ch05_02.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

handle runtime errors (exceptions). The code in
<em class="replaceable"><tt>block</tt></em> is compiled only once during the
compilation of the main program. If there is a syntax error in the
block, it will produce an error at compile time. If the code in
<em class="replaceable"><tt>block</tt></em> produces a runtime error (or if a
<tt class="literal">die</tt> statement is encountered), the
<tt class="literal">eval</tt> exits, and the error is placed in
<tt class="literal">$@</tt>. For example, the following code can be used to
trap a divide-by-zero error at runtime:
</p><blockquote><pre class="code">eval {
      $a = 10; $b = 0;
      $c = $a / $b;     # Causes runtime error
                        # Trapped by eval
     };
print $@;               # Prints "Illegal division by 0 at
                        # try.pl line 3"</pre></blockquote><p>As with any code in a block, a final semicolon is not required.</p></div>

<a name="INDEX-939" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="exec"><b>exec</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
exec <em class="emphasis">command</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-939" />Terminates the
currently running Perl script and executes the program named in
<em class="replaceable"><tt>command</tt></em>. The Perl program does not resume
after the <tt class="literal">exec</tt> unless the <tt class="literal">exec</tt>
cannot be run and produces an error. Unlike
<tt class="literal">system</tt>, the executed
<em class="replaceable"><tt>command</tt></em> is not forked off into a child
process. An <tt class="literal">exec</tt> completely replaces the script in
its current process.
</p><p><em class="replaceable"><tt>command</tt></em> may be a scalar containing a
string with the name of the program to run and any arguments. This
string is checked for shell metacharacters, and if there are any,
passes the string to <tt class="literal">/bin/sh/ -c</tt> for parsing.
Otherwise, the string is read as a program command, bypassing any
shell processing. The first word of the string is used as the program
name, with any remaining words used as arguments.
</p><p><em class="replaceable"><tt>command</tt></em> may also be a list value in which
the first element is parsed as the program name and remaining
elements as arguments. For example:
</p><blockquote><pre class="code">exec 'echo', 'Your arguments are: ', @ARGV;</pre></blockquote><p>The <tt class="literal">exec</tt> function is not implemented for Perl on
Win32 platforms.
</p></div>

<a name="INDEX-940" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="exists"><b>exists</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
exists $<em class="emphasis">hash</em>{$<em class="emphasis">key</em>}
</pre></td><td align="right" /></tr></table><a name="INDEX-940" />Returns true if the
specified hash key exists, even if the corresponding value is
undefined.
</p></div>

<a name="INDEX-941" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="exit"><b>exit</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
exit <em class="emphasis">status</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-941" />Exits the current
Perl process immediately with that value given by
<em class="replaceable"><tt>status</tt></em>. This could be the entire Perl
script you are running or only a child process created by
<tt class="literal">fork</tt>. Here's a fragment that lets
a user exit the program by typing <tt class="literal">x</tt> or
<tt class="literal">X</tt>:
</p><blockquote><pre class="code">$ans = &lt;STDIN&gt;;
exit 0 if $ans =~ /^[Xx]/;</pre></blockquote><p>If <em class="replaceable"><tt>status</tt></em> is omitted, the function exits
with <tt class="literal">0</tt>. You shouldn't use
<tt class="literal">exit</tt> to abort a subroutine if
there's any chance that someone might want to trap
whatever error happened. Use <tt class="literal">die</tt> instead, which
can be trapped by an <tt class="literal">eval</tt>.
</p></div>

<a name="INDEX-942" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="exp"><b>exp</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
exp <em class="emphasis">num</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-942" />Returns
<em class="emphasis">e</em> to the power of
<em class="replaceable"><tt>num</em>. If <em class="replaceable">num</tt></em> is
omitted, it gives <tt class="literal">exp($_)</tt>. To do general
exponentiation, use the <tt class="literal">**</tt> operator.
</p></div>

<a name="INDEX-943" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="fcntl"><b>fcntl</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
fcntl <em class="emphasis">filehandle</em>, <em class="emphasis">function</em>, <em class="emphasis">arg</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-943" />Calls
the file control <em class="replaceable"><tt>function</tt></em> (with the
function-specific <em class="replaceable"><tt>arg</tt></em>) to use on the file
or device opened with <em class="replaceable"><tt>filehandle</tt></em>.
<tt class="literal">fcntl</tt> calls Unix's
<tt class="literal">fcntl</tt> function (not available on Win32 platforms).
If the function is not implemented, the program exits with a fatal
error. <tt class="literal">fcntl</tt> sets file descriptors for a
filehandle. This built-in command is usable when you use the Fcntl
module in the standard distribution:
</p><blockquote><pre class="code">use Fcntl;</pre></blockquote><p>This module imports the correct <em class="replaceable"><tt>function</tt></em>definitions. See the description of the Fcntl module in <a href="ch08_01.htm">Chapter 8, "Standard Modules"</a>. 
</p><p>The return value of <tt class="literal">fcntl</tt> (and
<tt class="literal">ioctl</tt>) is as follows:
</p><a name="ch05-43-fm2xml" /><table border="1" cellpadding="3">



<tr>
<th>
<p>System call returns</p>
</th>
<th>
<p>Perl returns</p>
</th>
</tr>


<tr>
<td>
<p><tt class="literal">-1</tt></p>
</td>
<td>
<p>Undefined value</p>
</td>
</tr>
<tr>
<td>
<p><tt class="literal">0</tt></p>
</td>
<td>
<p>String <tt class="literal">"0 but true"</tt></p>
</td>
</tr>
<tr>
<td>
<p>Anything else</p>
</td>
<td>
<p>That number</p>
</td>
</tr>

</table>
<p>Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system.
</p></div>

<a name="INDEX-944" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="fileno"><b>fileno</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
fileno <em class="emphasis">filehandle</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-944" />Returns the file
descriptor for a filehandle. (A file descriptor is a small integer,
unlike the filehandle, which is a symbol.) It returns
<tt class="literal">undef</tt> if the handle is not open.
It's useful for constructing bitmaps for
<tt class="literal">select</tt> and for passing to certain obscure system
calls if <tt class="literal">syscall</tt> is implemented.
It's also useful for double-checking that the
<tt class="literal">open</tt> function gave you the file descriptor you
wanted.
</p></div>

<a name="INDEX-945" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="flock"><b>flock</b></font></td><td align="right"><i></i></td></tr></table><hr width="515" size="3" noshade="true" align="left" color="black" /><table width="515" border="0" cellpadding="5"><tr><td align="left"><pre>
flock <em class="emphasis">filehandle</em>, <em class="emphasis">operation</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-945" />Establishes or
removes a lock on a file opened with
<em class="replaceable"><tt>filehandle</tt></em>. This function calls one of the
Unix functions <tt class="literal">flock</tt>, <tt class="literal">lockf</tt>, or
the locking capabilities of <tt class="literal">fcntl</tt>, whichever your
system supports. If none of these functions exist on your system,
<tt class="literal">flock</tt> will produce a fatal error.
</p><p><em class="replaceable"><tt>operation</tt></em> is the type of locking function
to perform. The number by each operation name is the argument that
Perl's <tt class="literal">flock</tt> takes by default.
You may also use the operation names if you explicitly import them
from the Fcntl module with <tt class="literal">use Fcntl ":flock"</tt>.
</p><dl>
<dt><a name="<tt class="literal">LOCK_SH</tt> (1)"><b><tt class="literal">LOCK_SH</tt> (1)</b></dt>
<dd>
Establishes a shared lock on the file (read lock).</p>
</dd>


<dt><a name="<tt class="literal">LOCK_EX</tt> (2)"><b><tt class="literal">LOCK_EX</tt> (2)</b></dt>
<dd>
Establishes an exclusive lock on the file (write lock).</p>
</dd>


<dt><a name="<tt class="literal">LOCK_UN</tt> (8)"><b><tt class="literal">LOCK_UN</tt> (8)</b></dt>
<dd>