Server socket accepting connection only once

Question

I am learning to work with C sockets, and have created very basic client and server programs for the same

Client Program

#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>          // contains the type definitions related to sockets
#include <sys/socket.h>         // contains the socket API

#include <netinet/in.h>         // used to store the address information

int main() {
    int network_socket;         // the socket is represented by an integer value, which can be used later to access the socket
    network_socket = socket(AF_INET, SOCK_STREAM, 0);           // opening a socket, defining the domain of the socket, type of socket, and protocol used

    struct sockaddr_in server_address;          // creating an address structure for the server
    server_address.sin_family = AF_INET;        // specifying the type of socket as internet
    server_address.sin_port = htons(9002);      // specifying the port number
    server_address.sin_addr.s_addr = INADDR_ANY; // specifying the address 

    int connection_status = connect(network_socket, (struct sockaddr*) &server_address, sizeof(server_address));        // The function to establish a connection
    if(connection_status == -1) {
        printf("Connetion Failed");
    }

    char server_response[256];
    recv(network_socket, &server_response, sizeof(server_response), 0);

    printf("The server sent the data: %s", server_response);

    return 0;
}

Server Program

#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/socket.h>

#include <netinet/in.h>

int main() {
    char server_message[256] = "You have reached the server\n";

    int server_socket;
    server_socket = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in server_address;
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = INADDR_ANY;

    bind(server_socket, (struct sockaddr*) &server_address, sizeof(server_address));
    listen(server_socket, 5);       // 5 indicates the backlog, i.e. how many sockets can be waiting for this connection at a given point of time

    int client_socket;
    client_socket = accept(server_socket, NULL, NULL);

    send(client_socket, server_message, sizeof(server_message), 0);

    return 0;
}

The given programs work well for the first time that I compile and run both of them, and I get the desired response upon running the client program

The server sent the data: You have reached the server

But when I try running the client program once again, I get the following message

Connection FailedThe server sent the data:

Even when I terminate the running server program, re-compile it and run it again, the same thing happens

Answer 1

I modified your server to support multiple clients sequentially using a loop.For supporting multiple concurrent clients, you have to use threads or NIO (e.g., epoll() or select()).

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <error.h>
#include <errno.h>

#define ERROR  -1
#define BACKLOG 5
#define NOEXIT  0
#define EXIT    1

const char *server_message = "You have reached the server\n";

int open_server_socket(int port) {
    int server_socket = socket(AF_INET, SOCK_STREAM, 0);  
    if (server_socket==ERROR) error(EXIT,errno, "socket");
    
    struct sockaddr_in server_address;
    server_address.sin_family = AF_INET;
    server_address.sin_port = port;
    server_address.sin_addr.s_addr = INADDR_ANY;
      
    if (bind(server_socket, (struct sockaddr*) &server_address,
             sizeof(server_address)) ==ERROR) error(EXIT, errno, "bind");

    if (listen(server_socket, BACKLOG) ==  ERROR) error(EXIT, errno, "listen");
    
    return server_socket;
}

void handle_client(int server_socket) {
  int client_socket = accept(server_socket, NULL, NULL);
  if (client_socket == ERROR) goto END;
  
  if (write(client_socket, server_message, sizeof(server_message) == ERROR))
    goto END;
  
  close(client_socket); 
  
 END:
  if(errno) error(NOEXIT, errno, "client");
}

int main() {
  int server_socket = open_server_socket(9005); 
  while(true) handle_client(server_socket);
}

In addition, I have mildly addressed the lack of error handling issue that was pointed out by Some programmer dude.