ch05_02.htm

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

Removes a lock from the file.</p>
</dd>


<dt><a name="<tt class="literal">LOCK_NB</tt> (4)"><b><tt class="literal">LOCK_NB</tt> (4)</b></dt>
<dd>
Prevents <tt class="literal">flock</tt> from blocking while trying to
establish a lock with <tt class="literal">LOCK_SH</tt> or
<tt class="literal">LOCK_EX</tt> and instructs it to return immediately.
<tt class="literal">LOCK_NB</tt> must be <em class="emphasis">or</em>ed with the
other operation as an expression for the operation argument, i.e.,
<tt class="literal">(LOCK_EX |</tt> <tt class="literal">LOCK_NB)</tt>.
</p>
</dd>

</dl></div>

<a name="INDEX-946" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="fork"><b>fork</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>
fork
</pre></td><td align="right" /></tr></table><a name="INDEX-946" />Spawns a child
process that executes the code immediately following the
<tt class="literal">fork</tt> call until the process is terminated (usually
with an <tt class="literal">exit</tt>). The child process runs parallel to
the parent process and shares all the parent's
variables and open filehandles. The function returns the child pid to
the parent process and <tt class="literal">0</tt> to the child process on
success. If it fails, it returns the undefined value to the parent
process, and no child process is created. If you
<tt class="literal">fork</tt> your child processes, you'll
have to <tt class="literal">wait</tt> on their zombies when they die. 
The
<tt class="literal">fork</tt> function is unlikely to be implemented on any
operating system not resembling Unix, unless it purports POSIX
compliance.
</p></div>

<a name="INDEX-947" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="formline"><b>formline</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>
formline <em class="emphasis">picture</em>, <em class="emphasis">variables</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-947" />Internal function
used by formats, although you may also call it. It formats a list of
values (<em class="replaceable"><tt>variables</tt></em>) according to the
contents of <em class="replaceable"><tt>picture</tt></em>, placing the output
into the format output accumulator, <tt class="literal">$^A</tt>. When a
<tt class="literal">write</tt> is done, the contents of
<tt class="literal">$^A</tt> are written to a filehandle, but you can also
read <tt class="literal">$^A</tt> yourself and set <tt class="literal">$^A</tt>
back to "". Note that a format
typically does one <tt class="literal">formline</tt> per line of form, but
the <tt class="literal">formline</tt> function itself
doesn't care how many newlines are embedded in the
<em class="replaceable"><tt>picture</tt></em>. This means that the
<tt class="literal">~</tt> and <tt class="literal">~~</tt> tokens will treat the
entire <em class="replaceable"><tt>picture</tt></em> as a single line. Thus, you
may need to use multiple formlines to implement a single-record
format, such as the format compiler.
</p><p>Be careful if you put double quotes around the picture, since an
<tt class="literal">@</tt> character may be taken to mean the beginning of
an array name. <tt class="literal">formline</tt> always returns true. See
<a href="ch04_01.htm">Chapter 4, "The Perl Language"</a> for more information.
</p></div>

<a name="INDEX-948" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getc"><b>getc</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>
getc <em class="emphasis">filehandle</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-948" />Returns the next byte
from the input file attached to
<em class="replaceable"><tt>filehandle</tt></em>. At end-of-file, it returns a
null string. If <em class="replaceable"><tt>filehandle</tt></em> is omitted, the
function reads from STDIN. This operator is very slow, but is
occasionally useful for single-character input from the keyboard.
</p></div>

<a name="INDEX-949" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getgrent"><b>getgrent</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>
getgrent
</pre></td><td align="right" /></tr></table><a name="INDEX-949" />Returns the next
entry from the systems group file (usually
<em class="emphasis">/etc/group</em> on Unix systems) starting from the
top. Returns null when end-of-file is reached. The return value from
<tt class="literal">getgrent</tt> in list context is:
</p><blockquote><pre class="code">($name, $passwd, $gid, $members)</pre></blockquote><p>in which <tt class="literal">$members</tt> contains a space-separated list
of the login names of the members of the group. In scalar context,
<tt class="literal">getgrent</tt> returns only the group name.
</p></div>

<a name="INDEX-950" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getgrgid"><b>getgrgid</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>
getgrgid <em class="emphasis">gid</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-950" />Retrieves a group
file (<em class="emphasis">/etc/group</em>) entry by group number
<em class="replaceable"><tt>gid</tt></em>. The return value in list context is:
</p><blockquote><pre class="code">($name, $passwd, $gid, $members)</pre></blockquote><p>in which <tt class="literal">$members</tt> contains a space-separated list
of the login names of the members of the group. If you want to do
this repeatedly, consider caching the data in a hash using
<tt class="literal">getgrent</tt>. In scalar context,
<tt class="literal">getgrgid</tt> returns only the group name.
</p></div>

<a name="INDEX-951" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getgrnam"><b>getgrnam</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>
getgrnam <em class="emphasis">name</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-951" />Retrieves a group
file entry by the group name <em class="replaceable"><tt>name</tt></em>. The
return value in list context is:
</p><blockquote><pre class="code">($name, $passwd, $gid, $members)</pre></blockquote><p>in which <tt class="literal">$members</tt> contains a space-separated list
of the login names of the members of the group. In scalar context,
<tt class="literal">getgrnam</tt> returns only the numeric group ID.
</p></div>

<a name="INDEX-952" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="gethostbyaddr"><b>gethostbyaddr</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>
gethostbyaddr <em class="emphasis">address</em>, [<em class="emphasis">addrtype</em>]
</pre></td><td align="right" /></tr></table><a name="INDEX-952" />Retrieves
the hostname (and alternate addresses) of a packed binary network
<em class="replaceable"><tt>address</tt></em>.
(<em class="replaceable"><tt>addrtype</tt></em> indicates the type of address
given. Since <tt class="literal">gethostbyaddr</tt> is used almost solely
for Internet IP addresses, <em class="replaceable"><tt>addrtype</tt></em> is not
needed.) The return value in list context is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $length, @addrs)</pre></blockquote><p>in which <tt class="literal">@addrs</tt> is a list of packed binary
addresses. In the Internet domain, each address is four bytes long
and can be unpacked by something like:
</p><blockquote><pre class="code">($a, $b, $c, $d) = unpack('C4', $addrs[0]);</pre></blockquote><p>In scalar context, <tt class="literal">gethostbyaddr</tt> returns only the
hostname.
</p></div>

<a name="INDEX-953" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="gethostbyname"><b>gethostbyname</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>
gethostbyname <em class="emphasis">name</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-953" />Retrieves
the address (and other names) of a network hostname. The return value
in list context is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $length, @addrs)</pre></blockquote><p>in which <tt class="literal">@addrs</tt> is a list of raw addresses. In
scalar context, <tt class="literal">gethostbyname</tt> returns only the
host address.
</p></div>

<a name="INDEX-954" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="gethostent"><b>gethostent</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>
gethostent
</pre></td><td align="right" /></tr></table><a name="INDEX-954" />Retrieves the
next entry from your system's network hosts file
(usually <em class="emphasis">/etc/hosts</em> on Unix). The return value
from <tt class="literal">gethostent</tt> is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $length, @addrs)</pre></blockquote><p>in which <tt class="literal">@addrs</tt> is a list of raw addresses.
Scripts that use this function should not be considered portable.
</p></div>

<a name="INDEX-955" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getlogin"><b>getlogin</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>
getlogin
</pre></td><td align="right" /></tr></table><a name="INDEX-955" />Returns the
current login from <em class="emphasis">/etc/utmp</em> (Unix systems
only), if any. If null, use <tt class="literal">getpwuid</tt>. For example:
</p><blockquote><pre class="code">$login = getlogin || getpwuid($&lt;) || "Intruder!!";</pre></blockquote></div>

<a name="INDEX-956" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getnetbyaddr"><b>getnetbyaddr</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>
getnetbyaddr <em class="emphasis">address</em>, [<em class="emphasis">addrtype</em>]
</pre></td><td align="right" /></tr></table><a name="INDEX-956" />Retrieves the
network name or names of the given network
<em class="replaceable"><tt>address</tt></em>.
(<em class="replaceable"><tt>addrtype</tt></em> indicates the type of address.
Often, this function is used for IP addresses, in which the type is
not needed.) The return value in list context is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $net)</pre></blockquote><p>In scalar context, <tt class="literal">getnetbyaddr</tt> returns only the
network name.
</p></div>

<a name="INDEX-957" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getnetbyname"><b>getnetbyname</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>
getnetbyname <em class="emphasis">name</em>
</pre></td><td align="right" /></tr></table><a name="INDEX-957" />Retrieves the
network address of a network <em class="replaceable"><tt>name</tt></em>. The
return value in list context is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $net)</pre></blockquote><p>In scalar context, <tt class="literal">getnetbyname</tt> returns only the
network address.
</p></div>

<a name="INDEX-958" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getnetent"><b>getnetent</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>
getnetent
</pre></td><td align="right" /></tr></table><a name="INDEX-958" />Retrieves the
next line from your <em class="emphasis">/etc/networks</em> file, or
system equivalent. The return value in list context is:
</p><blockquote><pre class="code">($name, $aliases, $addrtype, $net)</pre></blockquote><p>In scalar context, <tt class="literal">getnetent</tt> returns only the
network name.
</p></div>

<a name="INDEX-959" /><div class="refentry"><table width="515" border="0" cellpadding="5"><tr><td align="left"><font size="+1"><a name="getpeername"><b>getpeername</b></font></td><td align="right"