carp.3

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.\" ========================================================================
.\"
.IX Title "Carp 3"
.TH Carp 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
carp    \- warn of errors (from perspective of caller)
.PP
cluck   \- warn of errors with stack backtrace
          (not exported by default)
.PP
croak   \- die of errors (from perspective of caller)
.PP
confess \- die of errors with stack backtrace
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    use Carp;
\&    croak "We\*(Aqre outta here!";
\&
\&    use Carp qw(cluck);
\&    cluck "This is how we got here!";
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The Carp routines are useful in your own modules because
they act like \fIdie()\fR or \fIwarn()\fR, 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 \f(CW\*(C`carp\*(C'\fR
or \f(CW\*(C`croak\*(C'\fR 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.
.PP
You can also alter the way the output and logic of \f(CW\*(C`Carp\*(C'\fR works, by
changing some global variables in the \f(CW\*(C`Carp\*(C'\fR namespace. See the
section on \f(CW\*(C`GLOBAL VARIABLES\*(C'\fR below.
.PP
Here is a more complete description of how c<carp> and c<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:
.IP "1." 4
Any call from a package to itself is safe.
.IP "2." 4
Packages claim that there won't be errors on calls to or from
packages explicitly marked as safe by inclusion in \f(CW@CARP_NOT\fR, or
(if that array is empty) \f(CW@ISA\fR.  The ability to override what
\&\f(CW@ISA\fR says is new in 5.8.
.IP "3." 4
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 \f(CW@ISA\fR
with \f(CW@CARP_NOT\fR, then this trust relationship is identical to,
\&\*(L"inherits from\*(R".
.IP "4." 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.)
.IP "5." 4
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 \f(CW\*(C`carp\*(C'\fR or \f(CW\*(C`croak\*(C'\fR.)
.IP "6." 4
\&\f(CW$Carp::CarpLevel\fR can be set to skip a fixed number of additional
call levels.  Using this is not recommended because it is very
difficult to get it to behave correctly.
.Sh "Forcing a Stack Trace"
.IX Subsection "Forcing a Stack Trace"
As a debugging aid, you can force Carp to treat a croak as a confess
and a carp as a cluck across \fIall\fR modules. In other words, force a
detailed stack trace to be given.  This can be very helpful when trying
to understand why, or from where, a warning or error is being generated.
.PP
This feature is enabled by 'importing' the non-existent symbol
\&'verbose'. You would typically enable it by saying
.PP
.Vb 1
\&    perl \-MCarp=verbose script.pl
.Ve
.PP
or by including the string \f(CW\*(C`MCarp=verbose\*(C'\fR in the \s-1PERL5OPT\s0
environment variable.
.PP
Alternately, you can set the global variable \f(CW$Carp::Verbose\fR to true.
See the \f(CW\*(C`GLOBAL VARIABLES\*(C'\fR section below.
.SH "GLOBAL VARIABLES"
.IX Header "GLOBAL VARIABLES"
.ie n .Sh "$Carp::MaxEvalLen"
.el .Sh "\f(CW$Carp::MaxEvalLen\fP"
.IX Subsection "$Carp::MaxEvalLen"
This variable determines how many characters of a string-eval are to
be shown in the output. Use a value of \f(CW0\fR to show all text.
.PP
Defaults to \f(CW0\fR.
.ie n .Sh "$Carp::MaxArgLen"
.el .Sh "\f(CW$Carp::MaxArgLen\fP"
.IX Subsection "$Carp::MaxArgLen"
This variable determines how many characters of each argument to a
function to print. Use a value of \f(CW0\fR to show the full length of the
argument.
.PP
Defaults to \f(CW64\fR.
.ie n .Sh "$Carp::MaxArgNums"
.el .Sh "\f(CW$Carp::MaxArgNums\fP"
.IX Subsection "$Carp::MaxArgNums"
This variable determines how many arguments to each function to show.
Use a value of \f(CW0\fR to show all arguments to a function call.
.PP
Defaults to \f(CW8\fR.
.ie n .Sh "$Carp::Verbose"
.el .Sh "\f(CW$Carp::Verbose\fP"
.IX Subsection "$Carp::Verbose"
This variable makes \f(CW\*(C`carp\*(C'\fR and \f(CW\*(C`cluck\*(C'\fR generate stack backtraces
just like \f(CW\*(C`cluck\*(C'\fR and \f(CW\*(C`confess\*(C'\fR.  This is how \f(CW\*(C`use Carp \*(Aqverbose\*(Aq\*(C'\fR
is implemented internally.
.PP
Defaults to \f(CW0\fR.
.ie n .Sh "%Carp::Internal"
.el .Sh "\f(CW%Carp::Internal\fP"
.IX Subsection "%Carp::Internal"
This says what packages are internal to Perl.  \f(CW\*(C`Carp\*(C'\fR will never
report an error as being from a line in a package that is internal to
Perl.  For example:
.PP
.Vb 3
\&    $Carp::Internal{ _\|_PACKAGE_\|_ }++;
\&    # time passes...
\&    sub foo { ... or confess("whatever") };
.Ve
.PP
would give a full stack backtrace starting from the first caller
outside of _\|_PACKAGE_\|_.  (Unless that package was also internal to
Perl.)
.ie n .Sh "%Carp::CarpInternal"
.el .Sh "\f(CW%Carp::CarpInternal\fP"
.IX Subsection "%Carp::CarpInternal"
This says which packages are internal to Perl's warning system.  For
generating a full stack backtrace this is the same as being internal
to Perl, the stack backtrace will not start inside packages that are
listed in \f(CW%Carp::CarpInternal\fR.  But it is slightly different for
the summary message generated by \f(CW\*(C`carp\*(C'\fR or \f(CW\*(C`croak\*(C'\fR.  There errors
will not be reported on any lines that are calling packages in
\&\f(CW%Carp::CarpInternal\fR.
.PP
For example \f(CW\*(C`Carp\*(C'\fR itself is listed in \f(CW%Carp::CarpInternal\fR.
Therefore the full stack backtrace from \f(CW\*(C`confess\*(C'\fR will not start
inside of \f(CW\*(C`Carp\*(C'\fR, and the short message from calling \f(CW\*(C`croak\*(C'\fR is
not placed on the line where \f(CW\*(C`croak\*(C'\fR was called.
.ie n .Sh "$Carp::CarpLevel"
.el .Sh "\f(CW$Carp::CarpLevel\fP"
.IX Subsection "$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 \f(CW\*(C`Carp\*(C'\fR'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 \f(CW\*(C`Carp\*(C'\fR 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.
.PP
Therefore it is best to avoid \f(CW$Carp::CarpLevel\fR.  Instead use
\&\f(CW@CARP_NOT\fR, \f(CW%Carp::Internal\fR and \f(CW%Carp::CarpInternal\fR>.
.PP
Defaults to \f(CW0\fR.
.SH "BUGS"
.IX Header "BUGS"
The Carp routines don't handle exception objects currently.
If called with a first argument that is a reference, they simply
call \fIdie()\fR or \fIwarn()\fR, as appropriate.