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Carp(3)               User Contributed Perl Documentation              Carp(3)

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 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 AT fysh.org> divested Carp into an
       independent distribution.
COPYRIGHT
       Copyright (C) 1994-2012 Larry Wall
       Copyright (C) 2011, 2012 Andrew Main (Zefram) <zefram AT fysh.org>
LICENSE
       This module is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.16.3                      2012-06-18                           Carp(3)