Connectionless client

The client uses the following socket or POSIX interface functions in this program example:

• socket(): create socket

• gethostbyname(): get the host name entry

• sendto(): send a message to a socket

• close(): close socket

Example: connectionless client

#include <stdio.h>
#include <sys/socket.h>
#include <ioctl.h>
#include <signal.h>
#include <netinet/in.h>
#include <netdb.h>
#define DATA " The sea is calm, the tide is full ..."
#define ERROR_EXIT(M) perror(M); exit(1)
#define TESTPORT 2222
int main(int argc, char **argv)
{
    int                 sock;
    struct sockaddr_in  to;
    struct hostent     *hp;
    /* Create the client socket to be sent from. */
    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        ERROR_EXIT("socket");
    }
    /* Construct the name of the server socket to be sent to. */
    if ((hp =gethostbyname(argv[1])) == NULL) {
        fprintf(stderr, "%s: unknown host\n", argv[1]);
        exit(1);
    }
    memcpy((char *)&to.sin_addr, (char *)hp->h_addr,hp->h_length);
    to.sin_family = AF_INET;
    to.sin_port = htons(TESTPORT);
    /* Send message. */
    if (sendto(sock, DATA, sizeof DATA, 0, (struct sockaddr *)&to, sizeof to) < 0) {
         ERROR_EXIT("sendto");
    }
    close(sock);
    return 0;
}

The following steps are executed in the program example:

• The socket() function creates a communications endpoint (client socket) and a corresponding descriptor.

• The IP address of the server is determined with gethostbyname(). The computer name is passed as a parameter when the program is called.

• Then the server address structure is initialized.

• The sendto() function sends a datagram from the client socket to the server socket. It returns the number of characters sent.

• The close() function closes the client socket.

The example program is only valid for the communications domain AF_INET. If it is modified according to the information in " Socket addressing " and " Creating a socket ", it is also valid for the AF_INET6 domain. Please also make sure that you use the  getaddrinfo()  or  getipnodebyname()  functions instead of  gethostbyname() .