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                <h1>Carp</h1>
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            <h1>Carp</h1>


  <!--    -->
<ul><li><a href="#NAME">NAME</a><li><a href="#SYNOPSIS">SYNOPSIS</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Forcing-a-Stack-Trace">Forcing a Stack Trace</a><li><a href="#Stack-Trace-formatting">Stack Trace formatting</a></ul><li><a href="#GLOBAL-VARIABLES">GLOBAL VARIABLES</a><ul><li><a href="#%24Carp%3a%3aMaxEvalLen">$Carp::MaxEvalLen</a><li><a href="#%24Carp%3a%3aMaxArgLen">$Carp::MaxArgLen</a><li><a href="#%24Carp%3a%3aMaxArgNums">$Carp::MaxArgNums</a><li><a href="#%24Carp%3a%3aVerbose">$Carp::Verbose</a><li><a href="#%24Carp%3a%3aRefArgFormatter">$Carp::RefArgFormatter</a><li><a href="#%40CARP_NOT">@CARP_NOT</a><li><a href="#%25Carp%3a%3aInternal">%Carp::Internal</a><li><a href="#%25Carp%3a%3aCarpInternal">%Carp::CarpInternal</a><li><a href="#%24Carp%3a%3aCarpLevel">$Carp::CarpLevel</a></ul><li><a href="#BUGS">BUGS</a><li><a href="#SEE-ALSO">SEE ALSO</a><li><a href="#CONTRIBUTING">CONTRIBUTING</a><li><a href="#AUTHOR">AUTHOR</a><li><a href="#COPYRIGHT">COPYRIGHT</a><li><a href="#LICENSE">LICENSE</a></ul><a name="NAME"></a><h1>NAME</h1>
<p>Carp - alternative warn and die for modules</p>
<a name="SYNOPSIS"></a><h1>SYNOPSIS</h1>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">Carp</span><span class="sc">;</span></li><li></li><li>    <span class="c"># warn user (from perspective of caller)</span></li><li>    <span class="w">carp</span> <span class="q">&quot;string trimmed to 80 chars&quot;</span><span class="sc">;</span></li><li></li><li>    <span class="c"># die of errors (from perspective of caller)</span></li><li>    <span class="w">croak</span> <span class="q">&quot;We&#39;re outta here!&quot;</span><span class="sc">;</span></li><li></li><li>    <span class="c"># die of errors with stack backtrace</span></li><li>    <span class="w">confess</span> <span class="q">&quot;not implemented&quot;</span><span class="sc">;</span></li><li></li><li>    <span class="c"># cluck, longmess and shortmess not exported by default</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">Carp</span> <span class="q">qw(cluck longmess shortmess)</span><span class="sc">;</span></li><li>    <span class="w">cluck</span> <span class="q">&quot;This is how we got here!&quot;</span><span class="sc">;</span> <span class="c"># warn with stack backtrace</span></li><li>    <span class="i">$long_message</span>   = <span class="i">longmess</span><span class="s">(</span> <span class="q">&quot;message from cluck() or confess()&quot;</span> <span class="s">)</span><span class="sc">;</span></li><li>    <span class="i">$short_message</span>  = <span class="i">shortmess</span><span class="s">(</span> <span class="q">&quot;message from carp() or croak()&quot;</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>The Carp routines are useful in your own modules because
they act like <code class="inline"><a class="l_k" href="functions/die.html">die()</a></code> or <code class="inline"><a class="l_k" href="functions/warn.html">warn()</a></code>, but with a message which is more
likely to be useful to a user of your module.  In the case of
<code class="inline"><span class="i">cluck</span><span class="s">(</span><span class="s">)</span></code>
 and <code class="inline"><span class="i">confess</span><span class="s">(</span><span class="s">)</span></code>
, that context is a summary of every
call in the call-stack; <code class="inline"><span class="i">longmess</span><span class="s">(</span><span class="s">)</span></code>
 returns the contents of the error
message.</p>
<p>For a shorter message you can use <code class="inline"><span class="i">carp</span><span class="s">(</span><span class="s">)</span></code>
 or <code class="inline"><span class="i">croak</span><span class="s">(</span><span class="s">)</span></code>
 which report the
error as being from where your module was called.  <code class="inline"><span class="i">shortmess</span><span class="s">(</span><span class="s">)</span></code>
 returns the
contents of this error message.  There is no guarantee that that is where the
error was, but it is a good educated guess.</p>
<p><code class="inline"><span class="w">Carp</span></code>
 takes care not to clobber the status variables <code class="inline"><span class="i">$!</span></code>
 and <code class="inline"><span class="i">$^E</span></code>

in the course of assembling its error messages.  This means that a
<code class="inline"><span class="i">$SIG</span>{<span class="w">__DIE__</span>}</code>
 or <code class="inline"><span class="i">$SIG</span>{<span class="w">__WARN__</span>}</code>
 handler can capture the error
information held in those variables, if it is required to augment the
error message, and if the code calling <code class="inline"><span class="w">Carp</span></code>
 left useful values there.
Of course, <code class="inline"><span class="w">Carp</span></code>
 can't guarantee the latter.</p>
<p>You can also alter the way the output and logic of <code class="inline"><span class="w">Carp</span></code>
 works, by
changing some global variables in the <code class="inline"><span class="w">Carp</span></code>
 namespace. See the
section on <code class="inline"><span class="w">GLOBAL</span> <span class="w">VARIABLES</span></code>
 below.</p>
<p>Here is a more complete description of how <code class="inline"><span class="w">carp</span></code>
 and <code class="inline"><span class="w">croak</span></code>
 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:</p>
<dl>
<dt>1.</dt><dd>
<p>Any call from a package to itself is safe.</p>
</dd>
<dt>2.</dt><dd>
<p>Packages claim that there won't be errors on calls to or from
packages explicitly marked as safe by inclusion in <code class="inline"><span class="i">@CARP_NOT</span></code>
, or
(if that array is empty) <code class="inline"><span class="i">@ISA</span></code>
.  The ability to override what
@ISA says is new in 5.8.</p>
</dd>
<dt>3.</dt><dd>
<p>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 <code class="inline"><span class="i">@ISA</span></code>

with <code class="inline"><span class="i">@CARP_NOT</span></code>
, then this trust relationship is identical to,
"inherits from".</p>
</dd>
<dt>4.</dt><dd>
<p>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.)</p>
</dd>
<dt>5.</dt><dd>
<p>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 <code class="inline"><span class="w">carp</span></code>
 or <code class="inline"><span class="w">croak</span></code>
.)</p>
</dd>
<dt>6.</dt><dd>
<p><code class="inline"><span class="i">$Carp::CarpLevel</span></code>
 can be set to skip a fixed number of additional
call levels.  Using this is not recommended because it is very
difficult to get it to behave correctly.</p>
</dd>
</dl>
<a name="Forcing-a-Stack-Trace"></a><h2>Forcing a Stack Trace</h2>
<p>As a debugging aid, you can force Carp to treat a croak as a confess
and a carp as a cluck across <i>all</i> modules. In other words, force a
detailed stack trace to be given.  This can be very helpful when trying
to understand why, or from where, a warning or error is being generated.</p>
<p>This feature is enabled by 'importing' the non-existent symbol
'verbose'. You would typically enable it by saying</p>
<pre class="verbatim"><ol><li>    <span class="w">perl</span> -<span class="w">MCarp</span>=<span class="w">verbose</span> <span class="w">script</span>.<span class="w">pl</span></li></ol></pre><p>or by including the string <code class="inline">-<span class="w">MCarp</span>=<span class="w">verbose</span></code>
 in the PERL5OPT
environment variable.</p>
<p>Alternately, you can set the global variable <code class="inline"><span class="i">$Carp::Verbose</span></code>
 to true.
See the <code class="inline"><span class="w">GLOBAL</span> <span class="w">VARIABLES</span></code>
 section below.</p>
<a name="Stack-Trace-formatting"></a><h2>Stack Trace formatting</h2>
<p>At each stack level, the subroutine's name is displayed along with
its parameters.  For simple scalars, this is sufficient.  For complex
data types, such as objects and other references, this can simply
display <code class="inline"><span class="q">&#39;HASH(0x1ab36d8)&#39;</span></code>
.</p>
<p>Carp gives two ways to control this.</p>
<dl>
<dt>1.</dt><dd>
<p>For objects, a method, <code class="inline"><span class="w">CARP_TRACE</span></code>
, will be called, if it exists.  If
this method doesn't exist, or it recurses into <code class="inline"><span class="w">Carp</span></code>
, or it otherwise
throws an exception, this is skipped, and Carp moves on to the next option,
otherwise checking stops and the string returned is used.  It is recommended
that the object's type is part of the string to make debugging easier.</p>
</dd>
<dt>2.</dt><dd>
<p>For any type of reference, <code class="inline"><span class="i">$Carp::RefArgFormatter</span></code>
 is checked (see below).
This variable is expected to be a code reference, and the current parameter
is passed in.  If this function doesn't exist (the variable is undef), or
it recurses into <code class="inline"><span class="w">Carp</span></code>
, or it otherwise throws an exception, this is
skipped, and Carp moves on to the next option, otherwise checking stops
and the string returned is used.</p>
</dd>
<dt>3.</dt><dd>
<p>Otherwise, if neither <code class="inline"><span class="w">CARP_TRACE</span></code>
 nor <code class="inline"><span class="i">$Carp::RefArgFormatter</span></code>
 is
available, stringify the value ignoring any overloading.</p>
</dd>
</dl>
<a name="GLOBAL-VARIABLES"></a><h1>GLOBAL VARIABLES</h1>
<a name="%24Carp%3a%3aMaxEvalLen"></a><h2>$Carp::MaxEvalLen</h2>
<p>This variable determines how many characters of a string-eval are to
be shown in the output. Use a value of <code class="inline"><span class="n">0</span></code>
 to show all text.</p>
<p>Defaults to <code class="inline"><span class="n">0</span></code>
.</p>
<a name="%24Carp%3a%3aMaxArgLen"></a><h2>$Carp::MaxArgLen</h2>
<p>This variable determines how many characters of each argument to a
function to print. Use a value of <code class="inline"><span class="n">0</span></code>
 to show the full length of the
argument.</p>
<p>Defaults to <code class="inline"><span class="n">64</span></code>
.</p>
<a name="%24Carp%3a%3aMaxArgNums"></a><h2>$Carp::MaxArgNums</h2>
<p>This variable determines how many arguments to each function to show.
Use a false value to show all arguments to a function call.  To suppress all
arguments, use <code class="inline"><span class="n">-1</span></code>
 or <code class="inline"><span class="q">&#39;0 but true&#39;</span></code>
.</p>
<p>Defaults to <code class="inline"><span class="n">8</span></code>
.</p>
<a name="%24Carp%3a%3aVerbose"></a><h2>$Carp::Verbose</h2>
<p>This variable makes <code class="inline"><span class="i">carp</span><span class="s">(</span><span class="s">)</span></code>
 and <code class="inline"><span class="i">croak</span><span class="s">(</span><span class="s">)</span></code>
 generate stack backtraces
just like <code class="inline"><span class="i">cluck</span><span class="s">(</span><span class="s">)</span></code>
 and <code class="inline"><span class="i">confess</span><span class="s">(</span><span class="s">)</span></code>
.  This is how <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">Carp</span> <span class="q">&#39;verbose&#39;</span></code>

is implemented internally.</p>
<p>Defaults to <code class="inline"><span class="n">0</span></code>
.</p>
<a name="%24Carp%3a%3aRefArgFormatter"></a><h2>$Carp::RefArgFormatter</h2>
<p>This variable sets a general argument formatter to display references.
Plain scalars and objects that implement <code class="inline"><span class="w">CARP_TRACE</span></code>
 will not go through
this formatter.  Calling <code class="inline"><span class="w">Carp</span></code>
 from within this function is not supported.</p>
<p>local $Carp::RefArgFormatter = sub {
    require Data::Dumper;
    Data::Dumper::Dump($_[0]); # not necessarily safe
};</p>
<a name="%40CARP_NOT"></a><h2>@CARP_NOT</h2>
<p>This variable, <i>in your package</i>, says which packages are <i>not</i> to be
considered as the location of an error. The <code class="inline"><span class="i">carp</span><span class="s">(</span><span class="s">)</span></code>
 and <code class="inline"><span class="i">cluck</span><span class="s">(</span><span class="s">)</span></code>

functions will skip over callers when reporting where an error occurred.</p>
<p>NB: This variable must be in the package's symbol table, thus:</p>
<pre class="verbatim"><ol><li>    <span class="c"># These work</span></li><li>    <a class="l_k" href="functions/our.html">our</a> <span class="i">@CARP_NOT</span><span class="sc">;</span> <span class="c"># file scope</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">vars</span> <span class="q">qw(@CARP_NOT)</span><span class="sc">;</span> <span class="c"># package scope</span></li><li>    <span class="i">@My::Package::CARP_NOT</span> = ... <span class="sc">;</span> <span class="c"># explicit package variable</span></li><li></li><li>    <span class="c"># These don&#39;t work</span></li><li><a name="xyz"></a>    sub <span class="m">xyz</span> <span class="s">{</span> ... <span class="i">@CARP_NOT</span> = ... <span class="s">}</span> <span class="c"># w/o declarations above</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@CARP_NOT</span><span class="sc">;</span> <span class="c"># even at top-level</span></li></ol></pre><p>Example of use:</p>
<pre class="verbatim"><ol><li>    package My::Carping::Package;</li><li>    use Carp;</li><li>    our @CARP_NOT;</li><li>    sub bar     { .... or _error('Wrong input') }</li><li>    sub _error  {</li><li>        # temporary control of where'ness, __PACKAGE__ is implicit</li><li>        local @CARP_NOT = qw(My::Friendly::Caller);</li><li>        carp(@_)</li><li>    }</li></ol></pre><p>This would make <code class="inline"><span class="w">Carp</span></code>
 report the error as coming from a caller not
in <code class="inline"><span class="w">My::Carping::Package</span></code>
, nor from <code class="inline"><span class="w">My::Friendly::Caller</span></code>
.</p>
<p>Also read the <a href="#DESCRIPTION">DESCRIPTION</a> section above, about how <code class="inline"><span class="w">Carp</span></code>
 decides
where the error is reported from.</p>
<p>Use <code class="inline"><span class="i">@CARP_NOT</span></code>
, instead of <code class="inline"><span class="i">$Carp::CarpLevel</span></code>
.</p>
<p>Overrides <code class="inline"><span class="w">Carp</span></code>
's use of <code class="inline"><span class="i">@ISA</span></code>
.</p>
<a name="%25Carp%3a%3aInternal"></a><h2>%Carp::Internal</h2>
<p>This says what packages are internal to Perl.  <code class="inline"><span class="w">Carp</span></code>
 will never
report an error as being from a line in a package that is internal to
Perl.  For example:</p>
<pre class="verbatim"><ol><li>    <span class="i">$Carp::Internal</span>{ <span class="s">(</span><span class="w">__PACKAGE__</span><span class="s">)</span> }++<span class="sc">;</span></li><li>    <span class="c"># time passes...</span></li><li><a name="foo"></a>    sub <span class="m">foo</span> <span class="s">{</span> ... or <span class="i">confess</span><span class="s">(</span><span class="q">&quot;whatever&quot;</span><span class="s">)</span> <span class="s">}</span><span class="sc">;</span></li></ol></pre><p>would give a full stack backtrace starting from the first caller
outside of __PACKAGE__.  (Unless that package was also internal to
Perl.)</p>
<a name="%25Carp%3a%3aCarpInternal"></a><h2>%Carp::CarpInternal</h2>
<p>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 <code class="inline"><span class="i">%Carp::CarpInternal</span></code>
.  But it is slightly different for
the summary message generated by <code class="inline"><span class="w">carp</span></code>
 or <code class="inline"><span class="w">croak</span></code>
.  There errors
will not be reported on any lines that are calling packages in
<code class="inline"><span class="i">%Carp::CarpInternal</span></code>
.</p>
<p>For example <code class="inline"><span class="w">Carp</span></code>
 itself is listed in <code class="inline"><span class="i">%Carp::CarpInternal</span></code>
.
Therefore the full stack backtrace from <code class="inline"><span class="w">confess</span></code>
 will not start
inside of <code class="inline"><span class="w">Carp</span></code>
, and the short message from calling <code class="inline"><span class="w">croak</span></code>
 is
not placed on the line where <code class="inline"><span class="w">croak</span></code>
 was called.</p>
<a name="%24Carp%3a%3aCarpLevel"></a><h2>$Carp::CarpLevel</h2>
<p>This variable determines how many additional call frames are to be
skipped that would not otherwise be when reporting where an error
occurred on a call to one of <code class="inline"><span class="w">Carp</span></code>
's functions.  It is fairly easy
to count these call frames on calls that generate a full stack
backtrace.  However it is much harder to do this accounting for calls
that generate a short message.  Usually people skip too many call
frames.  If they are lucky they skip enough that <code class="inline"><span class="w">Carp</span></code>
 goes all of
the way through the call stack, realizes that something is wrong, and
then generates a full stack backtrace.  If they are unlucky then the
error is reported from somewhere misleading very high in the call
stack.</p>
<p>Therefore it is best to avoid <code class="inline"><span class="i">$Carp::CarpLevel</span></code>
.  Instead use
<code class="inline"><span class="i">@CARP_NOT</span></code>
, <code class="inline"><span class="i">%Carp::Internal</span></code>
 and <code class="inline"><span class="i">%Carp::CarpInternal</span></code>
.</p>
<p>Defaults to <code class="inline"><span class="n">0</span></code>
.</p>
<a name="BUGS"></a><h1>BUGS</h1>
<p>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.</p>
<a name="SEE-ALSO"></a><h1>SEE ALSO</h1>
<p><a href="http://search.cpan.org/perldoc/Carp::Always">Carp::Always</a>,
<a href="http://search.cpan.org/perldoc/Carp::Clan">Carp::Clan</a></p>
<a name="CONTRIBUTING"></a><h1>CONTRIBUTING</h1>
<p><a href="Carp.html">Carp</a> is maintained by the perl 5 porters as part of the core perl 5
version control repository. Please see the <a href="perlhack.html">perlhack</a> perldoc for how to
submit patches and contribute to it.</p>
<a name="AUTHOR"></a><h1>AUTHOR</h1>
<p>The Carp module first appeared in Larry Wall's perl 5.000 distribution.
Since then it has been modified by several of the perl 5 porters.
Andrew Main (Zefram) &lt;zefram@fysh.org&gt; divested Carp into an independent
distribution.</p>
<a name="COPYRIGHT"></a><h1>COPYRIGHT</h1>
<p>Copyright (C) 1994-2013 Larry Wall</p>
<p>Copyright (C) 2011, 2012, 2013 Andrew Main (Zefram) &lt;zefram@fysh.org&gt;</p>
<a name="LICENSE"></a><h1>LICENSE</h1>
<p>This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.</p>




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