Linux Manual Pages

Free Software * Books

Source Code

Free Media

Linux

Carp

NAME
SYNOPSIS
DESCRIPTION
GLOBAL VARIABLES
BUGS

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 not exported by default
    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".
	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.

$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.

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.

Opportunity

Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.

Education

Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.