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Carp(3pm)                             Perl Programmers Reference Guide                             Carp(3pm)



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 not exported by default
           use Carp qw(cluck);
           cluck "This is how we got here!";

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, confess,
       and longmess that context is a summary of every call in the call-stack.  For a shorter message you
       can use "carp" or "croak" which report the error as being from where your module was called.  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 callstack
       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 v5.16.2                                     2012-10-25                                        Carp(3pm)