carp



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


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!";
           $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 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::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.

       Some of the Carp code assumes that Perl's basic character encoding is
       ASCII, and will go wrong on an EBCDIC platform.


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@fysh.org> divested Carp into an
       independent distribution.


COPYRIGHT

       Copyright (C) 1994-2013 Larry Wall

       Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) <zefram@fysh.org>


LICENSE

       This module is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

perl v5.20.2                      2014-11-17                          Carp(3p)

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