#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>

#define SOCKETNAME "MySocket"

int main(void) {
    struct sockaddr_un sa;
    (void)unlink(SOCKETNAME);
    strcpy(sa.sun_path, SOCKETNAME);
    sa.sun_family = AF_UNIX;
    int fd_skt, fd_client; char buf[100]; 
    int written; ssize_t readb;
    
    
    if (fork() == 0) { /* client */
        if ((fd_skt = socket(PF_UNIX, SOCK_STREAM, 0)) < 0) {
             perror("Message from socket [client] : ");
             exit(-1); }        
        printf ("[Client] ==> Socket %d has been created\n", fd_skt);
        // Since cliend and server run concurrently, it is possible for the client
        // to call connect before the creation of the socket file by the server (ENOENT).
        // In this case the function sleeps for a second and tries again. If something
        // else goes wrong, the function fails.
        
        while (connect(fd_skt, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
            if (errno == ENOENT) {
                sleep(1); continue; }
            else {
             perror("Message from connect [client]");
             exit(-2); }}
        // After connection the client and the server can communicate each other
        // using read and write functions 
        printf ("[Client] ==> Connection has been established .. let us write to server\n");
        written = write(fd_skt, "Hello!", 7);
        if (written==-1) {
             perror("Message from write [client]");
             exit(-3); }
        else printf ("[Client] ==> %d bytes written to server\n", written);
        printf ("[Client] ==> Now let us read from server\n");             
        readb = read(fd_skt, buf, sizeof(buf));
        if (readb==-1) {
             perror("Message from read [client]");
             exit(-4); } 
        else printf ("[Client] ==> %zd bytes read from server\n", readb);     
        printf("Client got %s\n", buf);
        close(fd_skt);
        exit(0); }
        
    else { /* server */
  

        if ((fd_skt = socket(PF_UNIX, SOCK_STREAM, 0)) < 0) {
             perror("Message from socket [server]");
             exit(-1); }    
             
        printf ("[Server] ==> Socket %d has been created\n", fd_skt);
        if (bind(fd_skt, (struct sockaddr *) &sa, sizeof(sa))){
            perror("Message from bind [server]");
            exit(-2); }    
        printf ("[Server] ==> Socket %d has been bound to address\n", fd_skt);                
        if (listen(fd_skt, 5)){
            perror("Message from listen [server]");
            exit(-3); }
        printf ("[Server] ==> Listening for incoming messages\n");    
        if ((fd_client = accept(fd_skt, NULL, 0))<0) {
            perror("Message from accept [server]"); 
            exit(-4); }
        printf ("[Server] ==> let us read from client via socket %d\n", fd_client);            
        readb = read(fd_client, buf, sizeof(buf));
        if (readb == -1) {
             perror("Message from read [server] : ");
             exit(-5); } 
        printf("Server got %s ==> %zd bytes read from client\n", buf, readb);
        printf ("[Server] ==> let us write to client\n");           
        if (write(fd_client, "Goodbye!", 9)==-1) {
             perror("Message from write [server]");
             exit(-6); } 
        close(fd_skt);
        close(fd_client);
        exit(0); }}