CGI::Carp(3) User Contributed Perl Documentation CGI::Carp(3)
NAME
CGI::Carp - CGI routines for writing to the HTTPD (or other) error log
SYNOPSIS
use CGI::Carp;
croak "We're outta here!";
confess "It was my fault: $!";
carp "It was your fault!";
warn "I'm confused";
die "I'm dying.\n";
use CGI::Carp qw(cluck);
cluck "I wouldn't do that if I were you";
use CGI::Carp qw(fatalsToBrowser);
die "Fatal error messages are now sent to browser";
DESCRIPTION
CGI scripts have a nasty habit of leaving warning messages in the error
logs that are neither time stamped nor fully identified. Tracking down
the script that caused the error is a pain. This fixes that. Replace
the usual
use Carp;
with
use CGI::Carp
The standard warn(), die (), croak(), confess() and carp() calls will
be replaced with functions that write time-stamped messages to the HTTP
server error log.
For example:
[Fri Nov 17 21:40:43 1995] test.pl: I'm confused at test.pl line 3.
[Fri Nov 17 21:40:43 1995] test.pl: Got an error message: Permission denied.
[Fri Nov 17 21:40:43 1995] test.pl: I'm dying.
REDIRECTING ERROR MESSAGES
By default, error messages are sent to STDERR. Most HTTPD servers
direct STDERR to the server's error log. Some applications may wish to
keep private error logs, distinct from the server's error log, or they
may wish to direct error messages to STDOUT so that the browser will
receive them.
The "carpout()" function is provided for this purpose. Since carpout()
is not exported by default, you must import it explicitly by saying
use CGI::Carp qw(carpout);
The carpout() function requires one argument, a reference to an open
filehandle for writing errors. It should be called in a "BEGIN" block
at the top of the CGI application so that compiler errors will be
caught. Example:
BEGIN {
use CGI::Carp qw(carpout);
open(LOG, ">>/usr/local/cgi-logs/mycgi-log") or
die("Unable to open mycgi-log: $!\n");
carpout(LOG);
}
carpout() does not handle file locking on the log for you at this
point. Also, note that carpout() does not work with in-memory file
handles, although a patch would be welcome to address that.
The real STDERR is not closed -- it is moved to CGI::Carp::SAVEERR.
Some servers, when dealing with CGI scripts, close their connection to
the browser when the script closes STDOUT and STDERR.
CGI::Carp::SAVEERR is there to prevent this from happening prematurely.
You can pass filehandles to carpout() in a variety of ways. The
"correct" way according to Tom Christiansen is to pass a reference to a
filehandle GLOB:
carpout(\*LOG);
This looks weird to mere mortals however, so the following syntaxes are
accepted as well:
carpout(LOG);
carpout(main::LOG);
carpout(main'LOG);
carpout(\LOG);
carpout(\'main::LOG');
... and so on
FileHandle and other objects work as well.
Use of carpout() is not great for performance, so it is recommended for
debugging purposes or for moderate-use applications. A future version
of this module may delay redirecting STDERR until one of the CGI::Carp
methods is called to prevent the performance hit.
MAKING PERL ERRORS APPEAR IN THE BROWSER WINDOW
If you want to send fatal (die, confess) errors to the browser, import
the special "fatalsToBrowser" subroutine:
use CGI::Carp qw(fatalsToBrowser);
die "Bad error here";
Fatal errors will now be echoed to the browser as well as to the log.
CGI::Carp arranges to send a minimal HTTP header to the browser so that
even errors that occur in the early compile phase will be seen.
Nonfatal errors will still be directed to the log file only (unless
redirected with carpout).
Note that fatalsToBrowser may not work well with mod_perl version 2.0
and higher.
Changing the default message
By default, the software error message is followed by a note to contact
the Webmaster by e-mail with the time and date of the error. If this
message is not to your liking, you can change it using the
set_message() routine. This is not imported by default; you should
import it on the use() line:
use CGI::Carp qw(fatalsToBrowser set_message);
set_message("It's not a bug, it's a feature!");
You may also pass in a code reference in order to create a custom error
message. At run time, your code will be called with the text of the
error message that caused the script to die. Example:
use CGI::Carp qw(fatalsToBrowser set_message);
BEGIN {
sub handle_errors {
my $msg = shift;
print "<h1>Oh gosh</h1>";
print "<p>Got an error: $msg</p>";
}
set_message(\&handle_errors);
}
In order to correctly intercept compile-time errors, you should call
set_message() from within a BEGIN{} block.
DOING MORE THAN PRINTING A MESSAGE IN THE EVENT OF PERL ERRORS
If fatalsToBrowser in conjunction with set_message does not provide you
with all of the functionality you need, you can go one step further by
specifying a function to be executed any time a script calls "die", has
a syntax error, or dies unexpectedly at runtime with a line like
"undef->explode();".
use CGI::Carp qw(set_die_handler);
BEGIN {
sub handle_errors {
my $msg = shift;
print "content-type: text/html\n\n";
print "<h1>Oh gosh</h1>";
print "<p>Got an error: $msg</p>";
#proceed to send an email to a system administrator,
#write a detailed message to the browser and/or a log,
#etc....
}
set_die_handler(\&handle_errors);
}
Notice that if you use set_die_handler(), you must handle sending HTML
headers to the browser yourself if you are printing a message.
If you use set_die_handler(), you will most likely interfere with the
behavior of fatalsToBrowser, so you must use this or that, not both.
Using set_die_handler() sets SIG{__DIE__} (as does fatalsToBrowser),
and there is only one SIG{__DIE__}. This means that if you are
attempting to set SIG{__DIE__} yourself, you may interfere with this
module's functionality, or this module may interfere with your module's
functionality.
SUPPRESSING PERL ERRORS APPEARING IN THE BROWSER WINDOW
A problem sometimes encountered when using fatalsToBrowser is when a
"die()" is done inside an "eval" body or expression. Even though the
fatalsToBrower support takes precautions to avoid this, you still may
get the error message printed to STDOUT. This may have some
undesireable effects when the purpose of doing the eval is to determine
which of several algorithms is to be used.
By setting $CGI::Carp::TO_BROWSER to 0 you can suppress printing the
"die" messages but without all of the complexity of using
"set_die_handler". You can localize this effect to inside "eval"
bodies if this is desireable: For example:
eval {
local $CGI::Carp::TO_BROWSER = 0;
die "Fatal error messages not sent browser"
}
# $@ will contain error message
MAKING WARNINGS APPEAR AS HTML COMMENTS
It is also possible to make non-fatal errors appear as HTML comments
embedded in the output of your program. To enable this feature, export
the new "warningsToBrowser" subroutine. Since sending warnings to the
browser before the HTTP headers have been sent would cause an error,
any warnings are stored in an internal buffer until you call the
warningsToBrowser() subroutine with a true argument:
use CGI::Carp qw(fatalsToBrowser warningsToBrowser);
use CGI qw(:standard);
print header();
warningsToBrowser(1);
You may also give a false argument to warningsToBrowser() to prevent
warnings from being sent to the browser while you are printing some
content where HTML comments are not allowed:
warningsToBrowser(0); # disable warnings
print "<script type=\"text/javascript\"><!--\n";
print_some_javascript_code();
print "//--></script>\n";
warningsToBrowser(1); # re-enable warnings
Note: In this respect warningsToBrowser() differs fundamentally from
fatalsToBrowser(), which you should never call yourself!
OVERRIDING THE NAME OF THE PROGRAM
CGI::Carp includes the name of the program that generated the error or
warning in the messages written to the log and the browser window.
Sometimes, Perl can get confused about what the actual name of the
executed program was. In these cases, you can override the program
name that CGI::Carp will use for all messages.
The quick way to do that is to tell CGI::Carp the name of the program
in its use statement. You can do that by adding
"name=cgi_carp_log_name" to your "use" statement. For example:
use CGI::Carp qw(name=cgi_carp_log_name);
. If you want to change the program name partway through the program,
you can use the "set_progname()" function instead. It is not exported
by default, you must import it explicitly by saying
use CGI::Carp qw(set_progname);
Once you've done that, you can change the logged name of the program at
any time by calling
set_progname(new_program_name);
You can set the program back to the default by calling
set_progname(undef);
Note that this override doesn't happen until after the program has
compiled, so any compile-time errors will still show up with the non-
overridden program name
CHANGE LOG
3.51 Added $CGI::Carp::TO_BROWSER
1.29 Patch from Peter Whaite to fix the unfixable problem of CGI::Carp
not behaving correctly in an eval() context.
1.05 carpout() added and minor corrections by Marc Hedlund
<hedlund AT best.com> on 11/26/95.
1.06 fatalsToBrowser() no longer aborts for fatal errors within
eval() statements.
1.08 set_message() added and carpout() expanded to allow for FileHandle
objects.
1.09 set_message() now allows users to pass a code REFERENCE for
really custom error messages. croak and carp are now
exported by default. Thanks to Gunther Birznieks for the
patches.
1.10 Patch from Chris Dean (ctdean AT cogit.com) to allow
module to run correctly under mod_perl.
1.11 Changed order of > and < escapes.
1.12 Changed die() on line 217 to CORE::die to avoid -w warning.
1.13 Added cluck() to make the module orthogonal with Carp.
More mod_perl related fixes.
1.20 Patch from Ilmari Karonen (perl AT itz.fi): Added
warningsToBrowser(). Replaced <CODE> tags with <PRE> in
fatalsToBrowser() output.
1.23 ineval() now checks both $^S and inspects the message for the
"eval" pattern
(hack alert!) in order to accommodate various combinations of Perl
and
mod_perl.
1.24 Patch from Scott Gifford (sgifford AT suspectclass.com): Add support
for overriding program name.
1.26 Replaced CORE::GLOBAL::die with the evil $SIG{__DIE__} because the
former isn't working in some people's hands. There is no such
thing
as reliable exception handling in Perl.
1.27 Replaced tell STDOUT with bytes=tell STDOUT.
AUTHORS
Copyright 1995-2002, Lincoln D. Stein. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
SEE ALSO
Carp, CGI::Base, CGI::BasePlus, CGI::Request, CGI::MiniSvr, CGI::Form,
CGI::Response.
perl v5.16.3 2012-11-03 CGI::Carp(3)
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