phpman > man > 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, longmess and shortmess not exported by default
        use Carp qw(cluck longmess shortmess);
        cluck "This is how we got here!"; # warn with stack backtrace
        $long_message   = longmess( "message from cluck() or confess()" );
        $short_message  = shortmess( "message from carp() or croak()" );

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()" and "confess()", that context is a summary of every call in the call-stack;
    "longmess()" returns the contents of the error message.

    For a shorter message you can use "carp()" or "croak()" which report the error as being from
    where your module was called. "shortmess()" returns the contents of this error message. There is
    no guarantee that that is where the error was, but it is a good educated guess.

    "Carp" takes care not to clobber the status variables $! and $^E in the course of assembling its
    error messages. This means that a $SIG{__DIE__} or $SIG{__WARN__} handler can capture the error
    information held in those variables, if it is required to augment the error message, and if the
    code calling "Carp" left useful values there. Of course, "Carp" can't guarantee the latter.

    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.

  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.

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 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.

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

CONTRIBUTING
    Carp is maintained by the perl 5 porters as part of the core perl 5 version control repository.
    Please see the perlhack perldoc for how to submit patches and contribute to it.

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) 1994-2013 Larry Wall

    Copyright (C) 2011, 2012, 2013 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.