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.

Stack Trace formatting

At each stack level, the subroutine's name is displayed along with its parameters. For simple scalars, this is sufficient. For complex data types, such as objects and other references, this can simply display 'HASH(0x1ab36d8)'.

Carp gives two ways to control this.

1.

For objects, a method, "CARP_TRACE", will be called, if it exists. If this method doesn't exist, or it recurses into "Carp", or it otherwise throws an exception, this is skipped, and Carp moves on to the next option, otherwise checking stops and the string returned is used. It is recommended that the object's type is part of the string to make debugging easier.

2.

For any type of reference, $Carp::RefArgFormatter is checked (see below). This variable is expected to be a code reference, and the current parameter is passed in. If this function doesn't exist (the variable is undef), or it recurses into "Carp", or it otherwise throws an exception, this is skipped, and Carp moves on to the next option, otherwise checking stops and the string returned is used.

3.

Otherwise, if neither "CARP_TRACE" nor $Carp::RefArgFormatter is available, stringify the value ignoring any overloading.

$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 false value to show all arguments to a function call. To suppress all arguments, use "-1" or '0 but true'.

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::RefArgFormatter

This variable sets a general argument formatter to display references. Plain scalars and objects that implement "CARP_TRACE" will not go through this formatter. Calling "Carp" from within this function is not supported.

    local $Carp::RefArgFormatter = sub {
        require Data::Dumper;
        Data::Dumper->Dump($_[0]); # not necessarily safe
    };

@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.