From Forum Nokia Wiki

The send, sendto, sendmsg methods helps in sending a message from a socket.

ssize_t send (int s, const void *msg, size_t len, int flags);
 
ssize_t sendto (int s, const void *msg, size_t len, int flags, const struct sockaddr *to,
                socklen_t tolen);
 
 ssize_t sendmsg (int s, const struct msghdr *msg, int flags);

The send function, and sendto and sendmsg system calls are used to transmit a message to another socket. The send function may be used only when the socket is in a connected state, while sendto and sendmsg may be used at any time.

Following code snippets shows the usage of send, sendto, sendmsg functions:

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Recv()
{
   struct sockaddr_in serv_addr;
   int sock_fd;
   char line[15] = "Hello world!";
   int size = 13;
   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   serv_addr.sin_port = htons(5000);
   sock_fd = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
   connect(sock_fd,(struct sockaddr*)&serv_addr,sizeof(serv_addr));
   send(sock_fd, line, size, 0);
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Sendto()
{
   sockaddr_in receiver_addr;
   int sock_fd;
   char line[15] = "Hello World!";
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
   receiver_addr.sin_family = AF_INET;
   receiver_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   receiver_addr.sin_port = htons(5000);
   sendto(sock_fd, line, 13, 0,(struct sockaddr*)&receiver_addr,sizeof(receiver_addr));
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void sendmsg()
{
   struct sockaddr_in receiver_addr;
   int sock_fd;
   char line[15] = "Hello World!";
   struct msghdr msg;
   struct iovec iov;
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
    
   receiver_addr.sin_family = AF_INET;
   receiver_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   receiver_addr.sin_port = htons(5000);
   msg.msg_name = &receiver_addr;
   msg.msg_namelen = sizeof(receiver_addr);
   msg.msg_iov = &iov;
   msg.msg_iovlen = 1;
   msg.msg_iov->iov_base = line;
   msg.msg_iov->iov_len = 13;
   msg.msg_control = 0;
   msg.msg_controllen = 0;
   msg.msg_flags = 0;
   sendmsg(sock_fd,&msg,0);
   close(sock_fd);
}

Wiki Links

Open C Sockets Overview