listen
Prototype:
#include <sys/socket.h>
#include <resolv.h>
int listen(int sockfd, int queue_len);
General Description: Convert the socket into a listening socket. This option is only available to SOCK_STREAM protocols. The call creates a queue of incoming connections.
Return Value: Zero (0) if everything goes well. If an error occurs, you can find the cause in errno.
Parameters
sockfd A SOCK_STREAM socket which has been bound to a port.
queue_len The maximum number of pending connections.
Possible Errors
EBADF The argument sockfd is not a valid descriptor.
ENOTSOCK The argument sockfd is not a socket.
EOPNOTSUPP The socket is not of a type that supports the listen() operation. If you supply any socket that is not SOCK_STREAM, you get this error.
Examples
int sockfd;
sockfd = socket(PF_INET, SOCK_STREAM, 0);
/*---set up address with bind()---*/
listen(sockfd, 10);  /* create a 10-pending queue */