PERLOPENTUT(1) Perl Programmers Reference Guide PERLOPENTUT(1)NAMEperlopentut - tutorial on opening things in Perl
DESCRIPTION
Perl has two simple, built-in ways to open files: the
shell way for convenience, and the C way for precision.
The choice is yours.
Open A la shell
Perl's open function was designed to mimic the way
command-line redirection in the shell works. Here are
some basic examples from the shell:
$ myprogram file1 file2 file3
$ myprogram < inputfile
$ myprogram > outputfile
$ myprogram >> outputfile
$ myprogram | otherprogram
$ otherprogram | myprogram
And here are some more advanced examples:
$ otherprogram | myprogram f1 - f2
$ otherprogram 2>&1 | myprogram -
$ myprogram <&3
$ myprogram >&4
Programmers accustomed to constructs like those above can
take comfort in learning that Perl directly supports these
familiar constructs using virtually the same syntax as the
shell.
Simple Opens
The open function takes two arguments: the first is a
filehandle, and the second is a single string comprising
both what to open and how to open it. open returns true
when it works, and when it fails, returns a false value
and sets the special variable $! to reflect the system
error. If the filehandle was previously opened, it will
be implicitly closed first.
For example:
open(INFO, "datafile") || die("can't open datafile: $!");
open(INFO, "< datafile") || die("can't open datafile: $!");
open(RESULTS,"> runstats") || die("can't open runstats: $!");
open(LOG, ">> logfile ") || die("can't open logfile: $!");
If you prefer the low-punctuation version, you could write
that this way:
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open INFO, "< datafile" or die "can't open datafile: $!";
open RESULTS,"> runstats" or die "can't open runstats: $!";
open LOG, ">> logfile " or die "can't open logfile: $!";
A few things to notice. First, the leading less-than is
optional. If omitted, Perl assumes that you want to open
the file for reading.
The other important thing to notice is that, just as in
the shell, any white space before or after the filename is
ignored. This is good, because you wouldn't want these to
do different things:
open INFO, "<datafile"
open INFO, "< datafile"
open INFO, "< datafile"
Ignoring surround whitespace also helps for when you read
a filename in from a different file, and forget to trim it
before opening:
$filename = <INFO>; # oops, \n still there
open(EXTRA, "< $filename") || die "can't open $filename: $!";
This is not a bug, but a feature. Because open mimics the
shell in its style of using redirection arrows to specify
how to open the file, it also does so with respect to
extra white space around the filename itself as well. For
accessing files with naughty names, see the section on
/"Dispelling the Dweomer.
Pipe Opens
In C, when you want to open a file using the standard I/O
library, you use the fopen function, but when opening a
pipe, you use the popen function. But in the shell, you
just use a different redirection character. That's also
the case for Perl. The open call remains the same--just
its argument differs.
If the leading character is a pipe symbol, C<open) starts
up a new command and open a write-only filehandle leading
into that command. This lets you write into that handle
and have what you write show up on that command's standard
input. For example:
open(PRINTER, "| lpr -Plp1") || die "cannot fork: $!";
print PRINTER "stuff\n";
close(PRINTER) || die "can't close lpr: $!";
If the trailing character is a pipe, you start up a new
command and open a read-only filehandle leading out of
that command. This lets whatever that command writes to
its standard output show up on your handle for reading.
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For example:
open(NET, "netstat -i -n |") || die "cannot fork: $!";
while (<NET>) { } # do something with input
close(NET) || die "can't close netstat: $!";
What happens if you try to open a pipe to or from a non-
existent command? In most systems, such an open will not
return an error. That's because in the traditional
fork/exec model, running the other program happens only in
the forked child process, which means that the failed exec
can't be reflected in the return value of open. Only a
failed fork shows up there. See the section on Why
doesn't open() return an error when a pipe open fails? in
the perlfaq8 manpage to see how to cope with this.
There's also an explanation in the perlipc manpage.
If you would like to open a bidirectional pipe, the
IPC::Open2 library will handle this for you. Check out
the section on Bidirectional Communication with Another
Process in the perlipc manpage
The Minus File
Again following the lead of the standard shell utilities,
Perl's open function treats a file whose name is a single
minus, "-", in a special way. If you open minus for
reading, it really means to access the standard input. If
you open minus for writing, it really means to access the
standard output.
If minus can be used as the default input or default
output? What happens if you open a pipe into or out of
minus? What's the default command it would run? The same
script as you're current running! This is actually a
stealth fork hidden inside an open call. See the section
on Safe Pipe Opens in the perlipc manpage for details.
Mixing Reads and Writes
It is possible to specify both read and write access. All
you do is add a "+" symbol in front of the redirection.
But as in the shell, using a less-than on a file never
creates a new file; it only opens an existing one. On the
other hand, using a greater-than always clobbers
(truncates to zero length) an existing file, or creates a
brand-new one if there isn't an old one. Adding a "+" for
read-write doesn't affect whether it only works on
existing files or always clobbers existing ones.
open(WTMP, "+< /usr/adm/wtmp")
|| die "can't open /usr/adm/wtmp: $!";
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open(SCREEN, "+> /tmp/lkscreen")
|| die "can't open /tmp/lkscreen: $!";
open(LOGFILE, "+>> /tmp/applog"
|| die "can't open /tmp/applog: $!";
The first one won't create a new file, and the second one
will always clobber an old one. The third one will create
a new file if necessary and not clobber an old one, and it
will allow you to read at any point in the file, but all
writes will always go to the end. In short, the first
case is substantially more common than the second and
third cases, which are almost always wrong. (If you know
C, the plus in Perl's open is historically derived from
the one in C's fopen(3S), which it ultimately calls.)
In fact, when it comes to updating a file, unless you're
working on a binary file as in the WTMP case above, you
probably don't want to use this approach for updating.
Instead, Perl's -i flag comes to the rescue. The
following command takes all the C, C++, or yacc source or
header files and changes all their foo's to bar's, leaving
the old version in the original file name with a ".orig"
tacked on the end:
$ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy]
This is a short cut for some renaming games that are
really the best way to update textfiles. See the second
question in the perlfaq5 manpage for more details.
Filters
One of the most common uses for open is one you never even
notice. When you process the ARGV filehandle using
<ARGV>, Perl actually does an implicit open on each file
in @ARGV. Thus a program called like this:
$ myprogram file1 file2 file3
Can have all its files opened and processed one at a time
using a construct no more complex than:
while (<>) {
# do something with $_
}
If @ARGV is empty when the loop first begins, Perl
pretends you've opened up minus, that is, the standard
input. In fact, $ARGV, the currently open file during
<ARGV> processing, is even set to "-" in these
circumstances.
You are welcome to pre-process your @ARGV before starting
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the loop to make sure it's to your liking. One reason to
do this might be to remove command options beginning with
a minus. While you can always roll the simple ones by
hand, the Getopts modules are good for this.
use Getopt::Std;
# -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o
getopts("vDo:");
# -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o}
getopts("vDo:", \%args);
Or the standard Getopt::Long module to permit named
arguments:
use Getopt::Long;
GetOptions( "verbose" => \$verbose, # --verbose
"Debug" => \$debug, # --Debug
"output=s" => \$output );
# --output=somestring or --output somestring
Another reason for preprocessing arguments is to make an
empty argument list default to all files:
@ARGV = glob("*") unless @ARGV;
You could even filter out all but plain, text files. This
is a bit silent, of course, and you might prefer to
mention them on the way.
@ARGV = grep { -f && -T } @ARGV;
If you're using the -n or -p command-line options, you
should put changes to @ARGV in a BEGIN{} block.
Remember that a normal open has special properties, in
that it might call fopen(3S) or it might called popen(3S),
depending on what its argument looks like; that's why it's
sometimes called "magic open". Here's an example:
$pwdinfo = `domainname` =~ /^(\(none\))?$/
? '< /etc/passwd'
: 'ypcat passwd |';
open(PWD, $pwdinfo)
or die "can't open $pwdinfo: $!";
This sort of thing also comes into play in filter
processing. Because <ARGV> processing employs the normal,
shell-style Perl open, it respects all the special things
we've already seen:
$ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
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That program will read from the file f1, the process cmd1,
standard input (tmpfile in this case), the f2 file, the
cmd2 command, and finally the f3 file.
Yes, this also means that if you have a file named "-"
(and so on) in your directory, that they won't be
processed as literal files by open. You'll need to pass
them as "./-" much as you would for the rm program. Or
you could use sysopen as described below.
One of the more interesting applications is to change
files of a certain name into pipes. For example, to
autoprocess gzipped or compressed files by decompressing
them with gzip:
@ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV;
Or, if you have the GET program installed from LWP, you
can fetch URLs before processing them:
@ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV;
It's not for nothing that this is called magic <ARGV>.
Pretty nifty, eh?
Open A la C
If you want the convenience of the shell, then Perl's open
is definitely the way to go. On the other hand, if you
want finer precision than C's simplistic fopen(3S)
provides, then you should look to Perl's sysopen, which is
a direct hook into the open(2) system call. That does
mean it's a bit more involved, but that's the price of
precision.
sysopen takes 3 (or 4) arguments.
sysopen HANDLE, PATH, FLAGS, [MASK]
The HANDLE argument is a filehandle just as with open.
The PATH is a literal path, one that doesn't pay attention
to any greater-thans or less-thans or pipes or minuses,
nor ignore white space. If it's there, it's part of the
path. The FLAGS argument contains one or more values
derived from the Fcntl module that have been or'd together
using the bitwise "|" operator. The final argument, the
MASK, is optional; if present, it is combined with the
user's current umask for the creation mode of the file.
You should usually omit this.
Although the traditional values of read-only, write-only,
and read-write are 0, 1, and 2 respectively, this is known
not to hold true on some systems. Instead, it's best to
load in the appropriate constants first from the Fcntl
module, which supplies the following standard flags:
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O_RDONLY Read only
O_WRONLY Write only
O_RDWR Read and write
O_CREAT Create the file if it doesn't exist
O_EXCL Fail if the file already exists
O_APPEND Append to the file
O_TRUNC Truncate the file
O_NONBLOCK Non-blocking access
Less common flags that are sometimes available on some
operating systems include O_BINARY, O_TEXT, O_SHLOCK,
O_EXLOCK, O_DEFER, O_SYNC, O_ASYNC, O_DSYNC, O_RSYNC,
O_NOCTTY, O_NDELAY and O_LARGEFILE. Consult your open(2)
manpage or its local equivalent for details.
Here's how to use sysopen to emulate the simple open calls
we had before. We'll omit the || die $! checks for
clarity, but make sure you always check the return values
in real code. These aren't quite the same, since open
will trim leading and trailing white space, but you'll get
the idea:
To open a file for reading:
open(FH, "< $path");
sysopen(FH, $path, O_RDONLY);
To open a file for writing, creating a new file if needed
or else truncating an old file:
open(FH, "> $path");
sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT);
To open a file for appending, creating one if necessary:
open(FH, ">> $path");
sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT);
To open a file for update, where the file must already
exist:
open(FH, "+< $path");
sysopen(FH, $path, O_RDWR);
And here are things you can do with sysopen that you
cannot do with a regular open. As you see, it's just a
matter of controlling the flags in the third argument.
To open a file for writing, creating a new file which must
not previously exist:
sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT);
To open a file for appending, where that file must already
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exist:
sysopen(FH, $path, O_WRONLY | O_APPEND);
To open a file for update, creating a new file if
necessary:
sysopen(FH, $path, O_RDWR | O_CREAT);
To open a file for update, where that file must not
already exist:
sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT);
To open a file without blocking, creating one if
necessary:
sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT);
Permissions A la mode
If you omit the MASK argument to sysopen, Perl uses the
octal value 0666. The normal MASK to use for executables
and directories should be 0777, and for anything else,
0666.
Why so permissive? Well, it isn't really. The MASK will
be modified by your process's current umask. A umask is a
number representing disabled permissions bits; that is,
bits that will not be turned on in the created files'
permissions field.
For example, if your umask were 027, then the 020 part
would disable the group from writing, and the 007 part
would disable others from reading, writing, or executing.
Under these conditions, passing sysopen 0666 would create
a file with mode 0640, since 0666 &~ 027 is 0640.
You should seldom use the MASK argument to sysopen().
That takes away the user's freedom to choose what
permission new files will have. Denying choice is almost
always a bad thing. One exception would be for cases
where sensitive or private data is being stored, such as
with mail folders, cookie files, and internal temporary
files.
Obscure Open Tricks
Re-Opening Files (dups)
Sometimes you already have a filehandle open, and want to
make another handle that's a duplicate of the first one.
In the shell, we place an ampersand in front of a file
descriptor number when doing redirections. For example,
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2>&1 makes descriptor 2 (that's STDERR in Perl) be
redirected into descriptor 1 (which is usually Perl's
STDOUT). The same is essentially true in Perl: a filename
that begins with an ampersand is treated instead as a file
descriptor if a number, or as a filehandle if a string.
open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!";
open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!";
That means that if a function is expecting a filename, but
you don't want to give it a filename because you already
have the file open, you can just pass the filehandle with
a leading ampersand. It's best to use a fully qualified
handle though, just in case the function happens to be in
a different package:
somefunction("&main::LOGFILE");
This way if somefunction() is planning on opening its
argument, it can just use the already opened handle. This
differs from passing a handle, because with a handle, you
don't open the file. Here you have something you can pass
to open.
If you have one of those tricky, newfangled I/O objects
that the C++ folks are raving about, then this doesn't
work because those aren't a proper filehandle in the
native Perl sense. You'll have to use fileno() to pull
out the proper descriptor number, assuming you can:
use IO::Socket;
$handle = IO::Socket::INET->new("www.perl.com:80");
$fd = $handle->fileno;
somefunction("&$fd"); # not an indirect function call
It can be easier (and certainly will be faster) just to
use real filehandles though:
use IO::Socket;
local *REMOTE = IO::Socket::INET->new("www.perl.com:80");
die "can't connect" unless defined(fileno(REMOTE));
somefunction("&main::REMOTE");
If the filehandle or descriptor number is preceded not
just with a simple "&" but rather with a "&=" combination,
then Perl will not create a completely new descriptor
opened to the same place using the dup(2) system call.
Instead, it will just make something of an alias to the
existing one using the fdopen(3S) library call This is
slightly more parsimonious of systems resources, although
this is less a concern these days. Here's an example of
that:
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$fd = $ENV{"MHCONTEXTFD"};
open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!";
If you're using magic <ARGV>, you could even pass in as a
command line argument in @ARGV something like
"<&=$MHCONTEXTFD", but we've never seen anyone actually do
this.
Dispelling the Dweomer
Perl is more of a DWIMmer language than something like
Java--where DWIM is an acronym for "do what I mean". But
this principle sometimes leads to more hidden magic than
one knows what to do with. In this way, Perl is also
filled with dweomer, an obscure word meaning an
enchantment. Sometimes, Perl's DWIMmer is just too much
like dweomer for comfort.
If magic open is a bit too magical for you, you don't have
to turn to sysopen. To open a file with arbitrary weird
characters in it, it's necessary to protect any leading
and trailing whitespace. Leading whitespace is protected
by inserting a "./" in front of a filename that starts
with whitespace. Trailing whitespace is protected by
appending an ASCII NUL byte ("\0") at the end off the
string.
$file =~ s#^(\s)#./$1#;
open(FH, "< $file\0") || die "can't open $file: $!";
This assumes, of course, that your system considers dot
the current working directory, slash the directory
separator, and disallows ASCII NULs within a valid
filename. Most systems follow these conventions,
including all POSIX systems as well as proprietary
Microsoft systems. The only vaguely popular system that
doesn't work this way is the proprietary Macintosh system,
which uses a colon where the rest of us use a slash.
Maybe sysopen isn't such a bad idea after all.
If you want to use <ARGV> processing in a totally boring
and non-magical way, you could do this first:
# "Sam sat on the ground and put his head in his hands.
# 'I wish I had never come here, and I don't want to see
# no more magic,' he said, and fell silent."
for (@ARGV) {
s#^([^./])#./$1#;
$_ .= "\0";
}
while (<>) {
# now process $_
}
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But be warned that users will not appreciate being unable
to use "-" to mean standard input, per the standard
convention.
Paths as Opens
You've probably noticed how Perl's warn and die functions
can produce messages like:
Some warning at scriptname line 29, <FH> chunk 7.
That's because you opened a filehandle FH, and had read in
seven records from it. But what was the name of the file,
not the handle?
If you aren't running with strict refs, or if you've turn
them off temporarily, then all you have to do is this:
open($path, "< $path") || die "can't open $path: $!";
while (<$path>) {
# whatever
}
Since you're using the pathname of the file as its handle,
you'll get warnings more like
Some warning at scriptname line 29, </etc/motd> chunk 7.
Single Argument Open
Remember how we said that Perl's open took two arguments?
That was a passive prevarication. You see, it can also
take just one argument. If and only if the variable is a
global variable, not a lexical, you can pass open just one
argument, the filehandle, and it will get the path from
the global scalar variable of the same name.
$FILE = "/etc/motd";
open FILE or die "can't open $FILE: $!";
while (<FILE>) {
# whatever
}
Why is this here? Someone has to cater to the hysterical
porpoises. It's something that's been in Perl since the
very beginning, if not before.
Playing with STDIN and STDOUT
One clever move with STDOUT is to explicitly close it when
you're done with the program.
END { close(STDOUT) || die "can't close stdout: $!" }
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If you don't do this, and your program fills up the disk
partition due to a command line redirection, it won't
report the error exit with a failure status.
You don't have to accept the STDIN and STDOUT you were
given. You are welcome to reopen them if you'd like.
open(STDIN, "< datafile")
|| die "can't open datafile: $!";
open(STDOUT, "> output")
|| die "can't open output: $!";
And then these can be read directly or passed on to
subprocesses. This makes it look as though the program
were initially invoked with those redirections from the
command line.
It's probably more interesting to connect these to pipes.
For example:
$pager = $ENV{PAGER} || "(less || more)";
open(STDOUT, "| $pager")
|| die "can't fork a pager: $!";
This makes it appear as though your program were called
with its stdout already piped into your pager. You can
also use this kind of thing in conjunction with an
implicit fork to yourself. You might do this if you would
rather handle the post processing in your own program,
just in a different process:
head(100);
while (<>) {
print;
}
sub head {
my $lines = shift || 20;
return unless $pid = open(STDOUT, "|-");
die "cannot fork: $!" unless defined $pid;
while (<STDIN>) {
print;
last if --$lines < 0;
}
exit;
}
This technique can be applied to repeatedly push as many
filters on your output stream as you wish.
Other I/O Issues
These topics aren't really arguments related to open or
sysopen, but they do affect what you do with your open
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files.
Opening Non-File Files
When is a file not a file? Well, you could say when it
exists but isn't a plain file. We'll check whether it's
a symbolic link first, just in case.
if (-l $file || ! -f _) {
print "$file is not a plain file\n";
}
What other kinds of files are there than, well, files?
Directories, symbolic links, named pipes, Unix-domain
sockets, and block and character devices. Those are all
files, too--just not plain files. This isn't the same
issue as being a text file. Not all text files are plain
files. Not all plain files are textfiles. That's why
there are separate -f and -T file tests.
To open a directory, you should use the opendir function,
then process it with readdir, carefully restoring the
directory name if necessary:
opendir(DIR, $dirname) or die "can't opendir $dirname: $!";
while (defined($file = readdir(DIR))) {
# do something with "$dirname/$file"
}
closedir(DIR);
If you want to process directories recursively, it's
better to use the File::Find module. For example, this
prints out all files recursively, add adds a slash to
their names if the file is a directory.
@ARGV = qw(.) unless @ARGV;
use File::Find;
find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV;
This finds all bogus symbolic links beneath a particular
directory:
find sub { print "$File::Find::name\n" if -l && !-e }, $dir;
As you see, with symbolic links, you can just pretend that
it is what it points to. Or, if you want to know what it
points to, then readlink is called for:
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if (-l $file) {
if (defined($whither = readlink($file))) {
print "$file points to $whither\n";
} else {
print "$file points nowhere: $!\n";
}
}
Named pipes are a different matter. You pretend they're
regular files, but their opens will normally block until
there is both a reader and a writer. You can read more
about them in the section on Named Pipes in the perlipc
manpage. Unix-domain sockets are rather different beasts
as well; they're described in the section on Unix-Domain
TCP Clients and Servers in the perlipc manpage.
When it comes to opening devices, it can be easy and it
can tricky. We'll assume that if you're opening up a
block device, you know what you're doing. The character
devices are more interesting. These are typically used
for modems, mice, and some kinds of printers. This is
described in the section on How do I read and write the
serial port? in the perlfaq8 manpage It's often enough to
open them carefully:
sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY)
# (O_NOCTTY no longer needed on POSIX systems)
or die "can't open /dev/ttyS1: $!";
open(TTYOUT, "+>&TTYIN")
or die "can't dup TTYIN: $!";
$ofh = select(TTYOUT); $| = 1; select($ofh);
print TTYOUT "+++at\015";
$answer = <TTYIN>;
With descriptors that you haven't opened using sysopen,
such as a socket, you can set them to be non-blocking
using fcntl:
use Fcntl;
fcntl(Connection, F_SETFL, O_NONBLOCK)
or die "can't set non blocking: $!";
Rather than losing yourself in a morass of twisting,
turning ioctls, all dissimilar, if you're going to
manipulate ttys, it's best to make calls out to the
stty(1) program if you have it, or else use the portable
POSIX interface. To figure this all out, you'll need to
read the termios(3) manpage, which describes the POSIX
interface to tty devices, and then the POSIX manpage,
which describes Perl's interface to POSIX. There are also
some high-level modules on CPAN that can help you with
these games. Check out Term::ReadKey and Term::ReadLine.
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What else can you open? To open a connection using
sockets, you won't use one of Perl's two open functions.
See the section on Sockets: Client/Server Communication in
the perlipc manpage for that. Here's an example. Once
you have it, you can use FH as a bidirectional filehandle.
use IO::Socket;
local *FH = IO::Socket::INET->new("www.perl.com:80");
For opening up a URL, the LWP modules from CPAN are just
what the doctor ordered. There's no filehandle interface,
but it's still easy to get the contents of a document:
use LWP::Simple;
$doc = get('http://www.sn.no/libwww-perl/');
Binary Files
On certain legacy systems with what could charitably be
called terminally convoluted (some would say broken) I/O
models, a file isn't a file--at least, not with respect to
the C standard I/O library. On these old systems whose
libraries (but not kernels) distinguish between text and
binary streams, to get files to behave properly you'll
have to bend over backwards to avoid nasty problems. On
such infelicitous systems, sockets and pipes are already
opened in binary mode, and there is currently no way to
turn that off. With files, you have more options.
Another option is to use the binmode function on the
appropriate handles before doing regular I/O on them:
binmode(STDIN);
binmode(STDOUT);
while (<STDIN>) { print }
Passing sysopen a non-standard flag option will also open
the file in binary mode on those systems that support it.
This is the equivalent of opening the file normally, then
calling binmodeing on the handle.
sysopen(BINDAT, "records.data", O_RDWR | O_BINARY)
|| die "can't open records.data: $!";
Now you can use read and print on that handle without
worrying about the system non-standard I/O library
breaking your data. It's not a pretty picture, but then,
legacy systems seldom are. CP/M will be with us until the
end of days, and after.
On systems with exotic I/O systems, it turns out that,
astonishingly enough, even unbuffered I/O using sysread
and syswrite might do sneaky data mutilation behind your
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back.
while (sysread(WHENCE, $buf, 1024)) {
syswrite(WHITHER, $buf, length($buf));
}
Depending on the vicissitudes of your runtime system, even
these calls may need binmode or O_BINARY first. Systems
known to be free of such difficulties include Unix, the
Mac OS, Plan9, and Inferno.
File Locking
In a multitasking environment, you may need to be careful
not to collide with other processes who want to do I/O on
the same files as others are working on. You'll often
need shared or exclusive locks on files for reading and
writing respectively. You might just pretend that only
exclusive locks exist.
Never use the existence of a file -e $file as a locking
indication, because there is a race condition between the
test for the existence of the file and its creation.
Atomicity is critical.
Perl's most portable locking interface is via the flock
function, whose simplicity is emulated on systems that
don't directly support it, such as SysV or WindowsNT. The
underlying semantics may affect how it all works, so you
should learn how flock is implemented on your system's
port of Perl.
File locking does not lock out another process that would
like to do I/O. A file lock only locks out others trying
to get a lock, not processes trying to do I/O. Because
locks are advisory, if one process uses locking and
another doesn't, all bets are off.
By default, the flock call will block until a lock is
granted. A request for a shared lock will be granted as
soon as there is no exclusive locker. A request for a
exclusive lock will be granted as soon as there is no
locker of any kind. Locks are on file descriptors, not
file names. You can't lock a file until you open it, and
you can't hold on to a lock once the file has been closed.
Here's how to get a blocking shared lock on a file,
typically used for reading:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename: $!";
flock(FH, LOCK_SH) or die "can't lock filename: $!";
# now read from FH
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You can get a non-blocking lock by using LOCK_NB.
flock(FH, LOCK_SH | LOCK_NB)
or die "can't lock filename: $!";
This can be useful for producing more user-friendly
behaviour by warning if you're going to be blocking:
use 5.004;
use Fcntl qw(:DEFAULT :flock);
open(FH, "< filename") or die "can't open filename: $!";
unless (flock(FH, LOCK_SH | LOCK_NB)) {
$| = 1;
print "Waiting for lock...";
flock(FH, LOCK_SH) or die "can't lock filename: $!";
print "got it.\n"
}
# now read from FH
To get an exclusive lock, typically used for writing, you
have to be careful. We sysopen the file so it can be
locked before it gets emptied. You can get a nonblocking
version using LOCK_EX | LOCK_NB.
use 5.004;
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "filename", O_WRONLY | O_CREAT)
or die "can't open filename: $!";
flock(FH, LOCK_EX)
or die "can't lock filename: $!";
truncate(FH, 0)
or die "can't truncate filename: $!";
# now write to FH
Finally, due to the uncounted millions who cannot be
dissuaded from wasting cycles on useless vanity devices
called hit counters, here's how to increment a number in a
file safely:
use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR | O_CREAT)
or die "can't open numfile: $!";
# autoflush FH
$ofh = select(FH); $| = 1; select ($ofh);
flock(FH, LOCK_EX)
or die "can't write-lock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0)
or die "can't rewind numfile : $!";
print FH $num+1, "\n"
or die "can't write numfile: $!";
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truncate(FH, tell(FH))
or die "can't truncate numfile: $!";
close(FH)
or die "can't close numfile: $!";
SEE ALSO
The open and sysopen function in perlfunc(1); the standard
open(2), dup(2), fopen(3), and fdopen(3) manpages; the
POSIX documentation.
AUTHOR and COPYRIGHT
Copyright 1998 Tom Christiansen.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic License. Any distribution of
this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
Irrespective of its distribution, all code examples in
these files are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
HISTORY
First release: Sat Jan 9 08:09:11 MST 1999
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