msg_server.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

void handle_request(int cfd){
    char prompt[] = "Enter message: ";
    char msg[200];
    write(cfd, prompt, strlen(prompt));
    read(cfd, msg, 1024);
    // process the message.
}

int main(int argc, char const *argv[])
{
    int PORT=0;
    if(argc > 1){
        PORT = atoi(argv[1]);                        // get port to listen on from first argument.
    }
    if(!PORT){
        PORT=8888;
    }
    int sfd,opt=1,cfd;
    if((sfd = socket(AF_INET, SOCK_STREAM, 0)) < 0){ // initialize tcp server socket file descriptor.
        printf("[!] socket failed.\n");
        exit(-1);
    }
    struct sockaddr_in s_addr;                      // struct to hold server socket configurations.
    if(setsockopt(sfd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt)) < 0){    // So that multiple clients can connect to server at same time.
        printf("[!] setsockopt failed.\n");
        exit(-1);
    }
    s_addr.sin_family=AF_INET;
    s_addr.sin_port=htons(PORT);
    s_addr.sin_addr.s_addr=INADDR_ANY;              // any available address (listens on all interfaces i.e. 0.0.0.0)
    if(bind(sfd,(struct sockaddr*)&s_addr, sizeof(s_addr)) < 0){    // bind to the port
        printf("[!] bind failed.\n");
        exit(-1);
    }
    if(listen(sfd,10) < 0){                         // start listening for connections.
        printf("[!] listen failed.\n");
        exit(-1);
    }
    char sip[INET_ADDRSTRLEN];
    inet_ntop(AF_INET, &(s_addr.sin_addr), sip, INET_ADDRSTRLEN); // server ip address
    printf("[i] Listening on %s:%d, sfd is %d\n", sip, PORT, sfd);
    struct sockaddr_in c_addr;                      // struct to hold client.
    socklen_t addr_len = sizeof(c_addr);
    while(1){
        if((cfd = accept(sfd,(struct sockaddr*)&c_addr,(socklen_t*)&addr_len)) < 0){ // accept connection from client
            printf("[!] accept failed.\n");
        }
        else{
            pid_t pid;
            if((pid=fork()) == 0){                  // fork, create new child process to handle the request.
                close(sfd);                         // close server file descriptor in child.
                char cip[INET_ADDRSTRLEN];
                int cport = ntohs(c_addr.sin_port); // client port
                inet_ntop(AF_INET, &(c_addr.sin_addr), cip, INET_ADDRSTRLEN);   // client ip address
                printf("[*] Accepted, cfd %d from %s:%d, pid: %d\n", cfd, cip, cport, getpid());
                handle_request(cfd);
                char closing[] = "Request complete, Closing...\n";
                write(cfd, closing, strlen(closing)); // closing message.
                close(cfd);
                exit(0);                            // exit child after client is served.
            }
            else{
                signal(SIGCHLD,SIG_IGN);            // ignore exit status of child to kill zombie child.
                close(cfd);                         // close connection in parent process.
            }
        }
    }
    close(sfd);                                     // program won't actually reach this.
    return 0;
}