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 undesirable
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 desirable: For example:
eval {
local $CGI::Carp::TO_BROWSER = 0;
die "Fatal error messages not sent browser"
}
# $@ will contain error message
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
TURNING OFF TIMESTAMPS IN MESSAGES¶
If your web server automatically adds a timestamp to each log line, you may not
need CGI::Carp to add its own. You can disable timestamping by importing
"noTimestamp":
use CGI::Carp qw(noTimestamp);
Alternatively you can set $CGI::Carp::NO_TIMESTAMP to 1.
Note that the name of the program is still automatically included in the
message.
The CGI.pm distribution is copyright 1995-2007, Lincoln D. Stein. It is
distributed under GPL and the Artistic License 2.0. It is currently maintained
by Lee Johnson with help from many contributors.
Address bug reports and comments to:
https://github.com/leejo/CGI.pm/issues
The original bug tracker can be found at:
https://rt.cpan.org/Public/Dist/Display.html?Queue=CGI.pm
When sending bug reports, please provide the version of CGI.pm, the version of
Perl, the name and version of your Web server, and the name and version of the
operating system you are using. If the problem is even remotely browser
dependent, please provide information about the affected browsers as well.
SEE ALSO¶
Carp, CGI::Base, CGI::BasePlus, CGI::Request, CGI::MiniSvr, CGI::Form,
CGI::Response.