How to use fork() to call subroutines in parallel using C - c

Question

I have the following code in which I'm running a server using a TCP port. I need to fork and place calls to a subroutine to invoke clients so that they can run in parallel and connect with the server.
As of now, I have just been able to make serial implementation in which calls are placed through a loop, I've just seem to have hit a road block, it will be great if someone can take the pain to go through the code and guide me.
The following is the main. The client subroutine just resides in client.c and seems to work fine. If need be I can paste that too.
main(int argc, char *argv[])
{
struct sockaddr_in manager, client;
struct hostent *cp;
int sockdescriptor, td;
int len;
char buf[BLEN];
int j;
int n;
int num_nodes;
pid_t pid;
key_t key;
int shmid;
int *port_num;
sockdescriptor = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
memset((char *) &manager, 0, sizeof(struct sockaddr_in));
manager.sin_family = AF_INET;
manager.sin_addr.s_addr = INADDR_ANY;
manager.sin_port = htons((u_short) 0); /* dynamically assigning port */
bind(sockdescriptor, (struct sockaddr *) &manager, sizeof(struct sockaddr_in));
len = sizeof(struct sockaddr_in);
listen(sockdescriptor, QUELEN);
/***************************** Getting port by getsockname() **********************************/
/* */
/* */
if(getsockname(sockdescriptor, (struct sockaddr *) &manager, &len) == -1){
perror("getsockname failed!");
return -1;
}
/* */
/* */
/**********************************************************************************************/
printf("manager port %d\n", (int) ntohs(manager.sin_port));
/********************************* Creating Shared Memory *************************************/
/* */
/* */
key = 1234;
if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0){
perror("shmget is broken!");
exit(1);
}
if ((port_num = shmat(shmid, NULL, 0)) == (int *) -1){
perror("shmat is broken!");
exit(1);
}
*port_num = (int) ntohs(manager.sin_port);
/* */
/* */
/**********************************************************************************************/
for(j = 0; j < num_nodes ; j++){
if((pid = fork()) == 0){ // child process
while(1) {
len = sizeof(struct sockaddr_in);
td = accept(sockdescriptor, (struct sockaddr *) &client, &len);
// close(sockdescriptor); //closing listening socket
cp = gethostbyaddr((char *) &client.sin_addr, sizeof(struct in_addr), AF_INET);
// printf("Connected from %s\n", cp->h_name);
client_num++;
printf("client %d port %d\n",client_num, *port_num);
sprintf(buf, "%d",nonce);
send(td, buf, strlen(buf), 0);
n = recv(td, buf, sizeof(buf), 0);
printf("client %d says %s\n",client_num, buf);
close(td); /* client request processed, close this client's socket */
close(sockdescriptor);
exit(0);
} // end of while loop
}
// else if((pid = fork()) > 0){
client_prog(); // Calls to this subroutine need to be via forked processes
// close(td);
// exit(0);
// } // else if ends here
} // end of the for loop

Answer 1

To have num_nodes clients, do:
for(j = 0; j<num_nodes; j++)
if (fork() == 0) {
close(sockdescriptor);
client_prog();
exit(0);
}
Then to have a separate process handling each client connection, such that multiple clients can proceed in parallel, I suggest that you replace the entire for loop with something like this:
while(1) {
td = accept(sockdescriptor, ...);
client_num++;
if (fork() == 0) {
close(sockdescriptor);
/* handle client interaction here */
send(...) / receive(...)
exit(0);
} else {
close(td);
}
}
Note that there is a race condition: If num_nodes is greater than SOMAXCONN, it is possible that connections are dropped. Spawning the server process first does not eliminate the race condition. This sort of thing is safer with pipes, that are pre-opened before fork.