Sockets Programming in C Using UDP Datagrams


Below you’ll find the code of a simple server-client program in C using UDP sockets for the transmission. Basically the client sends a message to the server, the server converts the message to uppercase and returns it to the client.

sockets-programming-c

If you want to see a simpler program first check this client-server program that only sends a “Hello World” message. The functions and structs are better explained there.

Second, you can also see this same (server converting message to uppercase) example using TCP here.

Server Code

/************* UDP SERVER CODE *******************/

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>

int main(){
  int udpSocket, nBytes;
  char buffer[1024];
  struct sockaddr_in serverAddr, clientAddr;
  struct sockaddr_storage serverStorage;
  socklen_t addr_size, client_addr_size;
  int i;

  /*Create UDP socket*/
  udpSocket = socket(PF_INET, SOCK_DGRAM, 0);

  /*Configure settings in address struct*/
  serverAddr.sin_family = AF_INET;
  serverAddr.sin_port = htons(7891);
  serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
  memset(serverAddr.sin_zero, '\0', sizeof serverAddr.sin_zero);  

  /*Bind socket with address struct*/
  bind(udpSocket, (struct sockaddr *) &serverAddr, sizeof(serverAddr));

  /*Initialize size variable to be used later on*/
  addr_size = sizeof serverStorage;

  while(1){
    /* Try to receive any incoming UDP datagram. Address and port of 
      requesting client will be stored on serverStorage variable */
    nBytes = recvfrom(udpSocket,buffer,1024,0,(struct sockaddr *)&serverStorage, &addr_size);

    /*Convert message received to uppercase*/
    for(i=0;i<nBytes-1;i++)
      buffer[i] = toupper(buffer[i]);

    /*Send uppercase message back to client, using serverStorage as the address*/
    sendto(udpSocket,buffer,nBytes,0,(struct sockaddr *)&serverStorage,addr_size);
  }

  return 0;
}

Client Code

/************* UDP CLIENT CODE *******************/

#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>

int main(){
  int clientSocket, portNum, nBytes;
  char buffer[1024];
  struct sockaddr_in serverAddr;
  socklen_t addr_size;

  /*Create UDP socket*/
  clientSocket = socket(PF_INET, SOCK_DGRAM, 0);

  /*Configure settings in address struct*/
  serverAddr.sin_family = AF_INET;
  serverAddr.sin_port = htons(7891);
  serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
  memset(serverAddr.sin_zero, '\0', sizeof serverAddr.sin_zero);  

  /*Initialize size variable to be used later on*/
  addr_size = sizeof serverAddr;

  while(1){
    printf("Type a sentence to send to server:\n");
    fgets(buffer,1024,stdin);
    printf("You typed: %s",buffer);

    nBytes = strlen(buffer) + 1;
    
    /*Send message to server*/
    sendto(clientSocket,buffer,nBytes,0,(struct sockaddr *)&serverAddr,addr_size);

    /*Receive message from server*/
                nBytes = recvfrom(clientSocket,buffer,1024,0,NULL, NULL);

    printf("Received from server: %s\n",buffer);

  }

  return 0;
}

9 thoughts on “Sockets Programming in C Using UDP Datagrams

  1. shiva

    In client program,what value should i give for “serverAddr.sin_addr.s_add” if i run these two programs in different machines.

    Reply
  2. Johannes Peter

    Similar to the TCP code, there’s an unused size variable, here called ‘client_addr_size’. What were you originally intending to do with it?

    Reply
  3. Krunal Sonparate

    Some libraries are missing in the above code to use some functionality
    In server Code
    #include
    #include
    In Client Code
    #include

    Reply
  4. Krunal Sonparate

    Correction in my above comment
    Your comment is awaiting moderation.
    Some libraries are missing in the above code to use some functionality
    In server Code
    #include <arpa/inet.h>
    #include <ctype.h>
    In Client Code
    #include <arpa/inet.h>

    Reply

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