1Carp(3) User Contributed Perl Documentation Carp(3)
2
6 Carp - alternative warn and die for modules
7
9 use Carp;
10 # warn user (from perspective of caller)
12 carp "string trimmed to 80 chars";
13 # die of errors (from perspective of caller)
15 croak "We're outta here!";
16 # die of errors with stack backtrace
18 confess "not implemented";
19 # cluck, longmess and shortmess not exported by default
21 use Carp qw(cluck longmess shortmess);
22 cluck "This is how we got here!"; # warn with stack backtrace
23 $long_message = longmess( "message from cluck() or confess()" );
24 $short_message = shortmess( "message from carp() or croak()" );
25
27 The Carp routines are useful in your own modules because they act like
28 die() or warn(), but with a message which is more likely to be useful
29 to a user of your module. In the case of cluck() and confess(), that
30 context is a summary of every call in the call-stack; longmess()
31 returns the contents of the error message.
32 For a shorter message you can use carp() or croak() which report the
34 error as being from where your module was called. shortmess() returns
35 the contents of this error message. There is no guarantee that that is
36 where the error was, but it is a good educated guess.
37 "Carp" takes care not to clobber the status variables $! and $^E in the
39 course of assembling its error messages. This means that a
40 $SIG{__DIE__} or $SIG{__WARN__} handler can capture the error
41 information held in those variables, if it is required to augment the
42 error message, and if the code calling "Carp" left useful values there.
43 Of course, "Carp" can't guarantee the latter.
44 You can also alter the way the output and logic of "Carp" works, by
46 changing some global variables in the "Carp" namespace. See the section
47 on "GLOBAL VARIABLES" below.
48 Here is a more complete description of how "carp" and "croak" work.
50 What they do is search the call-stack for a function call stack where
51 they have not been told that there shouldn't be an error. If every
52 call is marked safe, they give up and give a full stack backtrace
53 instead. In other words they presume that the first likely looking
54 potential suspect is guilty. Their rules for telling whether a call
55 shouldn't generate errors work as follows:
56 1. Any call from a package to itself is safe.
58 2. Packages claim that there won't be errors on calls to or from
60 packages explicitly marked as safe by inclusion in @CARP_NOT, or
61 (if that array is empty) @ISA. The ability to override what @ISA
62 says is new in 5.8.
63 3. The trust in item 2 is transitive. If A trusts B, and B trusts C,
65 then A trusts C. So if you do not override @ISA with @CARP_NOT,
66 then this trust relationship is identical to, "inherits from".
67 4. Any call from an internal Perl module is safe. (Nothing keeps user
69 modules from marking themselves as internal to Perl, but this
70 practice is discouraged.)
71 5. Any call to Perl's warning system (eg Carp itself) is safe. (This
73 rule is what keeps it from reporting the error at the point where
74 you call "carp" or "croak".)
75 6. $Carp::CarpLevel can be set to skip a fixed number of additional
77 call levels. Using this is not recommended because it is very
78 difficult to get it to behave correctly.
79 Forcing a Stack Trace
81 As a debugging aid, you can force Carp to treat a croak as a confess
82 and a carp as a cluck across all modules. In other words, force a
83 detailed stack trace to be given. This can be very helpful when trying
84 to understand why, or from where, a warning or error is being
85 generated.
86 This feature is enabled by 'importing' the non-existent symbol
88 'verbose'. You would typically enable it by saying
89 perl -MCarp=verbose script.pl
91 or by including the string "-MCarp=verbose" in the PERL5OPT environment
93 variable.
94 Alternately, you can set the global variable $Carp::Verbose to true.
96 See the "GLOBAL VARIABLES" section below.
97 Stack Trace formatting
99 At each stack level, the subroutine's name is displayed along with its
100 parameters. For simple scalars, this is sufficient. For complex data
101 types, such as objects and other references, this can simply display
102 'HASH(0x1ab36d8)'.
103 Carp gives two ways to control this.
105 1. For objects, a method, "CARP_TRACE", will be called, if it exists.
107 If this method doesn't exist, or it recurses into "Carp", or it
108 otherwise throws an exception, this is skipped, and Carp moves on
109 to the next option, otherwise checking stops and the string
110 returned is used. It is recommended that the object's type is part
111 of the string to make debugging easier.
112 2. For any type of reference, $Carp::RefArgFormatter is checked (see
114 below). This variable is expected to be a code reference, and the
115 current parameter is passed in. If this function doesn't exist
116 (the variable is undef), or it recurses into "Carp", or it
117 otherwise throws an exception, this is skipped, and Carp moves on
118 to the next option, otherwise checking stops and the string
119 returned is used.
120 3. Otherwise, if neither "CARP_TRACE" nor $Carp::RefArgFormatter is
122 available, stringify the value ignoring any overloading.
123
125 $Carp::MaxEvalLen
126 This variable determines how many characters of a string-eval are to be
127 shown in the output. Use a value of 0 to show all text.
128 Defaults to 0.
130 $Carp::MaxArgLen
132 This variable determines how many characters of each argument to a
133 function to print. Use a value of 0 to show the full length of the
134 argument.
135 Defaults to 64.
137 $Carp::MaxArgNums
139 This variable determines how many arguments to each function to show.
140 Use a false value to show all arguments to a function call. To
141 suppress all arguments, use -1 or '0 but true'.
142 Defaults to 8.
144 $Carp::Verbose
146 This variable makes carp() and croak() generate stack backtraces just
147 like cluck() and confess(). This is how "use Carp 'verbose'" is
148 implemented internally.
149 Defaults to 0.
151 $Carp::RefArgFormatter
153 This variable sets a general argument formatter to display references.
154 Plain scalars and objects that implement "CARP_TRACE" will not go
155 through this formatter. Calling "Carp" from within this function is
156 not supported.
157 local $Carp::RefArgFormatter = sub {
159 require Data::Dumper;
160 Data::Dumper->Dump($_[0]); # not necessarily safe
161 };
162 @CARP_NOT
164 This variable, in your package, says which packages are not to be
165 considered as the location of an error. The carp() and cluck()
166 functions will skip over callers when reporting where an error
167 occurred.
168 NB: This variable must be in the package's symbol table, thus:
170 # These work
172 our @CARP_NOT; # file scope
173 use vars qw(@CARP_NOT); # package scope
174 @My::Package::CARP_NOT = ... ; # explicit package variable
175 # These don't work
177 sub xyz { ... @CARP_NOT = ... } # w/o declarations above
178 my @CARP_NOT; # even at top-level
179 Example of use:
181 package My::Carping::Package;
183 use Carp;
184 our @CARP_NOT;
185 sub bar { .... or _error('Wrong input') }
186 sub _error {
187 # temporary control of where'ness, __PACKAGE__ is implicit
188 local @CARP_NOT = qw(My::Friendly::Caller);
189 carp(@_)
190 }
191 This would make "Carp" report the error as coming from a caller not in
193 "My::Carping::Package", nor from "My::Friendly::Caller".
194 Also read the "DESCRIPTION" section above, about how "Carp" decides
196 where the error is reported from.
197 Use @CARP_NOT, instead of $Carp::CarpLevel.
199 Overrides "Carp"'s use of @ISA.
201 %Carp::Internal
203 This says what packages are internal to Perl. "Carp" will never report
204 an error as being from a line in a package that is internal to Perl.
205 For example:
206 $Carp::Internal{ (__PACKAGE__) }++;
208 # time passes...
209 sub foo { ... or confess("whatever") };
210 would give a full stack backtrace starting from the first caller
212 outside of __PACKAGE__. (Unless that package was also internal to
213 Perl.)
214 %Carp::CarpInternal
216 This says which packages are internal to Perl's warning system. For
217 generating a full stack backtrace this is the same as being internal to
218 Perl, the stack backtrace will not start inside packages that are
219 listed in %Carp::CarpInternal. But it is slightly different for the
220 summary message generated by "carp" or "croak". There errors will not
221 be reported on any lines that are calling packages in
222 %Carp::CarpInternal.
223 For example "Carp" itself is listed in %Carp::CarpInternal. Therefore
225 the full stack backtrace from "confess" will not start inside of
226 "Carp", and the short message from calling "croak" is not placed on the
227 line where "croak" was called.
228 $Carp::CarpLevel
230 This variable determines how many additional call frames are to be
231 skipped that would not otherwise be when reporting where an error
232 occurred on a call to one of "Carp"'s functions. It is fairly easy to
233 count these call frames on calls that generate a full stack backtrace.
234 However it is much harder to do this accounting for calls that generate
235 a short message. Usually people skip too many call frames. If they
236 are lucky they skip enough that "Carp" goes all of the way through the
237 call stack, realizes that something is wrong, and then generates a full
238 stack backtrace. If they are unlucky then the error is reported from
239 somewhere misleading very high in the call stack.
240 Therefore it is best to avoid $Carp::CarpLevel. Instead use @CARP_NOT,
242 %Carp::Internal and %Carp::CarpInternal.
243 Defaults to 0.
245
247 The Carp routines don't handle exception objects currently. If called
248 with a first argument that is a reference, they simply call die() or
249 warn(), as appropriate.
250
252 Carp::Always, Carp::Clan
253
255 Carp is maintained by the perl 5 porters as part of the core perl 5
256 version control repository. Please see the perlhack perldoc for how to
257 submit patches and contribute to it.
258
260 The Carp module first appeared in Larry Wall's perl 5.000 distribution.
261 Since then it has been modified by several of the perl 5 porters.
262 Andrew Main (Zefram) <zefram@fysh.org> divested Carp into an
263 independent distribution.
264
266 Copyright (C) 1994-2013 Larry Wall
267 Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) <zefram@fysh.org>
269
271 This module is free software; you can redistribute it and/or modify it
272 under the same terms as Perl itself.
273perl v5.36.0 2023-01-20 Carp(3)