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NAME
Carp - alternative warn and die for modules
SYNOPSIS
use Carp;
# warn user (from perspective of caller)
carp "string trimmed to 80 chars";
# die of errors (from perspective of caller)
croak "We're outta here!";
# die of errors with stack backtrace
confess "not implemented";
# cluck, longmess and shortmess not exported by default
use Carp qw(cluck longmess shortmess);
cluck "This is how we got here!";
$long_message = longmess( "message from cluck() or confess()" );
$short_message = shortmess( "message from carp() or croak()" );
DESCRIPTION
The Carp routines are useful in your own modules because they act like "die()" or "warn()", but with a
message which is more likely to be useful to a user of your module. In the case of "cluck()" and
"confess()", that context is a summary of every call in the call-stack; "longmess()" returns the contents
of the error message.
For a shorter message you can use "carp()" or "croak()" which report the error as being from where your
module was called. "shortmess()" returns the contents of this error message. There is no guarantee that
that is where the error was, but it is a good educated guess.
You can also alter the way the output and logic of "Carp" works, by changing some global variables in the
"Carp" namespace. See the section on "GLOBAL VARIABLES" below.
Here is a more complete description of how "carp" and "croak" work. What they do is search the call-
stack for a function call stack where they have not been told that there shouldn't be an error. If every
call is marked safe, they give up and give a full stack backtrace instead. In other words they presume
that the first likely looking potential suspect is guilty. Their rules for telling whether a call
shouldn't generate errors work as follows:
1. Any call from a package to itself is safe.
2. Packages claim that there won't be errors on calls to or from packages explicitly marked as safe by
inclusion in @CARP_NOT, or (if that array is empty) @ISA. The ability to override what @ISA says is
new in 5.8.
3. The trust in item 2 is transitive. If A trusts B, and B trusts C, then A trusts C. So if you do not
override @ISA with @CARP_NOT, then this trust relationship is identical to, "inherits from".
4. Any call from an internal Perl module is safe. (Nothing keeps user modules from marking themselves
as internal to Perl, but this practice is discouraged.)
5. Any call to Perl's warning system (eg Carp itself) is safe. (This rule is what keeps it from
reporting the error at the point where you call "carp" or "croak".)
6. $Carp::CarpLevel can be set to skip a fixed number of additional call levels. Using this is not
recommended because it is very difficult to get it to behave correctly.
Forcing a Stack Trace
As a debugging aid, you can force Carp to treat a croak as a confess and a carp as a cluck across all
modules. In other words, force a detailed stack trace to be given. This can be very helpful when trying
to understand why, or from where, a warning or error is being generated.
This feature is enabled by 'importing' the non-existent symbol 'verbose'. You would typically enable it
by saying
perl -MCarp=verbose script.pl
or by including the string "-MCarp=verbose" in the PERL5OPT environment variable.
Alternately, you can set the global variable $Carp::Verbose to true. See the "GLOBAL VARIABLES" section
below.
GLOBAL VARIABLES
$Carp::MaxEvalLen
This variable determines how many characters of a string-eval are to be shown in the output. Use a value
of 0 to show all text.
Defaults to 0.
$Carp::MaxArgLen
This variable determines how many characters of each argument to a function to print. Use a value of 0 to
show the full length of the argument.
Defaults to 64.
$Carp::MaxArgNums
This variable determines how many arguments to each function to show. Use a value of 0 to show all
arguments to a function call.
Defaults to 8.
$Carp::Verbose
This variable makes "carp()" and "croak()" generate stack backtraces just like "cluck()" and "confess()".
This is how "use Carp 'verbose'" is implemented internally.
Defaults to 0.
@CARP_NOT
This variable, in your package, says which packages are not to be considered as the location of an error.
The "carp()" and "cluck()" functions will skip over callers when reporting where an error occurred.
NB: This variable must be in the package's symbol table, thus:
# These work
our @CARP_NOT; # file scope
use vars qw(@CARP_NOT); # package scope
@My::Package::CARP_NOT = ... ; # explicit package variable
# These don't work
sub xyz { ... @CARP_NOT = ... } # w/o declarations above
my @CARP_NOT; # even at top-level
Example of use:
package My::Carping::Package;
use Carp;
our @CARP_NOT;
sub bar { .... or _error('Wrong input') }
sub _error {
# temporary control of where'ness, __PACKAGE__ is implicit
local @CARP_NOT = qw(My::Friendly::Caller);
carp(@_)
}
This would make "Carp" report the error as coming from a caller not in "My::Carping::Package", nor from
"My::Friendly::Caller".
Also read the "DESCRIPTION" section above, about how "Carp" decides where the error is reported from.
Use @CARP_NOT, instead of $Carp::CarpLevel.
Overrides "Carp"'s use of @ISA.
%Carp::Internal
This says what packages are internal to Perl. "Carp" will never report an error as being from a line in
a package that is internal to Perl. For example:
$Carp::Internal{ (__PACKAGE__) }++;
# time passes...
sub foo { ... or confess("whatever") };
would give a full stack backtrace starting from the first caller outside of __PACKAGE__. (Unless that
package was also internal to Perl.)
%Carp::CarpInternal
This says which packages are internal to Perl's warning system. For generating a full stack backtrace
this is the same as being internal to Perl, the stack backtrace will not start inside packages that are
listed in %Carp::CarpInternal. But it is slightly different for the summary message generated by "carp"
or "croak". There errors will not be reported on any lines that are calling packages in
%Carp::CarpInternal.
For example "Carp" itself is listed in %Carp::CarpInternal. Therefore the full stack backtrace from
"confess" will not start inside of "Carp", and the short message from calling "croak" is not placed on
the line where "croak" was called.
$Carp::CarpLevel
This variable determines how many additional call frames are to be skipped that would not otherwise be
when reporting where an error occurred on a call to one of "Carp"'s functions. It is fairly easy to
count these call frames on calls that generate a full stack backtrace. However it is much harder to do
this accounting for calls that generate a short message. Usually people skip too many call frames. If
they are lucky they skip enough that "Carp" goes all of the way through the call stack, realizes that
something is wrong, and then generates a full stack backtrace. If they are unlucky then the error is
reported from somewhere misleading very high in the call stack.
Therefore it is best to avoid $Carp::CarpLevel. Instead use @CARP_NOT, %Carp::Internal and
%Carp::CarpInternal.
Defaults to 0.
BUGS
The Carp routines don't handle exception objects currently. If called with a first argument that is a
reference, they simply call die() or warn(), as appropriate.
SEE ALSO
Carp::Always, Carp::Clan
AUTHOR
The Carp module first appeared in Larry Wall's perl 5.000 distribution. Since then it has been modified
by several of the perl 5 porters. Andrew Main (Zefram) <zefram@fysh.org> divested Carp into an
independent distribution.
COPYRIGHT
Copyright (C) 1994-2012 Larry Wall
Copyright (C) 2011, 2012 Andrew Main (Zefram) <zefram@fysh.org>
LICENSE
This module is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.
perl v5.18.2 2014-01-06 Carp(3perl)