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.

$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.