Question

I’m referring the below code snippet from this link:

while (1)
 {
   newsockfd = accept(sockfd,
               (struct sockaddr *) &cli_addr, &clilen);
   if (newsockfd < 0)
     error("ERROR on accept");
   pid = fork();
   if (pid < 0)
     error("ERROR on fork");
   if (pid == 0)
   {
     close(sockfd);
     dostuff(newsockfd);
     exit(0);
   }
   else
     close(newsockfd);
 } /* end of while */

void dostuff (int sock)
{
   int n;
   char buffer[256];

   bzero(buffer,256);
   n = read(sock,buffer,255);
   if (n < 0) error("ERROR reading from socket");
   printf("Here is the message: %s\n",buffer);
   n = write(sock,"I got your message",18);
   if (n < 0) error("ERROR writing to socket");
}

After the fork() call, there would be two processes – Parent and child.

For the parent process, the else part holds true, and so it will close newsockfd. But newsockfd is used by child process for read and write system calls in dostuff method. Won’t the read and write system calls fail in this case?

Answer 1

No, because during fork all open file descriptors are copied and they are not the same descriptors, they just point to the same file.

From the fork(2) manpage:

The child inherits copies of the parent’s set of open file
descriptors. Each file descriptor in the child refers to the same open
file description (see open(2)) as the corresponding file descriptor in
the parent. This means that the two descriptors share open file status
flags, current file offset, and signal-driven I/O attributes (see the
description of F_SETOWN and F_SETSIG in fcntl(2)).