


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


NAME
       carp    - warn of errors (from perspective of caller)

       cluck   - warn of errors with stack backtrace
                 (not exported by default)

       croak   - die of errors (from perspective of caller)

       confess - die of errors with stack backtrace

SYNOPSIS
           use Carp;
           croak "We're outta here!";

           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".



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


       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.




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


   $Carp::Verbose
       This variable makes "carp" and "cluck" generate stack
       backtraces just like "cluck" and "confess".  This is how
       "use Carp 'verbose'" is implemented internally.

       Defaults to 0.

   %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.




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


       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.


















































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