Carp(3p) Perl Programmers Reference Guide Carp(3p)NAMEcarp - warn of errors (from perspective of caller)
cluck - warn of errors with stack backtrace
(not exported by default)
croak - die of errors (from perspective of caller)
confess - die of errors with stack backtrace
shortmess - return the message that carp and croak produce
longmess - return the message that cluck and confess produce
SYNOPSIS
use Carp;
croak "We're outta here!";
use Carp qw(cluck);
cluck "This is how we got here!";
print FH Carp::shortmess("This will have caller's details added");
print FH Carp::longmess("This will have stack backtrace added");
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 sum-
mary of every call in the call-stack. For a shorter message
you can use carp, croak or shortmess 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 shortmess works.
What it does is search the call-stack for a function call
stack where it hasn't been told that there shouldn't be an
error. If every call is marked safe, it then gives up and
gives a full stack backtrace instead. In other words it
presumes that the first likely looking potential suspect is
guilty. Its rules for telling whether a call shouldn't gen-
erate 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
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@CARP_NOT, or (if that array is empty) @ISA. The abil-
ity 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 Carp is safe. (This rule is what keeps it
from reporting the error where you call
carp/croak/shortmess.)
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 sym-
bol '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::CarpLevel
This variable determines how many call frames are to be
skipped when reporting where an error occurred on a call to
one of "Carp"'s functions. For example:
$Carp::CarpLevel = 1;
sub bar { .... or _error('Wrong input') }
sub _error { Carp::carp(@_) }
This would make Carp report the error as coming from "bar"'s
caller, rather than from "_error"'s caller, as it normally
would.
Defaults to 0.
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$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 argu-
ment 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 func-
tion call.
Defaults to 8.
$Carp::Verbose
This variable makes "Carp" use the "longmess" function at
all times. This effectively means that all calls to "carp"
become "cluck" and all calls to "croak" become "confess".
Note, this is analogous to using "use Carp 'verbose'".
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.
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