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<b>Linux</b></font><p>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<UL>
<LI><A HREF="#Heading1">- 59 -</A>
<UL>
<LI><A HREF="#Heading2">Network Programming</A>
<UL>
<LI><A HREF="#Heading3">Ports and Sockets</A>
<LI><A HREF="#Heading4">Socket Programming</A>
<UL>
<LI><A HREF="#Heading5">The socket() System Call</A>
<LI><A HREF="#Heading6">The bind() System Call</A>
<LI><A HREF="#Heading7">The listen() System Call</A>
<LI><A HREF="#Heading8">The accept() System Call</A>
<LI><A HREF="#Heading9">The setsockopt() and getsockopt() System Calls</A>
<LI><A HREF="#Heading10">The connect() System Call</A>
</UL>
<LI><A HREF="#Heading11">Listing 59.1. The server side for a socket-oriented protocol.</A>
<LI><A HREF="#Heading12">Listing 59.2. The client side function.</A>
<UL>
<LI><A HREF="#Heading13">Connectionless Socket Programming</A>
</UL>
<LI><A HREF="#Heading14">Listing 59.3. The server side.</A>
<LI><A HREF="#Heading15">NOTE</A>
<LI><A HREF="#Heading16">Record and File Locking</A>
<LI><A HREF="#Heading17">Interprocess Communications</A>
<LI><A HREF="#Heading18">Summary</A>
</UL>
</UL>
</UL>
<P>
<HR SIZE="4">
<H2 ALIGN="CENTER"><A NAME="Heading1<FONT COLOR="#000077">- 59 -</FONT></H2>
<H2 ALIGN="CENTER"><A NAME="Heading2<FONT COLOR="#000077">Network Programming</FONT></H2>
<P><I>by Kamran Husain and Tim Parker</I></P>
<P>IN THIS CHAPTER</P>
<UL>
<LI>Ports and Sockets
<P>
<LI>Socket Programming
<P>
<LI>Record and File Locking
<P>
<LI>Interprocess Communications
</UL>
<P><BR>
This chapter looks at the basic concepts you need for network programming:
<UL>
<LI>Ports and sockets
<P>
<LI>Record and file locking
<P>
<LI>Interprocess communications
</UL>
<P>It is impossible to tell you how to program applications for a network in just
a few pages. Indeed, the best available reference to network programming takes almost
800 pages in the first volume alone! If you really want to do network programming,
you need a lot of experience with compilers, TCP/IP, and network operating systems--and
you need a great deal of patience.</P>
<P>For details on TCP/IP, check the book Teach Yourself TCP/IP in 14 Days, by Tim
Parker (Sams Publishing).
<H3 ALIGN="CENTER"><A NAME="Heading3<FONT COLOR="#000077">Ports and Sockets</FONT></H3>
<P>Network programming relies on the use of sockets to accept and transmit information.
Although there is a lot of mystique about sockets, the concept is actually simple
to understand.</P>
<P>Most applications that use the two primary network protocols, Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP) have a port number that identifies
the application. A port number is used for each different application the machine
is handling, so it can keep track of those applications by numbers rather than names.
The port number makes it easier for the operating system to know how many applications
are using the system and which services are available.</P>
<P>In theory, port numbers can be assigned on individual machines by the system administrator,
but some conventions have been adopted to allow better communications. These conventions
enable the port number to identify the type of service that one system is requesting
from another. For this reason, most systems maintain a file of port numbers and their
corresponding services.</P>
<P>Port numbers are assigned starting from the number 1. Normally, port numbers above
255 are reserved for the private use of the local machine, but numbers between 1
and 255 are used for processes requested by remote applications or for networking
services.</P>
<P>Each network communications circuit into and out of the host computer's TCP application
layer is uniquely identified by a combination of two numbers, together called the
socket. The socket is composed of the IP address of the machine and the port number
used by the TCP software.</P>
<P>Because at least two machines are involved in network communications, there will
be a socket on both the sending and the receiving machine. Because the IP address
of each machine is unique and the port numbers are unique to each machine, socket
numbers are also unique across the network. This setup enables an application to
talk to another application across the network based entirely on the socket number.</P>
<P>The sending and receiving machines maintain a port table that lists all active
port numbers. The two machines involved have reversed entries for each session between
the two, a process called binding. In other words, if one machine has the source
port number 23 and the destination port number set at 25, the other machine has its
source port number set at 25 and the destination port number set at 23.
<H3 ALIGN="CENTER"><A NAME="Heading4<FONT COLOR="#000077">Socket Programming</FONT></H3>
<P>Linux supports BSD-style socket programming. Both connection-oriented and connectionless
types of sockets are supported. In connection-oriented communication, the server
and client establish a connection before any data is exchanged. In connectionless
communication, data is exchanged as part of a message. In either case, the server
always starts first, binds itself to a socket, and listens to messages. How the server
attempts to listen depends on the type of connection for which you have programmed
it.</P>
<P>You need to know about a few system calls:
<UL>
<LI><TT>socket()</TT>
<P>
<LI><TT>bind()</TT>
<P>
<LI><TT>listen()</TT>
<P>
<LI><TT>accept()</TT>
<P>
<LI> setsockopt() and getsockopt()
<P>
<LI><TT>connect()</TT>
<P>
<LI><TT>sendto()</TT>
<P>
<LI><BR>
recvfrom()
</UL>
<P>We will cover these system calls in the following examples.
<H4 ALIGN="CENTER"><A NAME="Heading5<FONT COLOR="#000077">The socket() System
Call</FONT></H4>
<P>The <TT>socket()</TT> system call creates a socket for the client or the server.
The <TT>socket</TT> function is defined as shown here:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int socket(int family, int type, int protocol)
</FONT></PRE>
<P>For Linux, you will have family<TT> = AF_UNIX</TT>. The type is either <TT>SOCK_STREAM</TT>
for reliable, though slower, communications or <TT>SOCK_DGRAM</TT> for faster, but
less reliable, communications. The protocol should be <TT>IPPROTO_TCP</TT> for <TT>SOCK_STREAM</TT>
and <TT>IPPROTO_UDP</TT> for <TT>SOCK_DGRAM</TT>.</P>
<P>The return value from this function is <TT>-1</TT> if there was an error; otherwise,
it's a socket descriptor. You will use this socket descriptor to refer to this socket
in all subsequent calls in your program.</P>
<P>Sockets are created without a name. Clients use the name of the socket to read
or write to it. This is where the <TT>bind</TT> function comes in.
<H4 ALIGN="CENTER"><A NAME="Heading6<FONT COLOR="#000077">The bind() System
Call</FONT></H4>
<P>The <TT>bind()</TT> system call assigns a name to an unnamed socket. The <TT>bind</TT>
function is defined like this:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int bind(int sockfd, struct sockaddr *saddr, int addrlen)
</FONT></PRE>
<P>The first item is a socket descriptor. The second is a structure with the name
to use, and the third item is the size of the structure.</P>
<P>Now that you have bound an address for your server or client, you can <TT>connect()</TT>
to it or listen on it. If your program is a server, it sets itself up to listen and
accept connections. Let's look at the function available for such an endeavor.
<H4 ALIGN="CENTER"><A NAME="Heading7<FONT COLOR="#000077">The listen() System
Call</FONT></H4>
<P>The <TT>listen()</TT> system call is used by the server. It is defined in the
following way:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int listen(int sockfd, int backlog);
</FONT></PRE>
<P>The sockfd is the descriptor of the socket. The backlog is the number of connections
that are pending at one time before any are rejected. Use the standard value of <TT>5</TT>
for backlog. A returned value of less than <TT>1</TT> indicates an error.</P>
<P>If this call is successful, you can accept connections.
<H4 ALIGN="CENTER"><A NAME="Heading8<FONT COLOR="#000077">The accept() System
Call</FONT></H4>
<P>The <TT>accept()</TT> system call is used by a server to accept any incoming messages
from clients' <TT>connect()</TT> calls. Be aware that this function does not return
if no connections are received. It is defined like this:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int accept(int sockfd, struct sockaddr *peeraddr, int addrlen)
</FONT></PRE>
<P>The parameters are the same as those for the <TT>bind</TT> call, with the exception
that the peeraddr points to information about the client that is making a connection
request. Based on the incoming message, the fields in the structure pointed at by
peeraddr are filled out.
<H4 ALIGN="CENTER"><A NAME="Heading9<FONT COLOR="#000077">The setsockopt()
and getsockopt() System Calls</FONT></H4>
<P>The socket libraries provided with Linux include a bug. The symptom of this bug
is that you cannot reuse a port number for a socket even if you closed the socket
properly. For example, say you write your own server that waits on a socket. This
server opens the socket and listens on it with no problems. However, for some reason
(a crash or normal termination), when the program is restarted, you are not able
to bind to the same port. The error codes from the <TT>bind()</TT> call will always
return an error indicating that the port you are attempting to connect to is already
bound to another process.</P>
<P>The problem is that the Linux kernel never marks the port as unused when the process
bound to a socket terminates. In most other UNIX systems, the port can be used again
by another invocation of the same or even another process.</P>
<P>The way to get around this problem in Linux is to use the <TT>setsockopt()</TT>
system call to set the options on a socket when it is opened and before a connection
is made on it. The <TT>setsockopt()</TT> sets options and the <TT>getsockopt()</TT>call
gets options for a given socket.</P>
<P>Here is the syntax for these calls:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int getsockopt(int sockfd, int level, int name, char *value, int *optlen)
</FONT></PRE>
<PRE><FONT COLOR="#0066FF">int setsockopt(int sockfd, int level, int name, char *value, int *optlen)
</FONT></PRE>
<P>The sockfd must be an open socket. The level is the protocol level to use for
the function (<TT>IPPROTO_TCP</TT> for TCP/IP and <TT>SOL_SOCKET</TT> for socket
level options), and the name of the option is as defined in the socket's man page.
The *value pointer points to a location where a value is stored for <TT>getsockopt()</TT>
or when a value is read for <TT>setsockopt()</TT>. The optlen parameter is a pointer
to an integer containing the length of the parameters in bytes; the value is set
by <TT>getsockopt()</TT> and must be set by the programmer when making a call via
<TT>setsockopt()</TT>.</P>
<P>The full man page with details of all the options is found in the man page <TT>setsockopt(2)</TT>.</P>
<P>Now back to the bug in Linux. When you open a socket, you must also call the <TT>setsockopt()</TT>
function with the following segment of code:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#ifdef LINUX
opt = 1; len = sizeof(opt);
setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&opt,&len);
#endif
</FONT></PRE>
<P>The <TT>#ifdef</TT> and <TT>#endif</TT> statements are necessary only if you want
to port the code over to systems other than Linux. Some UNIX systems might not support
or require the <TT>SO_REUSEADDR</TT> flag.
<H4 ALIGN="CENTER"><A NAME="Heading10<FONT COLOR="#000077">The connect() System
Call</FONT></H4>
<P>The <TT>connect()</TT> system call is used by clients to connect to a server in
a connection-oriented system. This <TT>connect()</TT> call should be made after the
<TT>bind()</TT> call. It is defined like this:<FONT COLOR="#0066FF"></FONT>
<PRE><FONT COLOR="#0066FF">#include<sys/types.h>
#include<sys/socket.h>
int connect(int sockfd, struct sockaddr *servsaddr, int addrlen)
</FONT></PRE>
<P>The parameters are the same as those for the <TT>bind</TT> call, with the exception
that the servsaddr points to information about the server that the client is connecting
to. The <TT>accept</TT> call creates a new socket for the server to work with the
request. This way, the server can <TT>fork()</TT> off a new process and wait for
more connections. On the server side of things, you would have code that looks like
that shown in Listing 59.1.
<H3 ALIGN="CENTER"><A NAME="Heading11<FONT COLOR="#000077">Listing 59.1. The
server side for a socket-oriented protocol.</FONT><FONT COLOR="#0066FF"></FONT></H3>
<PRE><FONT COLOR="#0066FF">#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <linux/net.h>
#define MY_PORT 6545
main(int argc, char *argv[])
{
int sockfd, newfd;
int cpid; /* child id */
struct sockaddr_in servaddr;
struct sockaddr_in clientInfo;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0) < 0)
{
myabort("Unable to create socket");
}
#ifdef LINUX
opt = 1; len = sizeof(opt);
setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&opt,&len);
#endif
bzero((char *)&servaddr, sizeof(servaddr));
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