send
Prototype: 
#include <sys/socket.h>
#include <resolv.h>
int send(int sockfd, char *buffer, int msg_len,
    int options);
General Description: Send a message to the connected peer, client or server. This call is like the write() system call, but permits you to define additional options for channel control.
Return Value: Like write(), the call returns the number of bytes written. The byte count may be less that msg_len. If call does not succeed in writing all required bytes, you may use a loop for successive writes. If negative, the call stores the error detail in errno.
Parameters
sockfd The socket channel. This may be a connected SOCK_DGRAM or SOCK_STREAM socket.
buffer The data to send.
msg_len The number of bytes to send.
options A set of flags to enable special message handling:
  • MSG_OOB - Send message out-of-band (urgent).
  • MSG_DONTROUTE - Send the message bypassing all routers. If unsuccessful, the network sends an error back.
  • MSG_DONTWAIT - Don't allow blocking. Similar to the fcntl() option call, but only applies to this call. If the call would block, the call returns with EWOULDBLOCK in errno.
  • MSG_NOSIGNAL - If the peer severs the connection, don't raise a SIGPIPE signal locally.
Possible Errors
EBADF An invalid descriptor was specified.
ENOTSOCK The argument sockfd is not a socket.
EFAULT An invalid user space address was specified for buffer.
EMSGSIZE The call can't complete, because the socket requires the message be sent atomically, and the size of the message to be sent has made this impossible.
EAGAIN The socket is marked non-blocking and the requested operation would block.
ENOBUFS The system was unable to allocate an internal memory block. The operation may succeed when buffers become available.
EINTR A signal occurred.
ENOMEM No memory available.
EINVAL Invalid argument passed.
EPIPE The local end has been shut down on a connected socket. In this case the process will also receive a SIGPIPE unless MSG_NOSIGNAL is set.
Examples 
int sockfd;
int bytes, bytes_wrote=0;
/*--- Create socket, connect to server/peer ---*/
while ( (bytes = send(sockfd, buffer, msg_len, 0)) > 0 )
    if ( (bytes_wrote += bytes) >= msg_len )
        break;
if ( bytes < 0 )
    perror("send");


int sockfd;
int bytes, bytes_wrote=0;
/*--- Create socket, connect to server ---*/
if ( send(sockfd, buffer, 1, MSG_OOB) != 1 )
    perror("Urgent message");