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
29 useful to a user of your module. In the case of "cluck()" and
30 "confess()", that context is a summary of every call in the call-stack;
31 "longmess()" returns the contents of the error message.
32 For a shorter message you can use "carp()" or "croak()" which report
34 the error as being from where your module was called. "shortmess()"
35 returns the contents of this error message. There is no guarantee that
36 that is 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
147 just like "cluck()" and "confess()". This is how "use Carp 'verbose'"
148 is 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 @CARP_NOT
163 This variable, in your package, says which packages are not to be
164 considered as the location of an error. The "carp()" and "cluck()"
165 functions will skip over callers when reporting where an error
166 occurred.
167 NB: This variable must be in the package's symbol table, thus:
169 # These work
171 our @CARP_NOT; # file scope
172 use vars qw(@CARP_NOT); # package scope
173 @My::Package::CARP_NOT = ... ; # explicit package variable
174 # These don't work
176 sub xyz { ... @CARP_NOT = ... } # w/o declarations above
177 my @CARP_NOT; # even at top-level
178 Example of use:
180 package My::Carping::Package;
182 use Carp;
183 our @CARP_NOT;
184 sub bar { .... or _error('Wrong input') }
185 sub _error {
186 # temporary control of where'ness, __PACKAGE__ is implicit
187 local @CARP_NOT = qw(My::Friendly::Caller);
188 carp(@_)
189 }
190 This would make "Carp" report the error as coming from a caller not in
192 "My::Carping::Package", nor from "My::Friendly::Caller".
193 Also read the "DESCRIPTION" section above, about how "Carp" decides
195 where the error is reported from.
196 Use @CARP_NOT, instead of $Carp::CarpLevel.
198 Overrides "Carp"'s use of @ISA.
200 %Carp::Internal
202 This says what packages are internal to Perl. "Carp" will never report
203 an error as being from a line in a package that is internal to Perl.
204 For example:
205 $Carp::Internal{ (__PACKAGE__) }++;
207 # time passes...
208 sub foo { ... or confess("whatever") };
209 would give a full stack backtrace starting from the first caller
211 outside of __PACKAGE__. (Unless that package was also internal to
212 Perl.)
213 %Carp::CarpInternal
215 This says which packages are internal to Perl's warning system. For
216 generating a full stack backtrace this is the same as being internal to
217 Perl, the stack backtrace will not start inside packages that are
218 listed in %Carp::CarpInternal. But it is slightly different for the
219 summary message generated by "carp" or "croak". There errors will not
220 be reported on any lines that are calling packages in
221 %Carp::CarpInternal.
222 For example "Carp" itself is listed in %Carp::CarpInternal. Therefore
224 the full stack backtrace from "confess" will not start inside of
225 "Carp", and the short message from calling "croak" is not placed on the
226 line where "croak" was called.
227 $Carp::CarpLevel
229 This variable determines how many additional call frames are to be
230 skipped that would not otherwise be when reporting where an error
231 occurred on a call to one of "Carp"'s functions. It is fairly easy to
232 count these call frames on calls that generate a full stack backtrace.
233 However it is much harder to do this accounting for calls that generate
234 a short message. Usually people skip too many call frames. If they
235 are lucky they skip enough that "Carp" goes all of the way through the
236 call stack, realizes that something is wrong, and then generates a full
237 stack backtrace. If they are unlucky then the error is reported from
238 somewhere misleading very high in the call stack.
239 Therefore it is best to avoid $Carp::CarpLevel. Instead use @CARP_NOT,
241 %Carp::Internal and %Carp::CarpInternal.
242 Defaults to 0.
244
246 The Carp routines don't handle exception objects currently. If called
247 with a first argument that is a reference, they simply call die() or
248 warn(), as appropriate.
249
251 Carp::Always, Carp::Clan
252
254 Carp is maintained by the perl 5 porters as part of the core perl 5
255 version control repository. Please see the perlhack perldoc for how to
256 submit patches and contribute to it.
257
259 The Carp module first appeared in Larry Wall's perl 5.000 distribution.
260 Since then it has been modified by several of the perl 5 porters.
261 Andrew Main (Zefram) <zefram@fysh.org> divested Carp into an
262 independent distribution.
263
265 Copyright (C) 1994-2013 Larry Wall
266 Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) <zefram@fysh.org>
268
270 This module is free software; you can redistribute it and/or modify it
271 under the same terms as Perl itself.
272perl v5.30.0 2019-07-26 Carp(3)