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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.