bind
Prototype: 
#include <sys/socket.h>
#include <resolv.h>
int bind(int sockfd, struct sockaddr* addr,
    int addrlen);
General Description: Defines a port or name to a socket. If you want a consistent port for an incoming connection, you have to bind your socket to a port. In most instances, the kernel automatically calls bind() for the socket if you do not call it explicitly. The kernel-generated port assignment may be different from execution to execution.
Return Value: Zero (0) if everything goes well. If an error occurs, you can find the cause in errno.
Parameters
sockfd The socket descriptor to bind.
addr The port assignment or name.
addrlen The length of addr, because addr may be different sizes.
Possible Errors
EBADF sockfd is not a valid descriptor.
EINVAL The socket is already bound to an address. This may change in the future: see .../linux/unix/socket.c for details.
EACCES The address is protected, and the user is not the super-user.
ENOTSOCK Argument is a descriptor for a file, not a socket.
Examples 
int sockfd;
struct sockaddr_in addr;
sockfd = socket(PF_INET, SOCKET_STREAM, 0);
bzero(&addr, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons(9999);  /* or whatever port you want */
/* to bind any network interface */
addr.sin_addr.s_addr = INADDR_ANY;
/* -or- to bind a specific interface, use this: */
/* inet_aton("128.1.1.1", &addr.sin_addr); */
if ( bind(sockfd, &addr, sizeof(addr)) != 0 )
    perror("bind");