Editing Berkeley sockets (section)

==Client-server example using UDP==
The [[User Datagram Protocol]] (UDP) is a [[connectionless]] protocol with no guarantee of delivery. UDP packets may arrive out of order, multiple times, or not at all. Because of this minimal design, UDP has considerably less overhead than TCP. Being connectionless means that there is no concept of a stream or permanent connection between two hosts. Such data are referred to as datagrams ([[datagram socket]]s).

UDP address space, the space of UDP port numbers (in ISO terminology, the [[TSAP]]s), is completely disjoint from that of TCP ports.

===Server===
An application may set up a UDP server on port number 7654 as follows. The programs contains an infinite loop that receives UDP datagrams with function ''recvfrom()''.

<syntaxhighlight lang="c" highlight="32,25,24">
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h> /* for close() for socket */ 
#include <stdlib.h>

int main(void)
{
  int sock;
  struct sockaddr_in sa; 
  char buffer[1024];
  ssize_t recsize;
  socklen_t fromlen;

  memset(&sa, 0, sizeof sa);
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = htonl(INADDR_ANY);
  sa.sin_port = htons(7654);
  fromlen = sizeof sa;

  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (bind(sock, (struct sockaddr *)&sa, sizeof sa) == -1) {
    perror("error bind failed");
    close(sock);
    exit(EXIT_FAILURE);
  }

  for (;;) {
    recsize = recvfrom(sock, (void*)buffer, sizeof buffer, 0, (struct sockaddr*)&sa, &fromlen);
    if (recsize < 0) {
      fprintf(stderr, "%s\n", strerror(errno));
      exit(EXIT_FAILURE);
    }
    printf("recsize: %d\n ", (int)recsize);
    sleep(1);
    printf("datagram: %.*s\n", (int)recsize, buffer);
  }
}
</syntaxhighlight>

===Client===
The following is a client program for sending a UDP packet containing the string "Hello World!" to address 127.0.0.1 at port number 7654.

<syntaxhighlight lang="c" highlight="40">
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h>
#include <arpa/inet.h>

int main(void)
{
  int sock;
  struct sockaddr_in sa;
  int bytes_sent;
  char buffer[200];
 
  strcpy(buffer, "hello world!");
 
  /* create an Internet, datagram, socket using UDP */
  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (sock == -1) {
      /* if socket failed to initialize, exit */
      printf("Error Creating Socket");
      exit(EXIT_FAILURE);
  }
 
  /* Zero out socket address */
  memset(&sa, 0, sizeof sa);
  
  /* The address is IPv4 */
  sa.sin_family = AF_INET;
 
   /* IPv4 addresses is a uint32_t, convert a string representation of the octets to the appropriate value */
  sa.sin_addr.s_addr = inet_addr("127.0.0.1");
  
  /* sockets are unsigned shorts, htons(x) ensures x is in network byte order, set the port to 7654 */
  sa.sin_port = htons(7654);
 
  bytes_sent = sendto(sock, buffer, strlen(buffer), 0,(struct sockaddr*)&sa, sizeof sa);
  if (bytes_sent < 0) {
    printf("Error sending packet: %s\n", strerror(errno));
    exit(EXIT_FAILURE);
  }
 
  close(sock); /* close the socket */
  return 0;
}
</syntaxhighlight>

In this code, ''buffer'' is a pointer to the data to be sent, and ''buffer_length'' specifies the size of the data.