Shell job control

Question

For my school project I am implementing a shell and I need help with job control. If we type a command, say cat &, then because of the & it should run in background, but it's not working. I have this code:

{
  int pid;  
  int status;  
  pid = fork();  
  if (pid == 0) {  
    fprintf(stderr, "Child Job pid = %d\n", getpid());  
    execvp(arg1, arg2);  
  } 
  pid=getpid();  
  fprintf(stderr, "Child Job pid is = %d\n", getpid());      
  waitpid(pid, &status, 0);  
}

Answer 1

Rather than just going straight to waiting, you should set up a signal handler for the SIGCHLD signal. SIGCHLD is sent whenever a child process stops or is terminated. Check out the GNU description of process completion.

The end of this article has a sample handler (which I've more or less copied and pasted below). Try modeling your code off of it.

 void sigchld_handler (int signum) {
     int pid, status, serrno;
     serrno = errno;
     while (1) {
         pid = waitpid(WAIT_ANY, &status, WNOHANG);
         if (pid < 0) {
             perror("waitpid");
             break;
         }
         if (pid == 0)
           break;
         /* customize here.
            notice_termination is in this case some function you would provide
            that would report back to your shell.
         */             
         notice_termination (pid, status);
     }
     errno = serrno;
 }

Another good source of information on this subject is Advanced Programming in the UNIX Environment, chapters 8 and 10.

Answer 2

The parent process is calling waitpid on the child, which will block until the child process changes state (i.e. terminates).