with either 0, 0x, or 0b, see the oct
entry elsewhere in this document.) If EXPR is
omitted, uses "$_".
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings
that would cause integer overflow trigger a warn
ing.
import There is no builtin "import" function. It is just
an ordinary method (subroutine) defined (or inher
ited) by modules that wish to export names to
another module. The "use" function calls the
"import" method for the package used. See also
the use entry elsewhere in this document, the
perlmod manpage, and the Exporter manpage.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within
another, but without the wildcard-like behavior of
a full regular-expression pattern match. It
returns the position of the first occurrence of
SUBSTR in STR at or after POSITION. If POSITION
is omitted, starts searching from the beginning of
the string. The return value is based at "0" (or
whatever you've set the "$[" variable to--but
don't do that). If the substring is not found,
returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is
omitted, uses "$_". You should not use this func
tion for rounding: one because it truncates
towards "0", and two because machine representa
tions of floating point numbers can sometimes pro
duce counterintuitive results. For example,
"int(-6.725/0.025)" produces -268 rather than the
correct -269; that's because it's really more like
-268.99999999999994315658 instead. Usually, the
"sprintf", "printf", or the "POSIX::floor" and
"POSIX::ceil" functions will serve you better than
will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably
first have to say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
to get the correct function definitions. If
ioctl.ph doesn't exist or doesn't have the correct
definitions you'll have to roll your own, based on
your C header files such as <sys/ioctl.h>. (There
is a Perl script called h2ph that comes with the
Perl kit that may help you in this, but it's non
trivial.) SCALAR will be read and/or written
depending on the FUNCTION--a pointer to the string
value of SCALAR will be passed as the third argu
ment of the actual "ioctl" call. (If SCALAR has
no string value but does have a numeric value,
that value will be passed rather than a pointer to
the string value. To guarantee this to be true,
add a "0" to the scalar before using it.) The
"pack" and "unpack" functions may be needed to
manipulate the values of structures used by
"ioctl".
The return value of "ioctl" (and "fcntl") is as
follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on
failure, yet you can still easily determine the
actual value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string ""0" but true" is exempt from
-w complaints about improper numeric conversions.
Here's an example of setting a filehandle named
"REMOTE" to be non-blocking at the system level.
You'll have to negotiate "$|" on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
join EXPR,LIST
Joins the separate strings of LIST into a single
string with fields separated by the value of EXPR,
and returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a
pattern as its first argument. Compare the split
entry elsewhere in this document.
keys HASH
Returns a list consisting of all the keys of the
named hash. (In scalar context, returns the num
ber of keys.) The keys are returned in an appar
ently random order. The actual random order is
subject to change in future versions of perl, but
it is guaranteed to be the same order as either
the "values" or "each" function produces (given
that the hash has not been modified). As a side
effect, it resets HASH's iterator.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original
keys in the hash, so modifying them will not
affect the original hash. Compare the values
entry elsewhere in this document.
To sort a hash by value, you'll need to use a
"sort" function. Here's a descending numeric sort
of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "keys" allows you to increase the
number of hash buckets allocated for the given
hash. This can gain you a measure of efficiency
if you know the hash is going to get big. (This
is similar to pre-extending an array by assigning
a larger number to $#array.) If you say
keys %hash = 200;
then "%hash" will have at least 200 buckets allo
cated for it--256 of them, in fact, since it
rounds up to the next power of two. These buckets
will be retained even if you do "%hash = ()", use
"undef %hash" if you want to free the storage
while "%hash" is still in scope. You can't shrink
the number of buckets allocated for the hash using
"keys" in this way (but you needn't worry about
doing this by accident, as trying has no effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns
the number of processes successfully signaled
(which is not necessarily the same as the number
actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the pro
cess. This is a useful way to check that the pro
cess is alive and hasn't changed its UID. See the
perlport manpage for notes on the portability of
this construct.
Unlike in the shell, if SIGNAL is negative, it
kills process groups instead of processes. (On
System V, a negative PROCESS number will also kill
process groups, but that's not portable.) That
means you usually want to use positive not nega
tive signals. You may also use a signal name in
quotes. See the Signals entry in the perlipc man
page for details.
last LABEL
last The "last" command is like the "break" statement
in C (as used in loops); it immediately exits the
loop in question. If the LABEL is omitted, the
command refers to the innermost enclosing loop.
The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which
returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or
map() operation.
Note that a block by itself is semantically iden
tical to a loop that executes once. Thus "last"
can be used to effect an early exit out of such a
block.
See also the continue entry elsewhere in this doc
ument for an illustration of how "last", "next",
and "redo" work.
lc EXPR
lc Returns an lowercased version of EXPR. This is
the internal function implementing the "\L" escape
in double-quoted strings. Respects current
LC_CTYPE locale if "use locale" in force. See the
perllocale manpage and the utf8 manpage.
If EXPR is omitted, uses "$_".
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character
lowercased. This is the internal function imple
menting the "\l" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale"
in force. See the perllocale manpage.
If EXPR is omitted, uses "$_".
length EXPR
length Returns the length in characters of the value of
EXPR. If EXPR is omitted, returns length of "$_".
Note that this cannot be used on an entire array
or hash to find out how many elements these have.
For that, use "scalar @array" and "scalar keys
%hash" respectively.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename.
Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call
does. Returns true if it succeeded, false other
wise. See the example in the Sockets:
Client/Server Communication entry in the perlipc
manpage.
local EXPR
You really probably want to be using "my" instead,
because "local" isn't what most people think of as
"local". See the Private Variables via my() entry
in the perlsub manpage for details.
A local modifies the listed variables to be local
to the enclosing block, file, or eval. If more
than one value is listed, the list must be placed
in parentheses. See the Temporary Values via
local() entry in the perlsub manpage for details,
including issues with tied arrays and hashes.
localtime EXPR
Converts a time as returned by the time function
to a 9-element list with the time analyzed for the
local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight
out of the C `struct tm'. $sec, $min, and $hour
are the seconds, minutes, and hours of the speci
fied time. $mday is the day of the month, and
$mon is the month itself, in the range "0..11"
with 0 indicating January and 11 indicating Decem
ber. $year is the number of years since 1900.
That is, $year is "123" in year 2023. $wday is
the day of the week, with 0 indicating Sunday and
3 indicating Wednesday. $yday is the day of the
year, in the range "0..364" (or "0..365" in leap
years.) $isdst is true if the specified time
occurs during daylight savings time, false other
wise.
Note that the $year element is not simply the last
two digits of the year. If you assume it is, then
you create non-Y2K-compliant programs--and you
wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is
simply:
$year += 1900;
And to get the last two digits of the year (e.g.,
'01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "localtime()" uses the current
time ("localtime(time)").
In scalar context, "localtime()" returns the
ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent, see the
perllocale manpage, but instead a Perl builtin.
Also see the "Time::Local" module (to convert the
second, minutes, hours, ... back to seconds since
the stroke of midnight the 1st of January 1970,
the value returned by time()), and the strftime(3)
and mktime(3) functions available via the POSIX
module. To get somewhat similar but locale depen
dent date strings, set up your locale environment
variables appropriately (please see the perllocale
manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the "%a" and "%b", the short forms of
the day of the week and the month of the year, may
not necessarily be three characters wide.
lock
lock I<THING>
This function places an advisory lock on a vari
able, subroutine, or referenced object contained
in THING until the lock goes out of scope. This
is a built-in function only if your version of
Perl was built with threading enabled, and if
you've said "use Threads". Otherwise a user-
defined function by this name will be called. See
the Thread manpage.
log EXPR
log Returns the natural logarithm (base e) of EXPR.
If EXPR is omitted, returns log of "$_". To get
the log of another base, use basic algebra: The
base-N log of a number is equal to the natural log
of that number divided by the natural log of N.
For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also the exp entry elsewhere in this document
for the inverse operation.
lstat FILEHANDLE
lstat EXPR
lstat Does the same thing as the "stat" function
(including setting the special "_" filehandle) but
stats a symbolic link instead of the file the sym
bolic link points to. If symbolic links are unim
plemented on your system, a normal "stat" is done.
If EXPR is omitted, stats "$_".
m// The match operator. See the perlop manpage.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of
LIST (locally setting "$_" to each element) and
returns the list value composed of the results of
each such evaluation. In scalar context, returns
the total number of elements so generated. Evalu
ates BLOCK or EXPR in list context, so each ele
ment of LIST may produce zero, one, or more ele
ments in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding
characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that "$_" is an alias to the list value, so
it can be used to modify the elements of the LIST.
While this is useful and supported, it can cause
bizarre results if the elements of LIST are not
variables. Using a regular "foreach" loop for
this purpose would be clearer in most cases. See
also the grep entry elsewhere in this document for
an array composed of those items of the original
list for which the BLOCK or EXPR evaluates to
true.
"{" starts both hash references and blocks, so
"map { ..." could be either the start of map BLOCK
LIST or map EXPR, LIST. Because perl doesn't look
ahead for the closing "}" it has to take a guess
at which its dealing with based what it finds just
after the "{". Usually it gets it right, but if it
doesn't it won't realize something is wrong until
it gets to the "}" and encounters the missing (or
unexpected) comma. The syntax error will be
reported close to the "}" but you'll need to
change something near the "{" such as using a
unary "+" to give perl some help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{"
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with
only 1 entry.
mkdir FILENAME,MASK
mkdir FILENAME
Creates the directory specified by FILENAME, with
permissions specified by MASK (as modified by
"umask"). If it succeeds it returns true, other
wise it returns false and sets "$!" (errno). If
omitted, MASK defaults to 0777.
In general, it is better to create directories
with permissive MASK, and let the user modify that
with their "umask", than it is to supply a
restrictive MASK and give the user no way to be
more permissive. The exceptions to this rule are
when the file or directory should be kept private
(mail files, for instance). The perlfunc(1) entry
on "umask" discusses the choice of MASK in more
detail.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is "IPC_STAT", then ARG must be a variable
which will hold the returned "msqid_ds" structure.
Returns like "ioctl": the undefined value for
error, ""0 but true"" for zero, or the actual
return value otherwise. See also the SysV IPC
entry in the perlipc manpage, "IPC::SysV", and
"IPC::Semaphore" documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2).
Returns the message queue id, or the undefined
value if there is an error. See also the SysV IPC
entry in the perlipc manpage and "IPC::SysV" and
"IPC::Msg" documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive
a message from message queue ID into variable VAR
with a maximum message size of SIZE. Note that
when a message is received, the message type as a
native long integer will be the first thing in
VAR, followed by the actual message. This packing
may be opened with "unpack("l! a*")". Taints the
variable. Returns true if successful, or false if
there is an error. See also the SysV IPC entry in
the perlipc manpage, "IPC::SysV", and
"IPC::SysV::Msg" documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the
message MSG to the message queue ID. MSG must
begin with the native long integer message type,
and be followed by the length of the actual mes
sage, and finally the message itself. This kind
of packing can be achieved with "pack("l! a*",
$type, $message)". Returns true if successful, or
false if there is an error. See also "IPC::SysV"
and "IPC::SysV::Msg" documentation.
my EXPR
my EXPR : ATTRIBUTES
A "my" declares the listed variables to be local
(lexically) to the enclosing block, file, or
"eval". If more than one value is listed, the
list must be placed in parentheses. See the Pri
vate Variables via my() entry in the perlsub man
page for details.
next LABEL
next The "next" command is like the "continue" state
ment in C; it starts the next iteration of the
loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the
above, it would get executed even on discarded
lines. If the LABEL is omitted, the command
refers to the innermost enclosing loop.
"next" cannot be used to exit a block which
returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or
map() operation.
Note that a block by itself is semantically iden
tical to a loop that executes once. Thus "next"
will exit such a block early.
See also the continue entry elsewhere in this
document for an illustration of how "last",
"next", and "redo" work.
no Module LIST
See the the use entry elsewhere in this document
function, which "no" is the opposite of.
oct EXPR
oct Interprets EXPR as an octal string and returns the
corresponding value. (If EXPR happens to start
off with "0x", interprets it as a hex string. If
EXPR starts off with "0b", it is interpreted as a
binary string.) The following will handle deci
mal, binary, octal, and hex in the standard Perl
or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses "$_". To go the other
way (produce a number in octal), use sprintf() or
printf():
$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string
such as "644" needs to be converted into a file
mode, for example. (Although perl will automati
cally convert strings into numbers as needed, this
automatic conversion assumes base 10.)
open FILEHANDLE,MODE,LIST
open FILEHANDLE,EXPR
open FILEHANDLE
Opens the file whose filename is given by EXPR,
and associates it with FILEHANDLE. If FILEHANDLE
is an expression, its value is used as the name of
the real filehandle wanted. (This is considered a
symbolic reference, so "use strict 'refs'" should
not be in effect.)
If EXPR is omitted, the scalar variable of the
same name as the FILEHANDLE contains the filename.
(Note that lexical variables--those declared with
"my"--will not work for this purpose; so if you're
using "my", specify EXPR in your call to open.)
See the perlopentut manpage for a kinder, gentler
explanation of opening files.
If MODE is "'<'" or nothing, the file is opened
for input. If MODE is "'>'", the file is trun
cated and opened for output, being created if nec
essary. If MODE is "'>>'", the file is opened for
appending, again being created if necessary. You
can put a "'+'" in front of the "'>'" or "'<'" to
indicate that you want both read and write access
to the file; thus "'+<'" is almost always pre
ferred for read/write updates--the "'+>'" mode
would clobber the file first. You can't usually
use either read-write mode for updating textfiles,
since they have variable length records. See the
-i switch in the perlrun manpage for a better
approach. The file is created with permissions of
"0666" modified by the process' "umask" value.
These various prefixes correspond to the fopen(3)
modes of "'r'", "'r+'", "'w'", "'w+'", "'a'", and
"'a+'".
In the 2-arguments (and 1-argument) form of the
call the mode and filename should be concatenated
(in this order), possibly separated by spaces. It
is possible to omit the mode if the mode is "'<'".
If the filename begins with "'|'", the filename is
interpreted as a command to which output is to be
piped, and if the filename ends with a "'|'", the
filename is interpreted as a command which pipes
output to us. See the Using open() for IPC entry
in the perlipc manpage for more examples of this.
(You are not allowed to "open" to a command that
pipes both in and out, but see the IPC::Open2 man
page, the IPC::Open3 manpage, and the Bidirec
tional Communication with Another Process entry in
the perlipc manpage for alternatives.)
If MODE is "'|-'", the filename is interpreted as
a command to which output is to be piped, and if
MODE is "'-|'", the filename is interpreted as a
command which pipes output to us. In the 2-argu
ments (and 1-argument) form one should replace
dash ("'-'") with the command. See the Using
open() for IPC entry in the perlipc manpage for
more examples of this. (You are not allowed to
"open" to a command that pipes both in and out,
but see the IPC::Open2 manpage, the IPC::Open3
manpage, and the Bidirectional Communication entry
in the perlipc manpage for alternatives.)
In the 2-arguments (and 1-argument) form opening
"'-'" opens STDIN and opening "'>-'" opens STDOUT.
Open returns nonzero upon success, the undefined
value otherwise. If the "open" involved a pipe,
the return value happens to be the pid of the sub
process.
If you're unfortunate enough to be running Perl on
a system that distinguishes between text files and
binary files (modern operating systems don't
care), then you should check out the binmode entry
elsewhere in this document for tips for dealing
with this. The key distinction between systems
that need "binmode" and those that don't is their
text file formats. Systems like Unix, MacOS, and
Plan9, which delimit lines with a single charac
ter, and which encode that character in C as
""\n"", do not need "binmode". The rest need it.
When opening a file, it's usually a bad idea to
continue normal execution if the request failed,
so "open" is frequently used in connection with
"die". Even if "die" won't do what you want (say,
in a CGI script, where you want to make a nicely
formatted error message (but there are modules
that can help with that problem)) you should
always check the return value from opening a file.
The infrequent exception is when working with an
unopened filehandle is actually what you want to
do.
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(DBASE, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
You may also, in the Bourne shell tradition, spec
ify an EXPR beginning with "'>&'", in which case
the rest of the string is interpreted as the name
of a filehandle (or file descriptor, if numeric)
to be duped and opened. You may use "&" after
">", ">>", "<", "+>", "+>>", and "+<". The mode
you specify should match the mode of the original
filehandle. (Duping a filehandle does not take
into account any existing contents of stdio
buffers.) Duping file handles is not yet sup
ported for 3-argument open().
Here is a script that saves, redirects, and
restores STDOUT and STDERR:
#!/usr/bin/perl
open(OLDOUT, ">&STDOUT");
open(OLDERR, ">&STDERR");
open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&OLDOUT");
open(STDERR, ">&OLDERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify "'<&=N'", where "N" is a number,
then Perl will do an equivalent of C's "fdopen" of
that file descriptor; this is more parsimonious of
file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
Note that this feature depends on the fdopen() C
library function. On many UNIX systems, fdopen()
is known to fail when file descriptors exceed a
certain value, typically 255. If you need more
file descriptors than that, consider rebuilding
Perl to use the "sfio" library.
If you open a pipe on the command "'-'", i.e.,
either "'|-'" or "'-|'" with 2-arguments (or
1-argument) form of open(), then there is an
implicit fork done, and the return value of open
is the pid of the child within the parent process,
and "0" within the child process. (Use
"defined($pid)" to determine whether the open was
successful.) The filehandle behaves normally for
the parent, but i/o to that filehandle is piped
from/to the STDOUT/STDIN of the child process. In
the child process the filehandle isn't opened--i/o
happens from/to the new STDOUT or STDIN. Typi
cally this is used like the normal piped open when
you want to exercise more control over just how
the pipe command gets executed, such as when you
are running setuid, and don't want to have to scan
shell commands for metacharacters. The following
triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
See the Safe Pipe Opens entry in the perlipc man
page for more examples of this.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before any operation
that may do a fork, but this may not be supported
on some platforms (see the perlport manpage). To
be safe, you may need to set "$|" ($AUTOFLUSH in
English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F.
See the section on "$^F" in the perlvar manpage.
Closing any piped filehandle causes the parent
process to wait for the child to finish, and
returns the status value in "$?".
The filename passed to 2-argument (or 1-argument)
form of open() will have leading and trailing
whitespace deleted, and the normal redirection
characters honored. This property, known as
"magic open", can often be used to good effect. A
user could specify a filename of "rsh cat file |",
or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary
weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading
and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems).
One should conscientiously choose between the
magic and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the
form ""rsh cat file |"", but will not work on a
filename which happens to have a trailing space,
while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your
system), then you should use the "sysopen" func
tion, which involves no such magic (but may use
subtly different filemodes than Perl open(), which
is mapped to C fopen()). This is another way to
protect your filenames from interpretation. For
example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" pack
age (or one of its subclasses, such as "IO::File"
or "IO::Socket"), you can generate anonymous file
handles that have the scope of whatever variables
hold references to them, and automatically close
whenever and however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See the seek entry elsewhere in this document for
some details about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
"readdir", "telldir", "seekdir", "rewinddir", and
"closedir". Returns true if successful. DIRHAN
DLEs have their own namespace separate from FILE
HANDLEs.
ord EXPR
ord Returns the numeric (ASCII or Unicode) value of
the first character of EXPR. If EXPR is omitted,
uses "$_". For the reverse, see the chr entry
elsewhere in this document. See the utf8 manpage
for more about Unicode.
our EXPR
An "our" declares the listed variables to be valid
globals within the enclosing block, file, or
"eval". That is, it has the same scoping rules as
a "my" declaration, but does not create a local
variable. If more than one value is listed, the
list must be placed in parentheses. The "our"
declaration has no semantic effect unless "use
strict vars" is in effect, in which case it lets
you use the declared global variable without qual
ifying it with a package name. (But only within
the lexical scope of the "our" declaration. In
this it differs from "use vars", which is package
scoped.)
An "our" declaration declares a global variable
that will be visible across its entire lexical
scope, even across package boundaries. The pack
age in which the variable is entered is determined
at the point of the declaration, not at the point
of use. This means the following behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20
Multiple "our" declarations in the same lexical
scope are allowed if they are in different pack
ages. If they happened to be in the same package,
Perl will emit warnings if you have asked for
them.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a
string using the rules given by the TEMPLATE. The
resulting string is the concatenation of the con
verted values. Typically, each converted value
looks like its machine-level representation. For
example, on 32-bit machines a converted integer
may be represented by a sequence of 4 bytes.
The TEMPLATE is a sequence of characters that give
the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A An ASCII string, will be space padded.
Z A null terminated (asciz) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value. Only does bytes. See U for Unicode.
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'. If you want
native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'. If you want
native-length longs, use the '!' suffix.)
n An unsigned short in "network" (big-endian) order.
N An unsigned long in "network" (big-endian) order.
v An unsigned short in "VAX" (little-endian) order.
V An unsigned long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to UTF-8 internally.
Works even if C<use utf8> is not in effect.
w A BER compressed integer. Its bytes represent an unsigned
integer in base 128, most significant digit first, with as
few digits as possible. Bit eight (the high bit) is set
on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
The following rules apply:
Each letter may optionally be followed by
a number giving a repeat count. With all
types except "a", "A", "Z", "b", "B", "h",
"H", and "P" the pack function will gobble
up that many values from the LIST. A "*"
for the repeat count means to use however
many items are left, except for "@", "x",
"X", where it is equivalent to "0", and
"u", where it is equivalent to 1 (or 45,
what is the same).
When used with "Z", "*" results in the
addition of a trailing null byte (so the
packed result will be one longer than the
byte "length" of the item).
The repeat count for "u" is interpreted as
the maximal number of bytes to encode per
line of output, with 0 and 1 replaced by
45.
The "a", "A", and "Z" types gobble just
one value, but pack it as a string of
length count, padding with nulls or spaces
as necessary. When unpacking, "A" strips
trailing spaces and nulls, "Z" strips
everything after the first null, and "a"
returns data verbatim. When packing, "a",
and "Z" are equivalent.
If the value-to-pack is too long, it is
truncated. If too long and an explicit
count is provided, "Z" packs only
"$count-1" bytes, followed by a null byte.
Thus "Z" always packs a trailing null byte
under all circumstances.
Likewise, the "b" and "B" fields pack a
string that many bits long. Each byte of
the input field of pack() generates 1 bit
of the result. Each result bit is based
on the least-significant bit of the corre
sponding input byte, i.e., on
"ord($byte)%2". In particular, bytes
""0"" and ""1"" generate bits 0 and 1, as
do bytes ""\0"" and ""\1"".
Starting from the beginning of the input
string of pack(), each 8-tuple of bytes is
converted to 1 byte of output. With for
mat "b" the first byte of the 8-tuple
determines the least-significant bit of a
byte, and with format "B" it determines
the most-significant bit of a byte.
If the length of the input string is not
exactly divisible by 8, the remainder is
packed as if the input string were padded
by null bytes at the end. Similarly, dur
ing unpack()ing the "extra" bits are
ignored.
If the input string of pack() is longer
than needed, extra bytes are ignored. A
"*" for the repeat count of pack() means
to use all the bytes of the input field.
On unpack()ing the bits are converted to a
string of ""0""s and ""1""s.
The "h" and "H" fields pack a string that
many nybbles (4-bit groups, representable
as hexadecimal digits, 0-9a-f) long.
Each byte of the input field of pack()
generates 4 bits of the result. For non-
alphabetical bytes the result is based on
the 4 least-significant bits of the input
byte, i.e., on "ord($byte)%16". In par
ticular, bytes ""0"" and ""1"" generate
nybbles 0 and 1, as do bytes ""\0"" and
""\1"". For bytes ""a".."f"" and
""A".."F"" the result is compatible with
the usual hexadecimal digits, so that
""a"" and ""A"" both generate the nybble
"0xa==10". The result for bytes
""g".."z"" and ""G".."Z"" is not well-
defined.
Starting from the beginning of the input
string of pack(), each pair of bytes is
converted to 1 byte of output. With for
mat "h" the first byte of the pair deter
mines the least-significant nybble of the
output byte, and with format "H" it deter
mines the most-significant nybble.
If the length of the input string is not
even, it behaves as if padded by a null
byte at the end. Similarly, during
unpack()ing the "extra" nybbles are
ignored.
If the input string of pack() is longer
than needed, extra bytes are ignored. A
"*" for the repeat count of pack() means
to use all the bytes of the input field.
On unpack()ing the bits are converted to a
string of hexadecimal digits.
The "p" type packs a pointer to a null-
terminated string. You are responsible
for ensuring the string is not a temporary
value (which can potentially get deallo
cated before you get around to using the
packed result). The "P" type packs a
pointer to a structure of the size indi
cated by the length. A NULL pointer is
created if the corresponding value for "p"
or "P" is "undef", similarly for unpack().
The "/" template character allows packing
and unpacking of strings where the packed
structure contains a byte count followed
by the string itself. You write length-
item"/"string-item.
The length-item can be any "pack" template
letter, and describes how the length value
is packed. The ones likely to be of most
use are integer-packing ones like "n" (for
Java strings), "w" (for ASN.1 or SNMP) and
"N" (for Sun XDR).
The string-item must, at present, be
""A*"", ""a*"" or ""Z*"". For "unpack"
the length of the string is obtained from
the length-item, but if you put in the '*'
it will be ignored.
unpack 'C/a', "\04Gurusamy"; gives 'Guru'
unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
The length-item is not returned explicitly
from "unpack".
Adding a count to the length-item letter
is unlikely to do anything useful, unless
that letter is "A", "a" or "Z". Packing
with a length-item of "a" or "Z" may
introduce ""\000"" characters, which Perl
does not regard as legal in numeric
strings.
The integer types "s", "S", "l", and "L"
may be immediately followed by a "!" suf
fix to signify native shorts or longs--as
you can see from above for example a bare
"l" does mean exactly 32 bits, the native
"long" (as seen by the local C compiler)
may be larger. This is an issue mainly in
64-bit platforms. You can see whether
using "!" makes any difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
"i!" and "I!" also work but only because
of completeness; they are identical to "i"
and "I".
The actual sizes (in bytes) of native
shorts, ints, longs, and long longs on the
platform where Perl was built are also
available via the Config manpage:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
(The "$Config{longlongsize}" will be unde
fine if your system does not support long
longs.)
The integer formats "s", "S", "i", "I",
"l", and "L" are inherently non-portable
between processors and operating systems
because they obey the native byteorder and
endianness. For example a 4-byte integer
0x12345678 (305419896 decimal) be ordered
natively (arranged in and handled by the
CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, the Intel and VAX CPUs are lit
tle-endian, while everybody else, for
example Motorola m68k/88k, PPC, Sparc, HP
PA, Power, and Cray are big-endian. Alpha
and MIPS can be either: Digital/Compaq
used/uses them in little-endian mode;
SGI/Cray uses them in big-endian mode.
The names `big-endian' and `little-endian'
are comic references to the classic "Gul
liver's Travels" (via the paper "On Holy
Wars and a Plea for Peace" by Danny Cohen,
USC/ISI IEN 137, April 1, 1980) and the
egg-eating habits of the Lilliputians.
Some systems may have even weirder byte
orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("C*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl
was built is also available via the Config
manpage:
use Config;
print $Config{byteorder}, "\n";
Byteorders "'1234'" and "'12345678'" are
little-endian, "'4321'" and "'87654321'"
are big-endian.
If you want portable packed integers use
the formats "n", "N", "v", and "V", their
byte endianness and size is known. See
also the perlport manpage.
Real numbers (floats and doubles) are in
the native machine format only; due to the
multiplicity of floating formats around,
and the lack of a standard "network" rep
resentation, no facility for interchange
has been made. This means that packed
floating point data written on one machine
may not be readable on another - even if
both use IEEE floating point arithmetic
(as the endian-ness of the memory repre
sentation is not part of the IEEE spec).
See also the perlport manpage.
Note that Perl uses doubles internally for
all numeric calculation, and converting
from double into float and thence back to
double again will lose precision (i.e.,
"unpack("f", pack("f", $foo)") will not in
general equal $foo).
If the pattern begins with a "U", the
resulting string will be treated as Uni
code-encoded. You can force UTF8 encoding
on in a string with an initial "U0", and
the bytes that follow will be interpreted
as Unicode characters. If you don't want
this to happen, you can begin your pattern
with "C0" (or anything else) to force Perl
not to UTF8 encode your string, and then
follow this with a "U*" somewhere in your
pattern.
You must yourself do any alignment or
padding by inserting for example enough
"'x'"es while packing. There is no way to
pack() and unpack() could know where the
bytes are going to or coming from. There
fore "pack" (and "unpack") handle their
output and input as flat sequences of
bytes.
A comment in a TEMPLATE starts with "#"
and goes to the end of line.
If TEMPLATE requires more arguments to
pack() than actually given, pack() assumes
additional """" arguments. If TEMPLATE
requires less arguments to pack() than
actually given, extra arguments are
ignored.
Examples:
$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# note: the above examples featuring "C" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
# $foo = pack("CCCC",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
The same template may generally also be used in
unpack().
package NAMESPACE
package Declares the compilation unit as being in the
given namespace. The scope of the package decla
ration is from the declaration itself through the
end of the enclosing block, file, or eval (the
same as the "my" operator). All further unquali
fied dynamic identifiers will be in this names
pace. A package statement affects only dynamic
variables--including those you've used "local"
on--but not lexical variables, which are created
with "my". Typically it would be the first decla
ration in a file to be included by the "require"
or "use" operator. You can switch into a package
in more than one place; it merely influences which
symbol table is used by the compiler for the rest
of that block. You can refer to variables and
filehandles in other packages by prefixing the
identifier with the package name and a double
colon: "$Package::Variable". If the package name
is null, the "main" package as assumed. That is,
"$::sail" is equivalent to "$main::sail" (as well
as to "$main'sail", still seen in older code).
If NAMESPACE is omitted, then there is no current
package, and all identifiers must be fully quali
fied or lexicals. This is stricter than "use
strict", since it also extends to function names.
See the Packages entry in the perlmod manpage for
more information about packages, modules, and
classes. See the perlsub manpage for other scop
ing issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corre
sponding system call. Note that if you set up a
loop of piped processes, deadlock can occur unless
you are very careful. In addition, note that
Perl's pipes use stdio buffering, so you may need
to set "$|" to flush your WRITEHANDLE after each
command, depending on the application.
See the IPC::Open2 manpage, the IPC::Open3 man
page, and the Bidirectional Communication entry in
the perlipc manpage for examples of such things.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptors as determined by the value of
$^F. See the section on "$^F" in the perlvar man
page.
pop ARRAY
pop Pops and returns the last value of the array,
shortening the array by one element. Has an
effect similar to
$ARRAY[$#ARRAY--]
If there are no elements in the array, returns the
undefined value (although this may happen at other
times as well). If ARRAY is omitted, pops the
"@ARGV" array in the main program, and the "@_"
array in subroutines, just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search
left off for the variable in question ("$_" is
used when the variable is not specified). May be
modified to change that offset. Such modification
will also influence the "\G" zero-width assertion
in regular expressions. See the perlre manpage
and the perlop manpage.
print FILEHANDLE LIST
print LIST
print Prints a string or a list of strings. Returns
true if successful. FILEHANDLE may be a scalar
variable name, in which case the variable contains
the name of or a reference to the filehandle, thus
introducing one level of indirection. (NOTE: If
FILEHANDLE is a variable and the next token is a
term, it may be misinterpreted as an operator
unless you interpose a "+" or put parentheses
around the arguments.) If FILEHANDLE is omitted,
prints by default to standard output (or to the
last selected output channel--see the select entry
elsewhere in this document). If LIST is also
omitted, prints "$_" to the currently selected
output channel. To set the default output channel
to something other than STDOUT use the select
operation. The current value of "$," (if any) is
printed between each LIST item. The current value
of "$\" (if any) is printed after the entire LIST
has been printed. Because print takes a LIST,
anything in the LIST is evaluated in list context,
and any subroutine that you call will have one or
more of its expressions evaluated in list context.
Also be careful not to follow the print keyword
with a left parenthesis unless you want the corre
sponding right parenthesis to terminate the argu
ments to the print--interpose a "+" or put paren
theses around all the arguments.
Note that if you're storing FILEHANDLES in an
array or other expression, you will have to use a
block returning its value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT,
LIST)", except that "$\" (the output record sepa
rator) is not appended. The first argument of the
list will be interpreted as the "printf" format.
If "use locale" is in effect, the character used
for the decimal point in formatted real numbers is
affected by the LC_NUMERIC locale. See the perl
locale manpage.
Don't fall into the trap of using a "printf" when
a simple "print" would do. The "print" is more
efficient and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string
(or "undef" if the function has no prototype).
FUNCTION is a reference to, or the name of, the
function whose prototype you want to retrieve.
If FUNCTION is a string starting with "CORE::",
the rest is taken as a name for Perl builtin. If
the builtin is not overridable (such as "qw//") or
its arguments cannot be expressed by a prototype
(such as "system") returns "undef" because the
builtin does not really behave like a Perl func
tion. Otherwise, the string describing the equiv
alent prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of
LIST onto the end of ARRAY. The length of ARRAY
increases by the length of LIST. Has the same
effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of
elements in the array.
q/STRING/
qq/STRING/
qr/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See the Regexp Quote-Like
Operators entry in the perlop manpage.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word"
characters backslashed. (That is, all characters
not matching "/[A-Za-z_0-9]/" will be preceded by
a backslash in the returned string, regardless of
any locale settings.) This is the internal func
tion implementing the "\Q" escape in double-quoted
strings.
If EXPR is omitted, uses "$_".
rand EXPR
rand Returns a random fractional number greater than or
equal to "0" and less than the value of EXPR.
(EXPR should be positive.) If EXPR is omitted,
the value "1" is used. Automatically calls
"srand" unless "srand" has already been called.
See also "srand".
(Note: If your rand function consistently returns
numbers that are too large or too small, then your
version of Perl was probably compiled with the
wrong number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into vari
able SCALAR from the specified FILEHANDLE.
Returns the number of bytes actually read, "0" at
end of file, or undef if there was an error.
SCALAR will be grown or shrunk to the length actu
ally read. If SCALAR needs growing, the new bytes
will be zero bytes. An OFFSET may be specified to
place the read data into some other place in
SCALAR than the beginning. The call is actually
implemented in terms of stdio's fread(3) call. To
get a true read(2) system call, see "sysread".
readdir DIRHANDLE
Returns the next directory entry for a directory
opened by "opendir". If used in list context,
returns all the rest of the entries in the direc
tory. If there are no more entries, returns an
undefined value in scalar context or a null list
in list context.
If you're planning to filetest the return values
out of a "readdir", you'd better prepend the
directory in question. Otherwise, because we
didn't "chdir" there, it would have been testing
the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
readline EXPR
Reads from the filehandle whose typeglob is con
tained in EXPR. In scalar context, each call
reads and returns the next line, until end-of-file
is reached, whereupon the subsequent call returns
undef. In list context, reads until end-of-file
is reached and returns a list of lines. Note that
the notion of "line" used here is however you may
have defined it with "$/" or "$INPUT_RECORD_SEPA
RATOR"). See the section on "$/" in the perlvar
manpage.
When "$/" is set to "undef", when readline() is in
scalar context (i.e. file slurp mode), and when an
empty file is read, it returns "''" the first
time, followed by "undef" subsequently.
This is the internal function implementing the
"<EXPR>" operator, but you can use it directly.
The "<EXPR>" operator is discussed in more detail
in the I/O Operators entry in the perlop manpage.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic
links are implemented. If not, gives a fatal
error. If there is some system error, returns the
undefined value and sets "$!" (errno). If EXPR is
omitted, uses "$_".
readpipe EXPR
EXPR is executed as a system command. The col
lected standard output of the command is returned.
In scalar context, it comes back as a single
(potentially multi-line) string. In list context,
returns a list of lines (however you've defined
lines with "$/" or "$INPUT_RECORD_SEPARATOR").
This is the internal function implementing the
"qx/EXPR/" operator, but you can use it directly.
The "qx/EXPR/" operator is discussed in more
detail in the I/O Operators entry in the perlop
manpage.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to
receive LENGTH bytes of data into variable SCALAR
from the specified SOCKET filehandle. SCALAR will
be grown or shrunk to the length actually read.
Takes the same flags as the system call of the
same name. Returns the address of the sender if
SOCKET's protocol supports this; returns an empty
string otherwise. If there's an error, returns
the undefined value. This call is actually imple
mented in terms of recvfrom(2) system call. See
the UDP: Message Passing entry in the perlipc man
page for examples.
redo LABEL
redo The "redo" command restarts the loop block without
evaluating the conditional again. The "continue"
block, if any, is not executed. If the LABEL is
omitted, the command refers to the innermost
enclosing loop. This command is normally used by
programs that want to lie to themselves about what
was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block which
returns a value such as "eval {}", "sub {}" or "do
{}", and should not be used to exit a grep() or
map() operation.
Note that a block by itself is semantically iden
tical to a loop that executes once. Thus "redo"
inside such a block will effectively turn it into
a looping construct.
See also the continue entry elsewhere in this doc
ument for an illustration of how "last", "next",
and "redo" work.
ref EXPR
ref Returns a true value if EXPR is a reference, false
otherwise. If EXPR is not specified, "$_" will be
used. The value returned depends on the type of
thing the reference is a reference to. Builtin
types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
If the referenced object has been blessed into a
package, then that package name is returned
instead. You can think of "ref" as a "typeof"
operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
print "r is a reference to something that isa hash.\n";
}
See also the perlref manpage.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEW
NAME will be clobbered. Returns true for success,
false otherwise.
Behavior of this function varies wildly depending
on your system implementation. For example, it
will usually not work across file system bound
aries, even though the system mv command sometimes
compensates for this. Other restrictions include
whether it works on directories, open files, or
pre-existing files. Check the perlport manpage
and either the rename(2) manpage or equivalent
system documentation for details.
require VERSION
require EXPR
require Demands some semantics specified by EXPR, or by
"$_" if EXPR is not supplied.
If a VERSION is specified as a literal of the form
v5.6.1, demands that the current version of Perl
("$^V" or $PERL_VERSION) be at least as recent as
that version, at run time. (For compatibility
with older versions of Perl, a numeric argument
will also be interpreted as VERSION.) Compare
with the use entry elsewhere in this document,
which can do a similar check at compile time.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.005_03; # float version allowed for compatibility
Otherwise, demands that a library file be included
if it hasn't already been included. The file is
included via the do-FILE mechanism, which is
essentially just a variety of "eval". Has seman
tics similar to the following subroutine:
sub require {
my($filename) = @_;
return 1 if $INC{$filename};
my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
delete $INC{$filename} if $@ || !$result;
die $@ if $@;
die "$filename did not return true value" unless $result;
return $result;
}
Note that the file will not be included twice
under the same specified name. The file must
return true as the last statement to indicate suc
cessful execution of any initialization code, so
it's customary to end such a file with "1;" unless
you're sure it'll return true otherwise. But it's
better just to put the "1;", in case you add more
statements.
If EXPR is a bareword, the require assumes a ".pm"
extension and replaces "::" with "/" in the file
name for you, to make it easy to load standard
modules. This form of loading of modules does not
risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the
"Foo/Bar.pm" file in the directories specified in
the "@INC" array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar"
file in the @INC array and will complain about not
finding "Foo::Bar" there. In this case you can
do:
eval "require $class";
For a yet-more-powerful import facility, see the
use entry elsewhere in this document and the
perlmod manpage.
reset EXPR
reset Generally used in a "continue" block at the end of
a loop to clear variables and reset "??" searches
so that they work again. The expression is
interpreted as a list of single characters
(hyphens allowed for ranges). All variables and
arrays beginning with one of those letters are
reset to their pristine state. If the expression
is omitted, one-match searches ("?pattern?") are
reset to match again. Resets only variables or
searches in the current package. Always returns
1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting ""A-Z"" is not recommended because
you'll wipe out your "@ARGV" and "@INC" arrays and
your "%ENV" hash. Resets only package vari
ables--lexical variables are unaffected, but they
clean themselves up on scope exit anyway, so
you'll probably want to use them instead. See the
my entry elsewhere in this document.
return EXPR
return Returns from a subroutine, "eval", or "do FILE"
with the value given in EXPR. Evaluation of EXPR
may be in list, scalar, or void context, depending
on how the return value will be used, and the con
text may vary from one execution to the next (see
"wantarray"). If no EXPR is given, returns an
empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in
a void context.
(Note that in the absence of a explicit "return",
a subroutine, eval, or do FILE will automatically
return the value of the last expression evalu
ated.)
reverse LIST
In list context, returns a list value consisting
of the elements of LIST in the opposite order. In
scalar context, concatenates the elements of LIST
and returns a string value with all characters in
the opposite order.
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
This operator is also handy for inverting a hash,
although there are some caveats. If a value is
duplicated in the original hash, only one of those
can be represented as a key in the inverted hash.
Also, this has to unwind one hash and build a
whole new one, which may take some time on a large
hash, such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the
directory for the "readdir" routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the
position of the LAST occurrence of SUBSTR in STR.
If POSITION is specified, returns the last occur
rence at or before that position.
rmdir FILENAME
rmdir Deletes the directory specified by FILENAME if
that directory is empty. If it succeeds it
returns true, otherwise it returns false and sets
"$!" (errno). If FILENAME is omitted, uses "$_".
s/// The substitution operator. See the perlop man
page.
scalar EXPR
Forces EXPR to be interpreted in scalar context
and returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an
expression to be interpolated in list context
because in practice, this is never needed. If you
really wanted to do so, however, you could use the
construction "@{[ (some expression) ]}", but usu
ally a simple "(some expression)" suffices.
Because "scalar" is unary operator, if you acci
dentally use for EXPR a parenthesized list, this
behaves as a scalar comma expression, evaluating
all but the last element in void context and
returning the final element evaluated in scalar
context. This is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See the perlop manpage for more details on unary
operators and the comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the "fseek"
call of "stdio". FILEHANDLE may be an expression
whose value gives the name of the filehandle. The
values for WHENCE are "0" to set the new position
to POSITION, "1" to set it to the current position
plus POSITION, and "2" to set it to EOF plus POSI
TION (typically negative). For WHENCE you may use
the constants "SEEK_SET", "SEEK_CUR", and
"SEEK_END" (start of the file, current position,
end of the file) from the Fcntl module. Returns
"1" upon success, "0" otherwise.
If you want to position file for "sysread" or
"syswrite", don't use "seek"--buffering makes its
effect on the file's system position unpredictable
and non-portable. Use "sysseek" instead.
Due to the rules and rigors of ANSI C, on some
systems you have to do a seek whenever you switch
between reading and writing. Amongst other
things, this may have the effect of calling
stdio's clearerr(3). A WHENCE of "1" ("SEEK_CUR")
is useful for not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating
"tail -f". Once you hit EOF on your read, and
then sleep for a while, you might have to stick in
a seek() to reset things. The "seek" doesn't
change the current position, but it does clear the
end-of-file condition on the handle, so that the
next "<FILE>" makes Perl try again to read some
thing. We hope.
If that doesn't work (some stdios are particularly
cantankerous), then you may need something more
like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" rou
tine on DIRHANDLE. POS must be a value returned
by "telldir". Has the same caveats about possible
directory compaction as the corresponding system
library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets
the current default filehandle for output, if
FILEHANDLE is supplied. This has two effects:
first, a "write" or a "print" without a filehandle
will default to this FILEHANDLE. Second, refer
ences to variables related to output will refer to
this output channel. For example, if you have to
set the top of form format for more than one out
put channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehan
dles as objects with methods, preferring to write
the last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit
masks specified, which can be constructed using
"fileno" and "vec", along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you
might wish to write a subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do
this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything use
ful in $timeleft, so calling select() in scalar
context just returns $nfound.
Any of the bit masks can also be undef. The time
out, if specified, is in seconds, which may be
fractional. Note: not all implementations are
capable of returning the$timeleft. If not, they
always return $timeleft equal to the supplied
$timeout.
You can effect a sleep of 250 milliseconds this
way:
select(undef, undef, undef, 0.25);
WARNING: One should not attempt to mix buffered
I/O (like "read" or <FH>) with "select", except as
permitted by POSIX, and even then only on POSIX
systems. You have to use "sysread" instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function "semctl". You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is IPC_STAT or GETALL, then ARG must be a
variable which will hold the returned semid_ds
structure or semaphore value array. Returns like
"ioctl": the undefined value for error, ""0 but
true"" for zero, or the actual return value other
wise. The ARG must consist of a vector of native
short integers, which may be created with
"pack("s!",(0)x$nsem)". See also the SysV IPC
entry in the perlipc manpage, "IPC::SysV",
"IPC::Semaphore" documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns
the semaphore id, or the undefined value if there
is an error. See also the SysV IPC entry in the
perlipc manpage, "IPC::SysV",
"IPC::SysV::Semaphore" documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform
semaphore operations such as signaling and wait
ing. OPSTRING must be a packed array of semop
structures. Each semop structure can be generated
with "pack("sss", $semnum, $semop, $semflag)".
The number of semaphore operations is implied by
the length of OPSTRING. Returns true if success
ful, or false if there is an error. As an exam
ple, the following code waits on semaphore $semnum
of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
See also the SysV IPC entry in the perlipc man
page, "IPC::SysV", and "IPC::SysV::Semaphore" doc
umentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Takes the same flags
as the system call of the same name. On uncon
nected sockets you must specify a destination to
send TO, in which case it does a C "sendto".
Returns the number of characters sent, or the
undefined value if there is an error. The C sys
tem call sendmsg(2) is currently unimplemented.
See the UDP: Message Passing entry in the perlipc
manpage for examples.
setpgrp PID,PGRP
Sets the current process group for the specified
PID, "0" for the current process. Will produce a
fatal error if used on a machine that doesn't
implement POSIX setpgid(2) or BSD setpgrp(2). If
the arguments are omitted, it defaults to "0,0".
Note that the BSD 4.2 version of "setpgrp" does
not accept any arguments, so only "setpgrp(0,0)"
is portable. See also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process
group, or a user. (See setpriority(2).) Will
produce a fatal error if used on a machine that
doesn't implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns unde
fined if there is an error. OPTVAL may be speci
fied as "undef" if you don't want to pass an
argument.
shift ARRAY
shift Shifts the first value of the array off and
returns it, shortening the array by 1 and moving
everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is
omitted, shifts the "@_" array within the lexical
scope of subroutines and formats, and the "@ARGV"
array at file scopes or within the lexical scopes
established by the "eval ''", "BEGIN {}", "INIT
{}", "CHECK {}", and "END {}" constructs.
See also "unshift", "push", and "pop". "shift"
and "unshift" do the same thing to the left end of
an array that "pop" and "push" do to the right
end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is "IPC_STAT", then ARG must be a variable
which will hold the returned "shmid_ds" structure.
Returns like ioctl: the undefined value for error,
""0" but true" for zero, or the actual return
value otherwise. See also the SysV IPC entry in
the perlipc manpage and "IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns
the shared memory segment id, or the undefined
value if there is an error. See also the SysV IPC
entry in the perlipc manpage and "IPC::SysV" docu
mentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment
ID starting at position POS for size SIZE by
attaching to it, copying in/out, and detaching
from it. When reading, VAR must be a variable
that will hold the data read. When writing, if
STRING is too long, only SIZE bytes are used; if
STRING is too short, nulls are written to fill out
SIZE bytes. Return true if successful, or false
if there is an error. shmread() taints the vari
able. See also the SysV IPC entry in the perlipc
manpage, "IPC::SysV" documentation, and the
"IPC::Shareable" module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indi
cated by HOW, which has the same interpretation as
in the system call of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell
the other side you're done writing but not done
reading, or vice versa. It's also a more
insistent form of close because it also disables
the file descriptor in any forked copies in other
processes.
sin EXPR
sin Returns the sine of EXPR (expressed in radians).
If EXPR is omitted, returns sine of "$_".
For the inverse sine operation, you may use the
"Math::Trig::asin" function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or
forever if no EXPR. May be interrupted if the
process receives a signal such as "SIGALRM".
Returns the number of seconds actually slept. You
probably cannot mix "alarm" and "sleep" calls,
because "sleep" is often implemented using
"alarm".
On some older systems, it may sleep up to a full
second less than what you requested, depending on
how it counts seconds. Most modern systems always
sleep the full amount. They may appear to sleep
longer than that, however, because your process
might not be scheduled right away in a busy multi
tasking system.
For delays of finer granularity than one second,
you may use Perl's "syscall" interface to access
setitimer(2) if your system supports it, or else
see the select entry elsewhere in this document
above. The Time::HiRes module from CPAN may also
help.
See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches
it to filehandle SOCKET. DOMAIN, TYPE, and PROTO
COL are specified the same as for the system call
of the same name. You should "use Socket" first
to get the proper definitions imported. See the
examples in the Sockets: Client/Server Communica
tion entry in the perlipc manpage.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor, as determined by the value of
$^F. See the section on "$^F" in the perlvar man
page.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the speci
fied domain, of the specified type. DOMAIN, TYPE,
and PROTOCOL are specified the same as for the
system call of the same name. If unimplemented,
yields a fatal error. Returns true if successful.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptors, as determined by the value of
$^F. See the section on "$^F" in the perlvar
manpage.
Some systems defined "pipe" in terms of "socket
pair", in which a call to "pipe(Rdr, Wtr)" is
essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See the perlipc manpage for an example of socket
pair use.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted list value.
If SUBNAME or BLOCK is omitted, "sort"s in stan
dard string comparison order. If SUBNAME is spec
ified, it gives the name of a subroutine that
returns an integer less than, equal to, or greater
than "0", depending on how the elements of the
list are to be ordered. (The "<=>" and "cmp"
operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsub
scripted), in which case the value provides the
name of (or a reference to) the actual subroutine
to use. In place of a SUBNAME, you can provide a
BLOCK as an anonymous, in-line sort subroutine.
If the subroutine's prototype is "($$)", the ele
ments to be compared are passed by reference in
"@_", as for a normal subroutine. This is slower
than unprototyped subroutines, where the elements
to be compared are passed into the subroutine as
the package global variables $a and $b (see exam
ple below). Note that in the latter case, it is
usually counter-productive to declare $a and $b as
lexicals.
In either case, the subroutine may not be recur
sive. The values to be compared are always passed
by reference, so don't modify them.
You also cannot exit out of the sort block or sub
routine using any of the loop control operators
described in the perlsyn manpage or with "goto".
When "use locale" is in effect, "sort LIST" sorts
LIST according to the current collation locale.
See the perllocale manpage.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
If you're using strict, you must not declare $a
and $b as lexicals. They are package globals.
That means if you're in the "main" package and
type
@articles = sort {$b <=> $a} @files;
then "$a" and "$b" are "$main::a" and "$main::b"
(or "$::a" and "$::b"), but if you're in the
"FooPack" package, it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If
it returns inconsistent results (sometimes saying
"$x[1]" is less than "$x[2]" and sometimes saying
the opposite, for example) the results are not
well-defined.
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and
LENGTH from an array, and replaces them with the
elements of LIST, if any. In list context,
returns the elements removed from the array. In
scalar context, returns the last element removed,
or "undef" if no elements are removed. The array
grows or shrinks as necessary. If OFFSET is nega
tive then it starts that far from the end of the
array. If LENGTH is omitted, removes everything
from OFFSET onward. If LENGTH is negative, leaves
that many elements off the end of the array. If
both OFFSET and LENGTH are omitted, removes every
thing.
The following equivalences hold (assuming "$[ ==
0"):
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y)
Example, assuming array lengths are passed before
arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits a string into a list of strings and returns
that list. By default, empty leading fields are
preserved, and empty trailing ones are deleted.
In scalar context, returns the number of fields
found and splits into the "@_" array. Use of
split in scalar context is deprecated, however,
because it clobbers your subroutine arguments.
If EXPR is omitted, splits the "$_" string. If
PATTERN is also omitted, splits on whitespace
(after skipping any leading whitespace). Anything
matching PATTERN is taken to be a delimiter sepa
rating the fields. (Note that the delimiter may
be longer than one character.)
If LIMIT is specified and positive, splits into no
more than that many fields (though it may split
into fewer). If LIMIT is unspecified or zero,
trailing null fields are stripped (which potential
users of "pop" would do well to remember). If
LIMIT is negative, it is treated as if an arbi
trarily large LIMIT had been specified.
A pattern matching the null string (not to be con
fused with a null pattern "//", which is just one
member of the set of patterns matching a null
string) will split the value of EXPR into separate
characters at each point it matches that way. For
example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
Empty leading (or trailing) fields are produced
when there positive width matches at the beginning
(or end) of the string; a zero-width match at the
beginning (or end) of the string does not produce
an empty field. For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'.
The LIMIT parameter can be used to split a line
partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted,
Perl supplies a LIMIT one larger than the number
of variables in the list, to avoid unnecessary
work. For the list above LIMIT would have been 4
by default. In time critical applications it
behooves you not to split into more fields than
you really need.
If the PATTERN contains parentheses, additional
list elements are created from each matching sub
string in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix
email message in $header, you could split it up
into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an
expression to specify patterns that vary at run
time. (To do runtime compilation only once, use
"/$variable/o".)
As a special case, specifying a PATTERN of space
("' '") will split on white space just as "split"
with no arguments does. Thus, "split(' ')" can be
used to emulate awk's default behavior, whereas
"split(/ /)" will give you as many null initial
fields as there are leading spaces. A "split" on
"/\s+/" is like a "split(' ')" except that any
leading whitespace produces a null first field. A
"split" with no arguments really does a "split('
', $_)" internally.
A PATTERN of "/^/" is treated as if it were
"/^/m", since it isn't much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf"
conventions of the C library function "sprintf".
See below for more details and see sprintf(3) or
printf(3) on your system for an explanation of the
general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting--it emu
lates the C function "sprintf", but it doesn't use
it (except for floating-point numbers, and even
then only the standard modifiers are allowed). As
a result, any non-standard extensions in your
local "sprintf" are not available from Perl.
Unlike "printf", "sprintf" does not do what you
probably mean when you pass it an array as your
first argument. The array is given scalar context,
and instead of using the 0th element of the array
as the format, Perl will use the count of elements
in the array as the format, which is almost never
useful.
Perl's "sprintf" permits the following univer
sally-known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-
supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward")
compatibility, Perl permits these unnecessary but
widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the
scientific notation by "%e", "%E", "%g" and "%G"
for numbers with the modulus of the exponent less
than 100 is system-dependent: it may be three or
less (zero-padded as necessary). In other words,
1.23 times ten to the 99th may be either "1.23e99"
or "1.23e099".
Perl permits the following universally-known flags
between the "%" and the conversion letter:
space prefix positive number with a space
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# prefix non-zero octal with "0", non-zero hex with "0x"
number minimum field width
.number "precision": digits after decimal point for
floating-point, max length for string, minimum length
for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
If no flags, interpret integer as C type "int" or "unsigned"
There are also two Perl-specific flags:
V interpret integer as Perl's standard integer type
v interpret string as a vector of integers, output as
numbers separated either by dots, or by an arbitrary
string received from the argument list when the flag
is preceded by C<*>
Where a number would appear in the flags, an
asterisk ("*") may be used instead, in which case
Perl uses the next item in the parameter list as
the given number (that is, as the field width or
precision). If a field width obtained through "*"
is negative, it has the same effect as the "-"
flag: left-justification.
The "v" flag is useful for displaying ordinal val
ues of characters in arbitrary strings:
printf "version is v%vd\n", $^V; # Perl's version
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %*vb\n", " ", $bits; # random bitstring
If "use locale" is in effect, the character used
for the decimal point in formatted real numbers is
affected by the LC_NUMERIC locale. See the perl
locale manpage.
If Perl understands "quads" (64-bit integers)
(this requires either that the platform natively
support quads or that Perl be specifically com
piled to support quads), the characters
d u o x X b i D U O
print quads, and they may optionally be preceded
by
ll L q
For example
%lld %16LX %qo
You can find out whether your Perl supports quads
via the Config manpage:
use Config;
($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
print "quads\n";
If Perl understands "long doubles" (this requires
that the platform support long doubles), the flags
e f g E F G
may optionally be preceded by
ll L
For example
%llf %Lg
You can find out whether your Perl supports long
doubles via the Config manpage:
use Config;
$Config{d_longdbl} eq 'define' && print "long doubles\n";
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omit
ted, returns square root of "$_". Only works on
non-negative operands, unless you've loaded the
standard Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
srand EXPR
srand Sets the random number seed for the "rand" opera
tor. If EXPR is omitted, uses a semi-random value
supplied by the kernel (if it supports the
/dev/urandom device) or based on the current time
and process ID, among other things. In versions
of Perl prior to 5.004 the default seed was just
the current "time". This isn't a particularly
good seed, so many old programs supply their own
seed value (often "time ^ $$" or "time ^ ($$ + ($$
<< 15))"), but that isn't necessary any more.
In fact, it's usually not necessary to call
"srand" at all, because if it is not called
explicitly, it is called implicitly at the first
use of the "rand" operator. However, this was not
the case in version of Perl before 5.004, so if
your script will run under older Perl versions, it
should call "srand".
Note that you need something much more random than
the default seed for cryptographic purposes.
Checksumming the compressed output of one or more
rapidly changing operating system status programs
is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see
the "Math::TrulyRandom" module in CPAN.
Do not call "srand" multiple times in your program
unless you know exactly what you're doing and why
you're doing it. The point of the function is to
"seed" the "rand" function so that "rand" can pro
duce a different sequence each time you run your
program. Just do it once at the top of your pro
gram, or you won't get random numbers out of
"rand"!
Frequently called programs (like CGI scripts) that
simply use
time ^ $$
for a seed can fall prey to the mathematical prop
erty that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
stat Returns a 13-element list giving the status info
for a file, either the file opened via FILEHANDLE,
or named by EXPR. If EXPR is omitted, it stats
"$_". Returns a null list if the stat fails.
Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem
types. Here are the meaning of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time (NOT creation time!) in seconds since the epoch
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
If stat is passed the special filehandle consist
ing of an underline, no stat is done, but the cur
rent contents of the stat structure from the last
stat or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device
number is negative under NFS.)
Because the mode contains both the file type and
its permissions, you should mask off the file type
portion and (s)printf using a ""%o"" if you want
to see the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value
indicating success or failure, and, if successful,
sets the information associated with the special
filehandle "_".
The File::stat module provides a convenient, by-
name access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*")
and functions ("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_setgid = S_ISDIR($mode);
You could write the last two using the "-u" and
"-d" operators. The commonly available S_IF* con
stants are
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the S_IF* functions are
S_IFMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with e.g. S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -s.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation
for more details about the S_* constants.
study SCALAR
study Takes extra time to study SCALAR ("$_" if unspeci
fied) in anticipation of doing many pattern
matches on the string before it is next modified.
This may or may not save time, depending on the
nature and number of patterns you are searching
on, and on the distribution of character frequen
cies in the string to be searched--you probably
want to compare run times with and without it to
see which runs faster. Those loops which scan for
many short constant strings (including the con
stant parts of more complex patterns) will benefit
most. You may have only one "study" active at a
time--if you study a different scalar the first is
"unstudied". (The way "study" works is this: a
linked list of every character in the string to be
searched is made, so we know, for example, where
all the "'k'" characters are. From each search
string, the rarest character is selected, based on
some static frequency tables constructed from some
C programs and English text. Only those places
that contain this "rarest" character are exam
ined.)
For example, here is a loop that inserts index
producing entries before any line containing a
certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only those locations
in "$_" that contain "f" will be looked at,
because "f" is rarer than "o". In general, this
is a big win except in pathological cases. The
only question is whether it saves you more time
than it took to build the linked list in the first
place.
Note that if you have to look for strings that you
don't know till runtime, you can build an entire
loop as a string and "eval" that to avoid recom
piling all your patterns all the time. Together
with undefining "$/" to input entire files as one
record, this can be very fast, often faster than
specialized programs like fgrep(1). The following
scans a list of files ("@files") for a list of
words ("@words"), and prints out the names of
those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub BLOCK
sub NAME
sub NAME BLOCK
This is subroutine definition, not a real function
per se. With just a NAME (and possibly prototypes
or attributes), it's just a forward declaration.
Without a NAME, it's an anonymous function decla
ration, and does actually return a value: the CODE
ref of the closure you just created. See the
perlsub manpage and the perlref manpage for
details.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it.
First character is at offset "0", or whatever
you've set "$[" to (but don't do that). If OFFSET
is negative (or more precisely, less than "$["),
starts that far from the end of the string. If
LENGTH is omitted, returns everything to the end
of the string. If LENGTH is negative, leaves that
many characters off the end of the string.
You can use the substr() function as an lvalue, in
which case EXPR must itself be an lvalue. If you
assign something shorter than LENGTH, the string
will shrink, and if you assign something longer
than LENGTH, the string will grow to accommodate
it. To keep the string the same length you may
need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is
partly outside the string, only the part within
the string is returned. If the substring is
beyond either end of the string, substr() returns
the undefined value and produces a warning. When
used as an lvalue, specifying a substring that is
entirely outside the string is a fatal error.
Here's an example showing the behavior for bound
ary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns '' (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is
to specify the replacement string as the 4th argu
ment. This allows you to replace parts of the
EXPR and return what was there before in one oper
ation, just as you can with splice().
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the
old filename. Returns "1" for success, "0" other
wise. On systems that don't support symbolic
links, produces a fatal error at run time. To
check for that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall LIST
Calls the system call specified as the first ele
ment of the list, passing the remaining elements
as arguments to the system call. If unimple
mented, produces a fatal error. The arguments are
interpreted as follows: if a given argument is
numeric, the argument is passed as an int. If
not, the pointer to the string value is passed.
You are responsible to make sure a string is pre-
extended long enough to receive any result that
might be written into a string. You can't use a
string literal (or other read-only string) as an
argument to "syscall" because Perl has to assume
that any string pointer might be written through.
If your integer arguments are not literals and
have never been interpreted in a numeric context,
you may need to add "0" to them to force them to
look like numbers. This emulates the "syswrite"
function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14
arguments to your system call, which in practice
should usually suffice.
Syscall returns whatever value returned by the
system call it calls. If the system call fails,
"syscall" returns "-1" and sets "$!" (errno).
Note that some system calls can legitimately
return "-1". The proper way to handle such calls
is to assign "$!=0;" before the call and check the
value of "$!" if syscall returns "-1".
There's a problem with "syscall(&SYS_pipe)": it
returns the file number of the read end of the
pipe it creates. There is no way to retrieve the
file number of the other end. You can avoid this
problem by using "pipe" instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILE
NAME, and associates it with FILEHANDLE. If FILE
HANDLE is an expression, its value is used as the
name of the real filehandle wanted. This function
calls the underlying operating system's "open"
function with the parameters FILENAME, MODE,
PERMS.
The possible values and flag bits of the MODE
parameter are system-dependent; they are available
via the standard module "Fcntl". See the documen
tation of your operating system's "open" to see
which values and flag bits are available. You may
combine several flags using the "|"-operator.
Some of the most common values are "O_RDONLY" for
opening the file in read-only mode, "O_WRONLY" for
opening the file in write-only mode, and "O_RDWR"
for opening the file in read-write mode, and.
For historical reasons, some values work on almost
every system supported by perl: zero means read-
only, one means write-only, and two means
read/write. We know that these values do not work
under OS/390 & VM/ESA Unix and on the Macintosh;
you probably don't want to use them in new code.
If the file named by FILENAME does not exist and
the "open" call creates it (typically because MODE
includes the "O_CREAT" flag), then the value of
PERMS specifies the permissions of the newly cre
ated file. If you omit the PERMS argument to
"sysopen", Perl uses the octal value "0666".
These permission values need to be in octal, and
are modified by your process's current "umask".
In many systems the "O_EXCL" flag is available for
opening files in exclusive mode. This is not
locking: exclusiveness means here that if the file
already exists, sysopen() fails. The "O_EXCL"
wins "O_TRUNC".
Sometimes you may want to truncate an already-
existing file: "O_TRUNC".
You should seldom if ever use "0644" as argument
to "sysopen", because that takes away the user's
option to have a more permissive umask. Better to
omit it. See the perlfunc(1) entry on "umask" for
more on this.
Note that "sysopen" depends on the fdopen() C
library function. On many UNIX systems, fdopen()
is known to fail when file descriptors exceed a
certain value, typically 255. If you need more
file descriptors than that, consider rebuilding
Perl to use the "sfio" library, or perhaps using
the POSIX:\fIs0:open() function.
See the perlopentut manpage for a kinder, gentler
explanation of opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into vari
able SCALAR from the specified FILEHANDLE, using
the system call read(2). It bypasses stdio, so
mixing this with other kinds of reads, "print",
"write", "seek", "tell", or "eof" can cause confu
sion because stdio usually buffers data. Returns
the number of bytes actually read, "0" at end of
file, or undef if there was an error. SCALAR will
be grown or shrunk so that the last byte actually
read is the last byte of the scalar after the
read.
An OFFSET may be specified to place the read data
at some place in the string other than the begin
ning. A negative OFFSET specifies placement at
that many bytes counting backwards from the end of
the string. A positive OFFSET greater than the
length of SCALAR results in the string being
padded to the required size with ""\0"" bytes
before the result of the read is appended.
There is no syseof() function, which is ok, since
eof() doesn't work very well on device files (like
ttys) anyway. Use sysread() and check for a
return value for 0 to decide whether you're done.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position using the system
call lseek(2). It bypasses stdio, so mixing this
with reads (other than "sysread"), "print",
"write", "seek", "tell", or "eof" may cause confu
sion. FILEHANDLE may be an expression whose value
gives the name of the filehandle. The values for
WHENCE are "0" to set the new position to POSI
TION, "1" to set the it to the current position
plus POSITION, and "2" to set it to EOF plus POSI
TION (typically negative). For WHENCE, you may
also use the constants "SEEK_SET", "SEEK_CUR", and
"SEEK_END" (start of the file, current position,
end of the file) from the Fcntl module.
Returns the new position, or the undefined value
on failure. A position of zero is returned as the
string ""0 but true""; thus "sysseek" returns true
on success and false on failure, yet you can still
easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except
that a fork is done first, and the parent process
waits for the child process to complete. Note
that argument processing varies depending on the
number of arguments. If there is more than one
argument in LIST, or if LIST is an array with more
than one value, starts the program given by the
first element of the list with arguments given by
the rest of the list. If there is only one scalar
argument, the argument is checked for shell
metacharacters, and if there are any, the entire
argument is passed to the system's command shell
for parsing (this is "/bin/sh -c" on Unix plat
forms, but varies on other platforms). If there
are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp",
which is more efficient.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before any operation
that may do a fork, but this may not be supported
on some platforms (see the perlport manpage). To
be safe, you may need to set "$|" ($AUTOFLUSH in
English) or call the "autoflush()" method of
"IO::Handle" on any open handles.
The return value is the exit status of the program
as returned by the "wait" call. To get the actual
exit value divide by 256. See also the exec entry
elsewhere in this document. This is not what you
want to use to capture the output from a command,
for that you should use merely backticks or
"qx//", as described in the section on "`STRING`"
in the perlop manpage. Return value of -1 indi
cates a failure to start the program (inspect $!
for the reason).
Like "exec", "system" allows you to lie to a pro
gram about its name if you use the "system PROGRAM
LIST" syntax. Again, see the exec entry elsewhere
in this document.
Because "system" and backticks block "SIGINT" and
"SIGQUIT", killing the program they're running
doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by
inspecting "$?" like this:
$exit_value = $? >> 8;
$signal_num = $? & 127;
$dumped_core = $? & 128;
When the arguments get executed via the system
shell, results and return codes will be subject to
its quirks and capabilities. See the section on
"`STRING`" in the perlop manpage and the exec
entry elsewhere in this document for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from vari
able SCALAR to the specified FILEHANDLE, using the
system call write(2). If LENGTH is not specified,
writes whole SCALAR. It bypasses stdio, so mixing
this with reads (other than "sysread())", "print",
"write", "seek", "tell", or "eof" may cause confu
sion because stdio usually buffers data. Returns
the number of bytes actually written, or "undef"
if there was an error. If the LENGTH is greater
than the available data in the SCALAR after the
OFFSET, only as much data as is available will be
written.
An OFFSET may be specified to write the data from
some part of the string other than the beginning.
A negative OFFSET specifies writing that many
bytes counting backwards from the end of the
string. In the case the SCALAR is empty you can
use OFFSET but only zero offset.
tell FILEHANDLE
tell Returns the current position for FILEHANDLE, or -1
on error. FILEHANDLE may be an expression whose
value gives the name of the actual filehandle. If
FILEHANDLE is omitted, assumes the file last read.
The return value of tell() for the standard
streams like the STDIN depends on the operating
system: it may return -1 or something else.
tell() on pipes, fifos, and sockets usually
returns -1.
There is no "systell" function. Use "sysseek(FH,
0, 1)" for that.
telldir DIRHANDLE
Returns the current position of the "readdir" rou
tines on DIRHANDLE. Value may be given to
"seekdir" to access a particular location in a
directory. Has the same caveats about possible
directory compaction as the corresponding system
library routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class
that will provide the implementation for the vari
able. VARIABLE is the name of the variable to be
enchanted. CLASSNAME is the name of a class
implementing objects of correct type. Any addi
tional arguments are passed to the "new" method of
the class (meaning "TIESCALAR", "TIEHANDLE",
"TIEARRAY", or "TIEHASH"). Typically these are
arguments such as might be passed to the
"dbm_open()" function of C. The object returned
by the "new" method is also returned by the "tie"
function, which would be useful if you want to
access other methods in CLASSNAME.
Note that functions such as "keys" and "values"
may return huge lists when used on large objects,
like DBM files. You may prefer to use the "each"
function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the fol
lowing methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
DESTROY this
UNTIE this
A class implementing an ordinary array should have
the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a file handle should have the
following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the fol
lowing methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be imple
mented. See the perltie manpage, the Tie::Hash
manpage, the Tie::Array manpage, the Tie::Scalar
manpage, and the Tie::Handle manpage.
Unlike "dbmopen", the "tie" function will not use
or require a module for you--you need to do that
explicitly yourself. See the DB_File manpage or
the Config module for interesting "tie" implemen
tations.
For further details see the perltie manpage, the
section on "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARI
ABLE (the same value that was originally returned
by the "tie" call that bound the variable to a
package.) Returns the undefined value if VARIABLE
isn't tied to a package.
time Returns the number of non-leap seconds since what
ever time the system considers to be the epoch
(that's 00:00:00, January 1, 1904 for MacOS, and
00:00:00 UTC, January 1, 1970 for most other sys
tems). Suitable for feeding to "gmtime" and
"localtime".
For measuring time in better granularity than one
second, you may use either the Time::HiRes module
from CPAN, or if you have gettimeofday(2), you may
be able to use the "syscall" interface of Perl,
see the perlfaq8 manpage for details.
times Returns a four-element list giving the user and
system times, in seconds, for this process and the
children of this process.
($user,$system,$cuser,$csystem) = times;
tr/// The transliteration operator. Same as "y///".
See the perlop manpage.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named
by EXPR, to the specified length. Produces a
fatal error if truncate isn't implemented on your
system. Returns true if successful, the undefined
value otherwise.
uc EXPR
uc Returns an uppercased version of EXPR. This is
the internal function implementing the "\U" escape
in double-quoted strings. Respects current
LC_CTYPE locale if "use locale" in force. See the
perllocale manpage. Under Unicode ("use utf8") it
uses the standard Unicode uppercase mappings. (It
does not attempt to do titlecase mapping on ini
tial letters. See "ucfirst" for that.)
If EXPR is omitted, uses "$_".
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character
in uppercase (titlecase in Unicode). This is the
internal function implementing the "\u" escape in
double-quoted strings. Respects current LC_CTYPE
locale if "use locale" in force. See the perllo
cale manpage and the utf8 manpage.
If EXPR is omitted, uses "$_".
umask EXPR
umask Sets the umask for the process to EXPR and returns
the previous value. If EXPR is omitted, merely
returns the current umask.
The Unix permission "rwxr-x---" is represented as
three sets of three bits, or three octal digits:
"0750" (the leading 0 indicates octal and isn't
one of the digits). The "umask" value is such a
number representing disabled permissions bits.
The permission (or "mode") values you pass "mkdir"
or "sysopen" are modified by your umask, so even
if you tell "sysopen" to create a file with per
missions "0777", if your umask is "0022" then the
file will actually be created with permissions
"0755". If your "umask" were "0027" (group can't
write; others can't read, write, or execute), then
passing "sysopen" "0666" would create a file with
mode "0640" ("0666 &~ 027" is "0640").
Here's some advice: supply a creation mode of
"0666" for regular files (in "sysopen") and one of
"0777" for directories (in "mkdir") and executable
files. This gives users the freedom of choice: if
they want protected files, they might choose pro
cess umasks of "022", "027", or even the particu
larly antisocial mask of "077". Programs should
rarely if ever make policy decisions better left
to the user. The exception to this is when writ
ing files that should be kept private: mail files,
web browser cookies, .rhosts files, and so on.
If umask(2) is not implemented on your system and
you are trying to restrict access for yourself
(i.e., (EXPR & 0700) > 0), produces a fatal error
at run time. If umask(2) is not implemented and
you are not trying to restrict access for your
self, returns "undef".
Remember that a umask is a number, usually given
in octal; it is not a string of octal digits. See
also the oct entry elsewhere in this document, if
all you have is a string.
undef EXPR
undef Undefines the value of EXPR, which must be an
lvalue. Use only on a scalar value, an array
(using "@"), a hash (using "%"), a subroutine
(using "&"), or a typeglob (using <*>). (Saying
"undef $hash{$key}" will probably not do what you
expect on most predefined variables or DBM list
values, so don't do that; see the delete manpage.)
Always returns the undefined value. You can omit
the EXPR, in which case nothing is undefined, but
you still get an undefined value that you could,
for instance, return from a subroutine, assign to
a variable or pass as a parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list
operator.
unlink LIST
unlink Deletes a list of files. Returns the number of
files successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: "unlink" will not delete directories unless
you are superuser and the -U flag is supplied to
Perl. Even if these conditions are met, be warned
that unlinking a directory can inflict damage on
your filesystem. Use "rmdir" instead.
If LIST is omitted, uses "$_".
unpack TEMPLATE,EXPR
"unpack" does the reverse of "pack": it takes a
string and expands it out into a list of values.
(In scalar context, it returns merely the first
value produced.)
The string is broken into chunks described by the
TEMPLATE. Each chunk is converted separately to a
value. Typically, either the string is a result
of "pack", or the bytes of the string represent a
C structure of some kind.
The TEMPLATE has the same format as in the "pack"
function. Here's a subroutine that does sub
string:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may
prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items
instead of the items themselves. Default is a
16-bit checksum. Checksum is calculated by sum
ming numeric values of expanded values (for string
fields the sum of "ord($char)" is taken, for bit
fields the sum of zeroes and ones).
For example, the following computes the same num
ber as the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32C*",<>) % 65535;
};
The following efficiently counts the number of set
bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care.
Since Perl has no way of checking whether the
value passed to "unpack()" corresponds to a valid
memory location, passing a pointer value that's
not known to be valid is likely to have disastrous
consequences.
If the repeat count of a field is larger than what
the remainder of the input string allows, repeat
count is decreased. If the input string is longer
than one described by the TEMPLATE, the rest is
ignored.
See the pack entry elsewhere in this document for
more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a pack
age. (See "tie".)
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite
of a "push", depending on how you look at it.
Prepends list to the front of the array, and
returns the new number of elements in the array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element
at a time, so the prepended elements stay in the
same order. Use "reverse" to do the reverse.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package
from the named module, generally by aliasing cer
tain subroutine or variable names into your pack
age. It is exactly equivalent to
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
VERSION, which can be specified as a literal of
the form v5.6.1, demands that the current version
of Perl ("$^V" or $PERL_VERSION) be at least as
recent as that version. (For compatibility with
older versions of Perl, a numeric literal will
also be interpreted as VERSION.) If the version
of the running Perl interpreter is less than VER
SION, then an error message is printed and Perl
exits immediately without attempting to parse the
rest of the file. Compare with the require entry
elsewhere in this document, which can do a similar
check at run time.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.005_03; # float version allowed for compatibility
This is often useful if you need to check the cur
rent Perl version before "use"ing library modules
that have changed in incompatible ways from older
versions of Perl. (We try not to do this more
than we have to.)
The "BEGIN" forces the "require" and "import" to
happen at compile time. The "require" makes sure
the module is loaded into memory if it hasn't been
yet. The "import" is not a builtin--it's just an
ordinary static method call into the "Module"
package to tell the module to import the list of
features back into the current package. The mod
ule can implement its "import" method any way it
likes, though most modules just choose to derive
their "import" method via inheritance from the
"Exporter" class that is defined in the "Exporter"
module. See the Exporter manpage. If no "import"
method can be found then the call is skipped.
If you do not want to call the package's "import"
method (for instance, to stop your namespace from
being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module
and LIST, then the "use" will call the VERSION
method in class Module with the given version as
an argument. The default VERSION method, inher
ited from the UNIVERSAL class, croaks if the given
version is larger than the value of the variable
"$Module::VERSION".
Again, there is a distinction between omitting
LIST ("import" called with no arguments) and an
explicit empty LIST "()" ("import" not called).
Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas
(compiler directives) are also implemented this
way. Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
Some of these pseudo-modules import semantics into
the current block scope (like "strict" or "inte
ger", unlike ordinary modules, which import sym
bols into the current package (which are effective
through the end of the file).
There's a corresponding "no" command that unim
ports meanings imported by "use", i.e., it calls
"unimport Module LIST" instead of "import".
no integer;
no strict 'refs';
no warnings;
If no "unimport" method can be found the call
fails with a fatal error.
See the perlmodlib manpage for a list of standard
modules and pragmas. See the perlrun manpage for
the "-M" and "-m" command-line options to perl
that give "use" functionality from the command-
line.
utime LIST
Changes the access and modification times on each
file of a list of files. The first two elements
of the list must be the NUMERICAL access and modi
fication times, in that order. Returns the number
of files successfully changed. The inode change
time of each file is set to the current time.
This code has the same effect as the "touch" com
mand if the files already exist:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values HASH
Returns a list consisting of all the values of the
named hash. (In a scalar context, returns the
number of values.) The values are returned in an
apparently random order. The actual random order
is subject to change in future versions of perl,
but it is guaranteed to be the same order as
either the "keys" or "each" function would produce
on the same (unmodified) hash.
Note that the values are not copied, which means
modifying them will modify the contents of the
hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
As a side effect, calling values() resets the
HASH's internal iterator. See also "keys",
"each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up
of elements of width BITS, and returns the value
of the element specified by OFFSET as an unsigned
integer. BITS therefore specifies the number of
bits that are reserved for each element in the bit
vector. This must be a power of two from 1 to 32
(or 64, if your platform supports that).
If BITS is 8, "elements" coincide with bytes of
the input string.
If BITS is 16 or more, bytes of the input string
are grouped into chunks of size BITS/8, and each
group is converted to a number as with
pack()/unpack() with big-endian formats "n"/"N"
(and analogously for BITS==64). See the section
on "pack" for details.
If bits is 4 or less, the string is broken into
bytes, then the bits of each byte are broken into
8/BITS groups. Bits of a byte are numbered in a
little-endian-ish way, as in "0x01", "0x02",
"0x04", "0x08", "0x10", "0x20", "0x40", "0x80".
For example, breaking the single input byte
"chr(0x36)" into two groups gives a list "(0x6,
0x3)"; breaking it into 4 groups gives "(0x2, 0x1,
0x3, 0x0)".
"vec" may also be assigned to, in which case
parentheses are needed to give the expression the
correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the
value 0 is returned. If an element off the end of
the string is written to, Perl will first extend
the string with sufficiently many zero bytes. It
is an error to try to write off the beginning of
the string (i.e. negative OFFSET).
The string should not contain any character with
the value > 255 (which can only happen if you're
using UTF8 encoding). If it does, it will be
treated as something which is not UTF8 encoded.
When the "vec" was assigned to, other parts of
your program will also no longer consider the
string to be UTF8 encoded. In other words, if you
do have such characters in your string, vec() will
operate on the actual byte string, and not the
conceptual character string.
Strings created with "vec" can also be manipulated
with the logical operators "|", "&", "^", and "~".
These operators will assume a bit vector operation
is desired when both operands are strings. See
the Bitwise String Operators entry in the perlop
manpage.
The following code will build up an ASCII string
saying "'PerlPerlPerl'". The comments show the
string after each step. Note that this code works
in the same way on big-endian or little-endian
machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of
0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be
used in place of the "*".
Here is an example to illustrate how the bits
actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it
is run, the above example should print the follow
ing table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait Behaves like the wait(2) system call on your sys
tem: it waits for a child process to terminate and
returns the pid of the deceased process, or "-1"
if there are no child processes. The status is
returned in "$?". Note that a return value of
"-1" could mean that child processes are being
automatically reaped, as described in the perlipc
manpage.
waitpid PID,FLAGS
Waits for a particular child process to terminate
and returns the pid of the deceased process, or
"-1" if there is no such child process. On some
systems, a value of 0 indicates that there are
processes still running. The status is returned
in "$?". If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1,&WNOHANG);
} until $kid == -1;
then you can do a non-blocking wait for all pend
ing zombie processes. Non-blocking wait is avail
able on machines supporting either the waitpid(2)
or wait4(2) system calls. However, waiting for a
particular pid with FLAGS of "0" is implemented
everywhere. (Perl emulates the system call by
remembering the status values of processes that
have exited but have not been harvested by the
Perl script yet.)
Note that on some systems, a return value of "-1"
could mean that child processes are being automat
ically reaped. See the perlipc manpage for
details, and for other examples.
wantarray
Returns true if the context of the currently exe
cuting subroutine is looking for a list value.
Returns false if the context is looking for a
scalar. Returns the undefined value if the con
text is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
This function should have been named wantlist()
instead.
warn LIST
Produces a message on STDERR just like "die", but
doesn't exit or throw an exception.
If LIST is empty and "$@" already contains a value
(typically from a previous eval) that value is
used after appending ""\t...caught"" to "$@".
This is useful for staying almost, but not
entirely similar to "die".
If "$@" is empty then the string ""Warning: Some
thing's wrong"" is used.
No message is printed if there is a
"$SIG{__WARN__}" handler installed. It is the
handler's responsibility to deal with the message
as it sees fit (like, for instance, converting it
into a "die"). Most handlers must therefore make
arrangements to actually display the warnings that
they are not prepared to deal with, by calling
"warn" again in the handler. Note that this is
quite safe and will not produce an endless loop,
since "__WARN__" hooks are not called from inside
one.
You will find this behavior is slightly different
from that of "$SIG{__DIE__}" handlers (which don't
suppress the error text, but can instead call
"die" again to change it).
Using a "__WARN__" handler provides a powerful way
to silence all warnings (even the so-called manda
tory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See the perlvar manpage for details on setting
"%SIG" entries, and for more examples. See the
Carp module for other kinds of warnings using its
carp() and cluck() functions.
write FILEHANDLE
write EXPR
write Writes a formatted record (possibly multi-line) to
the specified FILEHANDLE, using the format associ
ated with that file. By default the format for a
file is the one having the same name as the file
handle, but the format for the current output
channel (see the "select" function) may be set
explicitly by assigning the name of the format to
the "$~" variable.
Top of form processing is handled automatically:
if there is insufficient room on the current page
for the formatted record, the page is advanced by
writing a form feed, a special top-of-page format
is used to format the new page header, and then
the record is written. By default the top-of-page
format is the name of the filehandle with "_TOP"
appended, but it may be dynamically set to the
format of your choice by assigning the name to the
"$^" variable while the filehandle is selected.
The number of lines remaining on the current page
is in variable "$-", which can be set to "0" to
force a new page.
If FILEHANDLE is unspecified, output goes to the
current default output channel, which starts out
as STDOUT but may be changed by the "select" oper
ator. If the FILEHANDLE is an EXPR, then the
expression is evaluated and the resulting string
is used to look up the name of the FILEHANDLE at
run time. For more on formats, see the perlform
manpage.
Note that write is not the opposite of "read".
Unfortunately.
y/// The transliteration operator. Same as "tr///".
See the perlop manpage.
2001-04-07 perl v5.6.1 PERLFUNC(1)
