UDP Client Server using connect | C implementation

Last Updated : 09 Dec, 2025

UDP is a connectionless protocol. No connection is established between the client and the server. In UDP, the client does not form a connection like TCP; instead, it simply sends datagrams. Similarly, the server does not accept connections and only waits for incoming datagrams.

Although the connect() function can be used with UDP, it does not perform a three-way handshake like TCP. Instead, it only associates a default peer address (IP and port) with the socket. After calling connect(), the application can use send() and recv() instead of sendto() and recvfrom().

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, strlen(message), 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);
}