ch05_02.htm

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

concatenated into a single string for output. If
<em class="replaceable"><tt>message</tt></em> does not end with a newline
(<tt class="literal">\n</tt>), the current script filename, line number,
and input line number (if any) are appended to the message with a
newline. With no argument, the function outputs the string
<tt class="literal">Died</tt> as its default.
</p><p><tt class="literal">die</tt> exits the programs with the current value of
the <tt class="literal">$!</tt> variable, which contains the text
describing the most recent operating system error value. This value
can be used in the <em class="replaceable"><tt>message</tt></em> to describe
what the problem may have been.
</p><p><tt class="literal">die</tt> behaves differently inside an
<tt class="literal">eval</tt> statement. It places the error message in the
<tt class="literal">$@</tt> variable and aborts the
<tt class="literal">eval</tt>, which returns an undefined value. This use
of <tt class="literal">die</tt> can raise runtime exceptions that can be
caught at a higher level of the program.
</p></div>

<a name="INDEX-928" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="do"><b>do</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>
do {<em class="emphasis">block</em>}
</pre></td><td align="right" /></tr></table><a name="INDEX-928" />Executes
the sequence of commands in the <em class="replaceable"><tt>block</tt></em> and
returns the value of the last expression evaluated. When modified by
a loop modifier, Perl executes the <em class="replaceable"><tt>block</tt></em>once before testing the loop condition. (On other statements, the
loop modifiers test the conditional first.)
</p></div>

<a name="INDEX-929" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="dump"><b>dump</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>
dump <em class="emphasis">label</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-929" />During
program execution, causes an immediate core dump after code previous
to it has already been executed. Primarily, this is so that you can
use the <tt class="literal">undump</tt> program to turn your core dump into
an executable binary after having initialized all your variables at
the beginning of the program. Please note that you
shouldn't use <tt class="literal">dump( )</tt> or
<tt class="literal">undump( )</tt> with the availability of the Perl
Compiler (B modules) as part of your Perl distribution. In addition,
you're unlikely to have success with
<tt class="literal">dump</tt> or <tt class="literal">undump</tt> on most modern
Unix systems (that support dynamic loading from
<tt class="literal">libdl</tt>). <tt class="literal">dump</tt> arranges for the
revived binary, when run, to begin by executing a
<tt class="literal">goto</tt> label (with all the restrictions that
<tt class="literal">goto</tt> suffers). Think of the operation as a
<tt class="literal">goto</tt> with an intervening core dump and
reincarnation. If <em class="replaceable"><tt>label</tt></em> is omitted, the
function arranges for the program to restart from the top. Note that
any files opened at the time of the dump will no longer be open when
the program is reincarnated. See also the <em class="emphasis">-u</em>
command-line switch.
</p><p>As of Perl 5.8, <tt class="literal">dump</tt> is now largely obsolete,
partly because it's difficult to convert a core file
into an executable, and because the real compiler backends for
generating portable bytecode and compilable C code have superseded
it. That's why you should now invoke it as <tt class="literal">"CORE::dump(
)"</tt> if you don't want to be warned against a possible typo.
</p><p>The <tt class="literal">undump</tt> program is not available on all systems
and may not be compatible with specific ports of Perl.
</p></div>

<a name="INDEX-930" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="each"><b>each</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>
each %<em class="emphasis">hash</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-930" />Returns a two-element
list consisting of the key and value for the next element of a hash.
With successive calls to <tt class="literal">each</tt>, you can iterate
over the entire hash. Entries are returned in an indeterminate order.
When the hash is entirely read, a null list is returned. The next
call to <tt class="literal">each</tt> after that will start a new
iteration. The iterator can be reset either by reading all the
elements from the hash or by calling the <tt class="literal">keys</tt>
function in scalar context. You must not add elements to the hash
while iterating over it, although you are permitted to use
<tt class="literal">delete</tt>. In a scalar context,
<tt class="literal">each</tt> returns only the key.
</p><p>There is a single iterator for each hash, shared by all
<tt class="literal">each</tt>, <tt class="literal">keys</tt>, and
<tt class="literal">values</tt> function calls in the program. This means
that after a <tt class="literal">keys</tt> or <tt class="literal">values</tt>
call, the next <tt class="literal">each</tt> call will start again from the
beginning.
</p></div>

<a name="INDEX-931" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endgrent"><b>endgrent</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>
endgrent
</pre></td><td align="right" /></tr></table><a name="INDEX-931" />Closes the groups
file (usually <em class="emphasis">/etc/group</em> on Unix systems) if
open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-932" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endhostent"><b>endhostent</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>
endhostent
</pre></td><td align="right" /></tr></table><a name="INDEX-932" />Closes the
hosts file (usually <em class="emphasis">/etc/hosts</em> on Unix systems)
if open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-933" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endnetent"><b>endnetent</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>
endnetent
</pre></td><td align="right" /></tr></table><a name="INDEX-933" />Closes the
networks file (usually <em class="emphasis">/etc/networks</em> on Unix
systems) if open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-934" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endprotoent"><b>endprotoent</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>
endprotoent
</pre></td><td align="right" /></tr></table><a name="INDEX-934" />Closes the
prototypes file (usually <em class="emphasis">/etc/prototypes</em> on Unix
systems) if open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-935" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endpwent"><b>endpwent</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>
endpwent
</pre></td><td align="right" /></tr></table><a name="INDEX-935" />Closes the
password file (<em class="emphasis">/etc/passwd</em> or equivalent on Unix
systems) if open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-936" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="endservent"><b>endservent</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>
endservent
</pre></td><td align="right" /></tr></table><a name="INDEX-936" />Closes the
services file (usually <em class="emphasis">/etc/services</em> on Unix
systems) if open. Not implemented on Win32 systems.
</p></div>

<a name="INDEX-937" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="eof"><b>eof</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>
eof <em class="emphasis">filehandle</em>
eof( )
</pre></td><td align="right" /></tr></table><a name="INDEX-937" />Returns true if the
next read on <em class="replaceable"><tt>filehandle</tt></em> will return
end-of-file, or if <em class="replaceable"><tt>filehandle</tt></em> is not open.
<em class="replaceable"><tt>filehandle</tt></em> may be an expression with a
value that gives the real filehandle name. An <tt class="literal">eof</tt>
without an argument returns the end-of-file status for the last file
read. Empty parentheses <tt class="literal">( )</tt> may be used in
connection with the combined files listed on the command line. That
is, inside a <tt class="literal">while (&lt;&gt;)</tt> loop, <tt class="literal">eof(
)</tt> will detect the end of only the last of a group of files.
Use <tt class="literal">eof(ARGV)</tt> or <tt class="literal">eof</tt> (without
parentheses) to test each file in a <tt class="literal">while
(&lt;&gt;)</tt> loop. For example, the following code inserts
dashes just before the last line of the last file:
</p><blockquote><pre class="code">while (&lt;&gt;) {
    if (eof( )) {
        print "-" x 30, "\n";
    }
    print;
}</pre></blockquote></div>

<a name="INDEX-938" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="eval"><b>eval</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>
eval <em class="emphasis">string</em>
eval {<em class="emphasis">block</em>}
</pre></td><td align="right" /></tr></table><a name="INDEX-938" />Evaluates the
expression or code in its argument at runtime as a separate Perl
program within the context of the larger script. Any variable
settings remain afterward, as do any subroutine or format
definitions. The code of the <tt class="literal">eval</tt> is treated as a
block, so any locally scoped variables declared within the
<tt class="literal">eval</tt> last only until the <tt class="literal">eval</tt>
is done. (See also <a href="ch05_02.htm#local">local</a> and
<a href="ch05_02.htm#my">my</a>.) The value returned from an
<tt class="literal">eval</tt> is the value of the last expression
evaluated. Like subroutines, you may also use the
<tt class="literal">return</tt> function to return a value and exit the
<tt class="literal">eval</tt>.
</p><p>With <tt class="literal">eval</tt> <em class="replaceable"><tt>string</tt></em>, the
contents of <em class="replaceable"><tt>string</tt></em> are compiled and
executed at runtime. For example:
</p><blockquote><pre class="code">$a = 3, $b = 4;
$c = '$a * $b';
print (eval "$c"); # Prints 12</pre></blockquote><p>The string form of <tt class="literal">eval</tt> is useful for executing
strings produced at runtime from standard or other dynamic input
sources. If the string produces an error, either from syntax or at
runtime, the <tt class="literal">eval</tt> exits with the undefined value
and places the error in <tt class="literal">$@</tt>. If
<em class="replaceable"><tt>string</tt></em> is omitted, the operator evaluates
<tt class="literal">$_</tt>.
</p><p>The block form of <tt class="literal">eval</tt> is used in Perl programs to