Provided by: perl-doc_5.18.2-2ubuntu1.7_all 
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@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 accommodate various combinations of Perl and
mod_perl.
1.24 Patch from Scott Gifford (sgifford@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.