ch17_01.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

> connects a local socket to a (possibly remote) one, <CODE
CLASS="literal"
>listen</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004663-0"
></A
> readies a socket for connections from other sockets, and <CODE
CLASS="literal"
>accept</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004665-0"
></A
> receives the connections one by one. You can communicate over a stream socket with <CODE
CLASS="literal"
>print</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004666-0"
></A
> and <A
CLASS="indexterm"
NAME="ch17-idx-1000004669-0"
></A
><A
CLASS="indexterm"
NAME="ch17-idx-1000004669-1"
></A
>&lt; &gt; as well as with <CODE
CLASS="literal"
>syswrite</CODE
> and <CODE
CLASS="literal"
>sysread</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004667-0"
></A
><A
CLASS="indexterm"
NAME="ch17-idx-1000004667-1"
></A
>, or over a datagram socket with <CODE
CLASS="literal"
>send</CODE
> and <CODE
CLASS="literal"
>recv</CODE
>. (Perl does not currently support <EM
CLASS="emphasis"
>sendmsg</EM
>(2).)</P
><P
CLASS="para"
>A typical server calls <CODE
CLASS="literal"
>socket</CODE
>, <CODE
CLASS="literal"
>bind</CODE
>, and <CODE
CLASS="literal"
>listen</CODE
>, then loops in a blocking <CODE
CLASS="literal"
>accept</CODE
> call that waits for incoming connections (see <A
CLASS="xref"
HREF="ch17_03.htm"
TITLE="Writing a TCP Server"
>Recipe 17.2</A
> and <A
CLASS="xref"
HREF="ch17_06.htm"
TITLE="Setting Up a UDP Server"
>Recipe 17.5</A
>). A typical client calls <CODE
CLASS="literal"
>socket</CODE
> and <CODE
CLASS="literal"
>connect</CODE
> (see <A
CLASS="xref"
HREF="ch17_02.htm"
TITLE="Writing a TCP Client"
>Recipe 17.1</A
> and <A
CLASS="xref"
HREF="ch17_05.htm"
TITLE="Setting Up a UDP Client"
>Recipe 17.4</A
>). Datagram clients are special. They don't have to <CODE
CLASS="literal"
>connect</CODE
> to send data because they can specify the destination as an argument to <CODE
CLASS="literal"
>send</CODE
>.</P
><P
CLASS="para"
>When you <CODE
CLASS="literal"
>bind</CODE
> , <CODE
CLASS="literal"
>connect</CODE
>, or <CODE
CLASS="literal"
>send</CODE
> to a specific destination, you must supply a socket name. An Internet domain socket name is a host (an IP address packed with <CODE
CLASS="literal"
>inet_aton</CODE
>) and a port (a number), packed into a C-style structure with <CODE
CLASS="literal"
>sockaddr_in</CODE
>:</P
><PRE
CLASS="programlisting"
>use Socket;

$packed_ip   = inet_aton(&quot;208.146.240.1&quot;);
$socket_name = sockaddr_in($port, $packed_ip);</PRE
><P
CLASS="para"
>A Unix domain socket name is a filename packed into a C structure with <CODE
CLASS="literal"
>sockaddr_un</CODE
>:</P
><PRE
CLASS="programlisting"
>use Socket;

$socket_name = sockaddr_un(&quot;/tmp/mysock&quot;);</PRE
><P
CLASS="para"
>To take a packed socket name and turn it back into a filename or host and port, call <CODE
CLASS="literal"
>sockaddr_un</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004677-0"
></A
><A
CLASS="indexterm"
NAME="ch17-idx-1000004677-1"
></A
> or <CODE
CLASS="literal"
>sockaddr_in</CODE
> in list context:</P
><PRE
CLASS="programlisting"
>($port, $packed_ip) = sockaddr_in($socket_name);    # for PF_INET sockets
($filename)         = sockaddr_un($socket_name);    # for PF_UNIX sockets</PRE
><P
CLASS="para"
>Use <CODE
CLASS="literal"
>inet_ntoa</CODE
><A
CLASS="indexterm"
NAME="ch17-idx-1000004678-0"
></A
> to turn a packed <A
CLASS="indexterm"
NAME="ch17-idx-1000004906-0"
></A
>IP address back into an ASCII string. It stands for "numbers to ASCII" and <CODE
CLASS="literal"
>inet_aton</CODE
> stands for "ASCII to numbers."</P
><PRE
CLASS="programlisting"
>$ip_address = inet_ntoa($packed_ip);
$packed_ip  = inet_aton(&quot;204.148.40.9&quot;);
$packed_ip  = inet_aton(&quot;www.oreilly.com&quot;);</PRE
><P
CLASS="para"
>Most recipes use Internet domain sockets in their examples, but nearly everything that applies to the Internet domain also applies to the Unix domain. <A
CLASS="xref"
HREF="ch17_07.htm"
TITLE="Using UNIX Domain Sockets"
>Recipe 17.6</A
> explains the differences and pitfalls.</P
><P
CLASS="para"
>Sockets are the basis of network services. We provide three ways to write servers: one where a child process is created for each incoming connection (<A
CLASS="xref"
HREF="ch17_12.htm"
TITLE="Forking Servers"
>Recipe 17.11</A
>), one where the server forks in advance (<A
CLASS="xref"
HREF="ch17_13.htm"
TITLE="Pre-Forking Servers"
>Recipe 17.12</A
>), and one where the server process doesn't fork at all (<A
CLASS="xref"
HREF="ch17_14.htm"
TITLE="Non-Forking Servers"
>Recipe 17.13</A
>).</P
><P
CLASS="para"
>Some servers need to listen to many IP addresses at once, which we demonstrate in <A
CLASS="xref"
HREF="ch17_15.htm"
TITLE="Writing a Multi-Homed Server"
>Recipe 17.14</A
>. Well-behaved servers clean up and restart when they get a HUP signal; <A
CLASS="xref"
HREF="ch17_17.htm"
TITLE="Restarting a Server on Demand"
>Recipe 17.16</A
> shows how to implement that behavior in Perl. We also show how to put a name to both ends of a connection; see <A
CLASS="xref"
HREF="ch17_08.htm"
TITLE="Identifying the Other End of a Socket"
>Recipe 17.7</A
> and <A
CLASS="xref"
HREF="ch17_09.htm"
TITLE="Finding Your Own Name and Address"
>Recipe 17.8</A
>.</P
><P
CLASS="para"
><EM
CLASS="emphasis"
>Unix Network Programming</EM
> and the three-volume <EM
CLASS="emphasis"
>TCP/IP Illustrated</EM
> by W. Richard Stevens are indispensable for the serious socket programmer. If you want to learn the basics about sockets, it's hard to beat the original and classic reference, <CITE
CLASS="citetitle"
>An Advanced 4.4BSD Interprocess Communication Tutorial.</CITE
> It's written for C, but almost everything is directly applicable to Perl. It's available in <EM
CLASS="emphasis"
>/usr/share/doc</EM
> on most BSD-derived Unix systems. We also recommend you look at <CITE
CLASS="citetitle"
>The Unix Programming Frequently Asked Questions List</CITE
> (Gierth and Horgan), and <CITE
CLASS="citetitle"
>Programming UNIX Sockets in C&nbsp;- Frequently Asked Questions </CITE
>(Metcalf and Gierth), both of which are posted periodically to the <A
CLASS="systemitem.newsgroup"
HREF="news:comp.unix.answers"
>comp.unix.answers</A
> newsgroup.</P
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