Address binding

As was mentioned in ``Basics'', binding addresses to sockets in the Internet domain can be complex. As a brief reminder, these associations are composed of local and foreign addresses, and local and foreign ports. Port numbers are allocated out of separate spaces, one for each system and one for each domain on that system. Through the bind system call, a process may specify half of an association, the <local address, local port> part, while the connect and accept primitives are used to complete a socket's association by specifying the <foreign address, foreign port> part. Since the association is created in two steps the association uniqueness requirement mentioned previously could be violated unless care is taken. Further, it is unrealistic to expect user programs always to know proper values to use for the local address and local port since a host may reside on multiple networks and the set of allocated port numbers is not directly accessible to a user.

To simplify local address binding in the Internet domain the notion of a wildcard address has been provided. When an address is specified as INADDR_ANY (a manifest constant defined in netinet/in.h), the system interprets the address as any valid address. For example, to bind a specific port number to a socket, but leave the local address unspecified, the following code might be used:

   #include <sys/types.h>
   #include <netinet/in.h>
    ...
   struct sockaddr_in sin;
    ...
   s = socket(AF_INET, SOCK_STREAM, 0);
   sin.sin_len = sizeof(sin);
   sin.sin_family = AF_INET;
   sin.sin_addr.s_addr = htonl(INADDR_ANY);
   sin.sin_port = htons(MYPORT);
   bind(s, (struct sockaddr *) &sin, sizeof(sin));

In a similar fashion, a local port may be left unspecified (specified as zero), in which case the system will select an appropriate port number for it. For example, to bind a specific local address to a socket, but to leave the local port number unspecified:

   hp = gethostbyname(hostname);
   if (hp == NULL) {
   	...
   }
   memcpy((char *) sin.sin_addr, hp->h_addr, hp->h_length);
   sin.sin_port = htons(0);
   bind(s, (struct sockaddr *) &sin, sizeof(sin));

   int lport = IPPORT_RESERVED - 1;
   int s;
   	...
   s = rresvport(&lport);
   if (s < 0) {
   	if (errno == EAGAIN)
   		fprintf(stderr, "socket: all ports in use\n");
   	else
   		perror("rresvport: socket");
   	...
   }

In certain cases the algorithm used by the system in selecting port numbers is unsuitable for an application. This is because associations are created in a two step process. For example, the Internet file transfer protocol, FTP, specifies that data connections must always originate from the same local port. However, duplicate associations are avoided by connecting to different foreign ports. In this situation the system would disallow binding the same local address and port number to a socket if a previous data connection's socket still existed. To override the default port selection algorithm, an option call must be performed before address binding:

    ...
   int	on = 1;
    ...
   setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
   bind(s, (struct sockaddr *) &sin, sizeof(sin));