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<h1>require</h1>
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require
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<h1>require</h1>
<div class="mod_az_list">
<a href="../index-functions.html">Perl functions A-Z</a> |
<a href="../index-functions-by-cat.html">Perl functions by category</a> |
<a href="../perlfunc.html">The 'perlfunc' manpage</a>
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<!-- -->
<ul>
<li><a name="require-VERSION"></a><b>require VERSION</b>
<p></p>
</li>
</ul>
<ul>
<li><a name="require-EXPR"></a><b>require EXPR</b>
</li>
<li><a name="require"></a><b>require</b>
<p>Demands a version of Perl specified by VERSION, or demands some semantics
specified by EXPR or by <code class="inline"><span class="i">$_</span></code>
if EXPR is not supplied.</p>
<p>VERSION may be either a numeric argument such as 5.006, which will be
compared to <code class="inline"><span class="i">$]</span></code>
, or a literal of the form v5.6.1, which will be compared
to <code class="inline"><span class="i">$^V</span></code>
(aka $PERL_VERSION). A fatal error is produced at run time if
VERSION is greater than the version of the current Perl interpreter.
Compare with <a href="../functions/use.html">"use"</a>, which can do a similar check at compile time.</p>
<p>Specifying VERSION as a literal of the form v5.6.1 should generally be
avoided, because it leads to misleading error messages under earlier
versions of Perl that do not support this syntax. The equivalent numeric
version should be used instead.</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/require.html">require</a> <span class="v">v5.6.1</span><span class="sc">;</span> <span class="c"># run time version check</span></li><li> <a class="l_k" href="../functions/require.html">require</a> <span class="v">5.6.1</span><span class="sc">;</span> <span class="c"># ditto</span></li><li> <a class="l_k" href="../functions/require.html">require</a> <span class="n">5.006_001</span><span class="sc">;</span> <span class="c"># ditto; preferred for backwards compatibility</span></li></ol></pre><p>Otherwise, <code class="inline"><a class="l_k" href="../functions/require.html">require</a></code> demands that a library file be included if it
hasn't already been included. The file is included via the do-FILE
mechanism, which is essentially just a variety of <code class="inline"><a class="l_k" href="../functions/eval.html">eval</a></code> with the
caveat that lexical variables in the invoking script will be invisible
to the included code. Has semantics similar to the following subroutine:</p>
<pre class="verbatim"><ol><li><a name="require"></a> sub <span class="m">require</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$filename</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li> if <span class="s">(</span><a class="l_k" href="../functions/exists.html">exists</a> <span class="i">$INC</span>{<span class="i">$filename</span>}<span class="s">)</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/return.html">return</a> <span class="n">1</span> if <span class="i">$INC</span>{<span class="i">$filename</span>}<span class="sc">;</span></li><li> <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Compilation failed in require"</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$realfilename</span><span class="cm">,</span><span class="i">$result</span><span class="s">)</span><span class="sc">;</span></li><li> <span class="j">ITER:</span> <span class="s">{</span></li><li> foreach <span class="i">$prefix</span> <span class="s">(</span><span class="i">@INC</span><span class="s">)</span> <span class="s">{</span></li><li> <span class="i">$realfilename</span> = <span class="q">"$prefix/$filename"</span><span class="sc">;</span></li><li> if <span class="s">(</span>-f <span class="i">$realfilename</span><span class="s">)</span> <span class="s">{</span></li><li> <span class="i">$INC</span>{<span class="i">$filename</span>} = <span class="i">$realfilename</span><span class="sc">;</span></li><li> <span class="i">$result</span> = <a class="l_k" href="../functions/do.html">do</a> <span class="i">$realfilename</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/last.html">last</a> <span class="j">ITER</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li> <span class="s">}</span></li><li> <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Can't find $filename in \@INC"</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li> if <span class="s">(</span><span class="i">$@</span><span class="s">)</span> <span class="s">{</span></li><li> <span class="i">$INC</span>{<span class="i">$filename</span>} = <a class="l_k" href="../functions/undef.html">undef</a><span class="sc">;</span></li><li> <a class="l_k" href="../functions/die.html">die</a> <span class="i">$@</span><span class="sc">;</span></li><li> <span class="s">}</span> elsif <span class="s">(</span>!<span class="i">$result</span><span class="s">)</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/delete.html">delete</a> <span class="i">$INC</span>{<span class="i">$filename</span>}<span class="sc">;</span></li><li> <a class="l_k" href="../functions/die.html">die</a> <span class="q">"$filename did not return true value"</span><span class="sc">;</span></li><li> <span class="s">}</span> else <span class="s">{</span></li><li> <a class="l_k" href="../functions/return.html">return</a> <span class="i">$result</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li> <span class="s">}</span></li></ol></pre><p>Note that the file will not be included twice under the same specified
name.</p>
<p>The file must return true as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with <code class="inline"><span class="n">1</span><span class="sc">;</span></code>
unless you're sure it'll return true
otherwise. But it's better just to put the <code class="inline"><span class="n">1</span><span class="sc">;</span></code>
, in case you add more
statements.</p>
<p>If EXPR is a bareword, the require assumes a "<i>.pm</i>" extension and
replaces "<i>::</i>" with "<i>/</i>" in the filename for you,
to make it easy to load standard modules. This form of loading of
modules does not risk altering your namespace.</p>
<p>In other words, if you try this:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/require.html">require</a> <span class="w">Foo::Bar</span><span class="sc">;</span> <span class="c"># a splendid bareword</span></li></ol></pre><p>The require function will actually look for the "<i>Foo/Bar.pm</i>" file in the
directories specified in the <code class="inline"><span class="i">@INC</span></code>
array.</p>
<p>But if you try this:</p>
<pre class="verbatim"><ol><li> <span class="i">$class</span> = <span class="q">'Foo::Bar'</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/require.html">require</a> <span class="i">$class</span><span class="sc">;</span> <span class="c"># $class is not a bareword</span></li><li> <span class="c">#or</span></li><li> <a class="l_k" href="../functions/require.html">require</a> <span class="q">"Foo::Bar"</span><span class="sc">;</span> <span class="c"># not a bareword because of the ""</span></li></ol></pre><p>The require function will look for the "<i>Foo::Bar</i>" file in the @INC array and
will complain about not finding "<i>Foo::Bar</i>" there. In this case you can do:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/eval.html">eval</a> <span class="q">"require $class"</span><span class="sc">;</span></li></ol></pre><p>Now that you understand how <code class="inline"><a class="l_k" href="../functions/require.html">require</a></code> looks for files in the case of a
bareword argument, there is a little extra functionality going on behind
the scenes. Before <code class="inline"><a class="l_k" href="../functions/require.html">require</a></code> looks for a "<i>.pm</i>" extension, it will
first look for a similar filename with a "<i>.pmc</i>" extension. If this file
is found, it will be loaded in place of any file ending in a "<i>.pm</i>"
extension.</p>
<p>You can also insert hooks into the import facility, by putting directly
Perl code into the @INC array. There are three forms of hooks: subroutine
references, array references and blessed objects.</p>
<p>Subroutine references are the simplest case. When the inclusion system
walks through @INC and encounters a subroutine, this subroutine gets
called with two parameters, the first being a reference to itself, and the
second the name of the file to be included (e.g. "<i>Foo/Bar.pm</i>"). The
subroutine should return nothing, or a list of up to three values in the
following order:</p>
<ol>
<li>
<p>A filehandle, from which the file will be read.</p>
</li>
<li>
<p>A reference to a subroutine. If there is no filehandle (previous item),
then this subroutine is expected to generate one line of source code per
call, writing the line into <code class="inline"><span class="i">$_</span></code>
and returning 1, then returning 0 at
"end of file". If there is a filehandle, then the subroutine will be
called to act as a simple source filter, with the line as read in <code class="inline"><span class="i">$_</span></code>
.
Again, return 1 for each valid line, and 0 after all lines have been
returned.</p>
</li>
<li>
<p>Optional state for the subroutine. The state is passed in as <code class="inline"><span class="i">$_</span>[<span class="n">1</span>]</code>
. A
reference to the subroutine itself is passed in as <code class="inline"><span class="i">$_</span>[<span class="n">0</span>]</code>
.</p>
</li>
</ol>
<p>If an empty list, <code class="inline"><a class="l_k" href="../functions/undef.html">undef</a></code>, or nothing that matches the first 3 values above
is returned then <code class="inline"><a class="l_k" href="../functions/require.html">require</a></code> will look at the remaining elements of @INC.
Note that this file handle must be a real file handle (strictly a typeglob,
or reference to a typeglob, blessed or unblessed) - tied file handles will be
ignored and return value processing will stop there.</p>
<p>If the hook is an array reference, its first element must be a subroutine
reference. This subroutine is called as above, but the first parameter is
the array reference. This enables to pass indirectly some arguments to
the subroutine.</p>
<p>In other words, you can write:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/push.html">push</a> <span class="i">@INC</span><span class="cm">,</span> \<span class="i">&my_sub</span><span class="sc">;</span></li><li><a name="my_sub"></a> sub <span class="m">my_sub</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$coderef</span><span class="cm">,</span> <span class="i">$filename</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span> <span class="c"># $coderef is \&my_sub</span></li><li> ...</li><li> <span class="s">}</span></li></ol></pre><p>or:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/push.html">push</a> <span class="i">@INC</span><span class="cm">,</span> <span class="s">[</span> \<span class="i">&my_sub</span><span class="cm">,</span> <span class="i">$x</span><span class="cm">,</span> <span class="i">$y</span><span class="cm">,</span> ... <span class="s">]</span><span class="sc">;</span></li><li><a name="my_sub"></a> sub <span class="m">my_sub</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$arrayref</span><span class="cm">,</span> <span class="i">$filename</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li> <span class="c"># Retrieve $x, $y, ...</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">@parameters</span> = <span class="i">@$arrayref</span>[<span class="n">1</span>..<span class="i">$#$arrayref</span>]<span class="sc">;</span></li><li> ...</li><li> <span class="s">}</span></li></ol></pre><p>If the hook is an object, it must provide an INC method that will be
called as above, the first parameter being the object itself. (Note that
you must fully qualify the sub's name, as unqualified <code class="inline"><span class="w">INC</span></code>
is always forced
into package <code class="inline"><span class="w">main</span></code>
.) Here is a typical code layout:</p>
<pre class="verbatim"><ol><li> <span class="c"># In Foo.pm</span></li><li><a name="package-Foo"></a> package <span class="i">Foo</span><span class="sc">;</span></li><li><a name="new"></a> sub <span class="m">new</span> <span class="s">{</span> ... <span class="s">}</span></li><li><a name="Foo::INC"></a> sub <span class="m">Foo::INC</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$self</span><span class="cm">,</span> <span class="i">$filename</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li> ...</li><li> <span class="s">}</span></li><li></li><li> <span class="c"># In the main program</span></li><li> <a class="l_k" href="../functions/push.html">push</a> <span class="i">@INC</span><span class="cm">,</span> <span class="w">Foo</span><span class="w">->new</span><span class="s">(</span>...<span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Note that these hooks are also permitted to set the %INC entry
corresponding to the files they have loaded. See <a href="../perlvar.html#%25INC">"%INC" in perlvar</a>.</p>
<p>For a yet-more-powerful import facility, see <a href="../functions/use.html">"use"</a> and <a href="../perlmod.html">perlmod</a>.</p>
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