UDP Client Server using connect | C implementation

Last Updated : 17 Apr, 2018

UDP is a connection less protocol. There is no connection is established between client and server. Creating Standard UDP Client/Server is discussed here Prerequisite : Socket Programming in C/CPP | UDP server-client implementation In UDP, the client does not form a connection with the server like in TCP and instead, It just sends a datagram. Similarly, the server need not to accept a connection and just waits for datagrams to arrive. We can call a function called connect() in UDP but it does not result anything like it does in TCP. There is no 3 way handshake. It just checks for any immediate errors and store the peer's IP address and port number. connect() is storing peers address so no need to pass server address and server address length arguments in sendto().

Necessary Functions :

int connect(int sockfd, const struct sockaddr *servaddr,
               socklen_t addrlen);
returns : 0 if OK -1 on error
arguments :
sockfd : File descriptor of socket to be connected.
struct sockaddr *servaddr : server address structure.
addrlen : length of server address structure.

Below is the implementation showing message transfer between server-client : UDP Server code :

// server program for udp connection
#include <stdio.h>
#include <strings.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include<netinet/in.h>
#define PORT 5000
#define MAXLINE 1000

// Driver code
int main()
{   
    char buffer[100];
    char *message = "Hello Client";
    int listenfd, len;
    struct sockaddr_in servaddr, cliaddr;
    bzero(&servaddr, sizeof(servaddr));

    // Create a UDP Socket
    listenfd = socket(AF_INET, SOCK_DGRAM, 0);        
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(PORT);
    servaddr.sin_family = AF_INET; 
 
    // bind server address to socket descriptor
    bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr));
     
    //receive the datagram
    len = sizeof(cliaddr);
    int n = recvfrom(listenfd, buffer, sizeof(buffer),
            0, (struct sockaddr*)&cliaddr,&len); //receive message from server
    buffer[n] = '\0';
    puts(buffer);
         
    // send the response
    sendto(listenfd, message, MAXLINE, 0,
          (struct sockaddr*)&cliaddr, sizeof(cliaddr));
}

UDP Client code :

// udp client driver program
#include <stdio.h>
#include <strings.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include<netinet/in.h>
#include<unistd.h>
#include<stdlib.h>

#define PORT 5000
#define MAXLINE 1000

// Driver code
int main()
{   
    char buffer[100];
    char *message = "Hello Server";
    int sockfd, n;
    struct sockaddr_in servaddr;
    
    // clear servaddr
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    servaddr.sin_port = htons(PORT);
    servaddr.sin_family = AF_INET;
    
    // create datagram socket
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    
    // connect to server
    if(connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0)
    {
        printf("\n Error : Connect Failed \n");
        exit(0);
    }

    // request to send datagram
    // no need to specify server address in sendto
    // connect stores the peers IP and port
    sendto(sockfd, message, MAXLINE, 0, (struct sockaddr*)NULL, sizeof(servaddr));
    
    // waiting for response
    recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr*)NULL, NULL);
    puts(buffer);

    // close the descriptor
    close(sockfd);
}