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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
And the standard warn(), die (), croak(), confess() and carp() calls will
automagically be replaced with functions that write out nicely 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, which should be 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.
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, ask to
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).
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.
MAKING WARNINGS APPEAR AS HTML COMMENTS
It is now 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!
CHANGE LOG
1.05 carpout() added and minor corrections by Marc Hedlund
<hedlund@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@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@itz.pp.sci.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 accomodate various combinations of Perl and
mod_perl.
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.
Address bug reports and comments to: lstein@cshl.org
SEE ALSO
Carp, CGI::Base, CGI::BasePlus, CGI::Request, CGI::MiniSvr, CGI::Form,
CGI::Response
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