MOD_PERL_TRAPS(User Contributed Perl DocumentatiMOD_PERL_TRAPS(1)NAMEmod_perl_traps - common/known mod_perl traps
DESCRIPTION
In the CGI environment, the server starts a single exter
nal process (Perl interpreter) per HTTP request which runs
single script in that process space. When the request is
over, the process goes away everything is cleaned up and a
fresh script is started for the next request. mod_perl
brings Perl inside of the HTTP server not only for speedup
of CGI scripts, but also for access to server functional
ity that CGI scripts do not and/or cannot have. Now that
we're inside the server, each process will likely handle
more than one Perl script and keep it "compiled" in memory
for longer than a single HTTP request. This new location
and longer lifetime of Perl execution brings with it some
common traps. This document is here to tell you what they
are and how to prevent them. The descriptions here are
short, please consult the mod_perl FAQ for more detail.
If you trip over something not documented here, please
send a message to the mod_perl list.
Migrating from CGI
Be sure to have read the cgi_to_mod_perl manpage
Scripts under Apache::Registry are not run in package
main, they are run in a unique namespace based on the
requested uri.
Apache::Registry scripts cannot contain __END__ or
__DATA__ tokens
Output of "system", "exec" and "open PIPE, "|program""
calls will not be sent to the browser unless you Perl
was configured with sfio.
Perl's exit() built-in function cannot be used in
mod_perl scripts. The Apache::exit() function should
be used instead. Apache::exit() automatically over
rides the built-in exit() for Apache::Registry and
Apache::PerlRun scripts.
Your script *will not* run from the command line if
your script makes any direct calls to Apache->methods.
See Apache::FakeRequest.
Apache::Registry
undefined subroutine &Apache::Registry::handler
Interaction with certain modules causes the shortcut
configuration to break, if you see this message change
your configuration from this:
<Location /perl>
PerlHandler Apache::Registry
...
</Location>
To this:
PerlModule Apache::Registry
<Location /perl>
PerlHandler Apache::Registry::handler
...
</Location>
Using CGI.pm and CGI::*
CGI.pm users must have version 2.39 of the package or
higher, earlier versions will not work under mod_perl.
If you use the "SendHeaders()" function, be sure to
call $req_obj->cgi->done when you are done with a
request, just as you would under CGI::MiniSrv.
Perl Modules and Extensions
Files pulled in via "use" or "require" statements are
not automatically reloaded when changed on disk. See
the Apache::StatINC or the Apache::Reload module to
add this functionality.
Undefined subroutines
A common trap with required files may result in an
error message similar to this in the error_log:
[Thu Sep 11 11:03:06 1997] Undefined subroutine
&Apache::ROOT::perl::test_2epl::some_function called at
/opt/www/apache/perl/test.pl line 79.
As the above items explains, a file pulled in via
"require" will only happen once per-process (unless
%INC is modified). If the file does not contain a
"package" declaration, the file's subroutines and
variables will be created in the current package.
Under CGI, this is commonly package "main". However,
Apache::Registry scripts are compiled into a unique
package name (base on the uri). So, if multiple
scripts in the same process try to require the same
file, which does not declare a package, only one
script will actually be able to see the subroutines.
The solution is to read the perlmodlib manpage, the
perlmod manpage and related perl documentation and re-
work your required file into a module which exports
functions or defines a method interface. Or something
more simple, along these lines:
#required_file.pl
package Test;
sub some_function {...}
...
__END__
Now, have your scripts say:
require "required_file.pl";
Test::some_function();
Undefined subroutine &Foo::Bar::handler called at PerlHan
dler subroutine `Foo::Bar' line 1.
You mistyped the module name in the 'package' line in
your module.
""Use of uninitialized value""
Because of eval context, you may see warnings with
useless filename/line, example:
Use of uninitialized value at (eval 80) line 12.
Use of uninitialized value at (eval 80) line 43.
Use of uninitialized value at (eval 80) line 44.
To track down where this eval is really happening, try
using a __WARN__ handler to give you a stack trace:
use Carp ();
local $SIG{__WARN__} = \&Carp::cluck;
""Callback called exit""
""Out of memory!""
If something goes really wrong with your code, Perl
may die with an "Out of memory!" message and or "Call
back called exit". A common cause of this are never-
ending loops, deep recursion or calling an undefined
subroutine. Here's one way to catch the problem: See
Perl's INSTALL document for this item:
-DPERL_EMERGENCY_SBRK
If PERL_EMERGENCY_SBRK is defined, running out of mem
ory need not be a fatal error: a memory pool can allo
cated by assigning to the special variable $^M. See
perlvar(1) for more details.
If you compile with that option and add 'use
Apache::Debug level => 4;' to your PerlScript, it will
allocate the $^M emergency pool and the $SIG{__DIE__}
handler will call Carp::confess, giving you a stack
trace which should reveal where the problem is.
See the Apache::Resource module for prevention of
spinning httpds.
If you wish to use a module that is normally linked
static with your Perl, it must be listed in static_ext
in Perl's Config.pm to be linked with httpd during the
mod_perl build.
Can't load '$Config{sitearchexp}/auto/Foo/Foo.so' for mod
ule Foo...
When starting httpd some people have reported seeing
an error along the lines of:
[Thu Jul 9 17:33:42 1998] [error] Can't load
'/usr/local/ap/lib/perl5/site_perl/sun4-solaris/auto/DBI/DBI.so' for
module DBI: ld.so.1: src/httpd: fatal: relocation error: file
/usr/local/ap/lib/perl5/site_perl/sun4-solaris/auto/DBI/DBI.so: symbol
Perl_sv_undef: referenced symbol not found at
/usr/local/ap/lib/perl5/sun4-solaris/5.00404/DynaLoader.pm line 166.
Or similar for the IO module or whatever dynamic mod
ule mod_perl tries to pull in first. The solution is
to re-configure, re-build and re-install Perl and
dynamic modules with the following flags when
Configure asks for "additional LD flags":
-Xlinker --export-dynamic
or
-Xlinker -E
This problem is only known to be caused by installing
gnu ld under Solaris.
Other known causes of this problem:
OS distributions that ship with a (broken) binary Perl
installation.
The `perl' program and `libperl.a' library are somehow
built with different binary compatiblity flags.
The solution to these problems is to rebuild Perl and
extension modules from a fresh source tree. Tip for
running Perl's Configure script, use the `"-des"'
flags to accepts defaults and `"-D"' flag to override
certain attributes:
% ./Configure -des -Dcc=gcc ... && make test && make install
Read Perl's INSTALL doc for more details.
Clashes with other Apache C modules
mod_auth_dbm
If you are a user of mod_auth_dbm or mod_auth_db, you
may need to edit Perl's "Config" module. When Perl is
configured it attempts to find libraries for ndbm,
gdbm, db, etc., for the *DBM*_File modules. By
default, these libraries are linked with Perl and
remembered by the Config module. When mod_perl is
configured with apache, the ExtUtils::Embed module
returns these libraries to be linked with httpd so
Perl extensions will work under mod_perl. However,
the order in which these libraries are stored in Con
fig.pm, may confuse "mod_auth_db*". If "mod_auth_db*"
does not work with mod_perl, take a look at this order
with the following command:
% perl -V:libs
If "-lgdbm" or "-ldb" is before "-lndbm", example:
libs='-lnet -lnsl_s -lgdbm -lndbm -ldb -ldld -lm -lc -lndir -lcrypt';
Edit Config.pm and move "-lgdbm" and "-ldb" to the end
of the list. Here's how to find Config.pm:
% perl -MConfig -e 'print "$Config{archlibexp}/Config.pm\n"'
Another solution for building
Apache/mod_perl+mod_auth_dbm under Solaris is to
remove the DBM and NDBM "emulation" from libgdbm.a.
Seems Solaris already provides its own DBM and NDBM,
and there's no reason to build GDBM with them (for us
anyway).
In our Makefile for GDBM, we changed
OBJS = $(DBM_OF) $(NDBM_OF) $(GDBM_OF)
to
OBJS = $(GDBM_OF)
Rebuild libgdbm, then Apache/mod_perl.
REGULAR EXPRESSIONS
COMPILED REGULAR EXPRESSIONS
When using a regular expression that contains an interpo
lated Perl variable, if it is known that the variable (or
variables) will not vary during the execution of the pro
gram, a standard optimization technique consists of adding
the "o" modifier to the regexp pattern, to direct the com
piler to build the internal table once, for the entire
lifetime of the script, rather than every time the pattern
is executed. Consider:
my $pat = '^foo$'; # likely to be input from an HTML form field
foreach( @list ) {
print if /$pat/o;
}
This is usually a big win in loops over lists, or when
using "grep" or "map".
In long-lived "mod_perl" scripts, however, this can pose a
problem if the variable changes according to the invoca
tion. The first invocation of a fresh httpd child will
compile the table and perform the search correctly, how
ever, all subsequent uses by the httpd child will continue
to match the original pattern, regardless of the current
contents of the Perl variables the pattern is dependent
on. Your script will appear broken.
There are two solutions to this problem.
The first is to use "eval q//", to force the code to be
evaluated each time. Just make sure that the "eval" block
covers the entire loop of processing, and not just the
pattern match itself.
The above code fragment would be rewritten as:
my $pat = '^foo$';
eval q{
foreach( @list ) {
print if /$pat/o;
}
}
Just saying
eval q{ print if /$pat/o; };
is going to be a horribly expensive proposition.
You use this approach if you require more than one pattern
match operator in a given section of code. If the section
contains only one operator (be it an "m//" or "s///"), you
can rely on the property of the null pattern, that reuses
the last pattern seen. This leads to the second solution,
which also eliminates the use of "eval".
The above code fragment becomes:
my $pat = '^foo$';
"something" =~ /$pat/; # dummy match (MUST NOT FAIL!)
foreach( @list ) {
print if //;
}
The only gotcha is that the dummy match that boots the
regular expression engine must absolutely, positively suc
ceed, otherwise the pattern will not be cached, and the //
will match everything. If you can't count on fixed text to
ensure the match succeeds, you have two possibilities.
If you can guaranteee that the pattern variable contains
no meta-characters (things like "*", "+", "^", "$"...),
you can use the dummy match:
"$pat" =~ /\Q$pat\E/; # guaranteed if no meta-characters present
If there is a possibility that the pattern can contain
meta-characters, you should search for the pattern or the
unsearchable "\377" character as follows:
"\377" =~ /$pat|^[\377]$/; # guarenteed if meta-characters present
References
The Camel Book, 2nd edition, p. 538 (p. 356 in the 1st edition).
AUTHORS
Doug MacEachern, with contributions from Jens Heunemann
<heunemann2@janet.de>, David Landgren <david@land
gren.net>, Mark Mills <mark@ntr.net>, Randal Schwartz
<merlyn@stonehenge.com> and Ask Bjoern Hansen <ask@devel
ooper.com>
2002-03-28 mod_perl-1.27 MOD_PERL_TRAPS(1)
