The wait System Calls

• The simplest such function is called simply wait. It blocks the calling process until one of its child processes exits (or an error occurs).
• It returns a status code via an integer pointer argument, from which you can extract information about how the child process exited.
  • For instance, the WEXITSTATUS macro extracts the child process's exit code.
  • You can use the WIFEXITED macro to determine from a child process's exit status whether that process exited normally (via the exit function or returning from main) or died from an unhandled signal.

  int main ()
  {
  int child_status;
  /* The argument list to pass to the "ls" command. */
  char* arg_list[] = {
  "ls", /* argv[0], the name of the program. */
  "-l",
  "/",
  NULL /* The argument list must end with a NULL. */
  };
  /* Spawn a child process running the "ls" command. Ignore the
  returned child process ID. */
  spawn ("ls", arg_list);
  /* Wait for the child process to complete. */
  wait (&child_status);
  if (WIFEXITED (child_status))
  printf ("the child process exited normally, with exit code %d\n",
  WEXITSTATUS (child_status));
  else
  printf ("the child process exited abnormally\n");
  return 0;
  }
  

• The waitpid function can be used to wait for a specific child process to exit instead of any child process.
• The wait3 function returns CPU usage statistics about the exiting child process, and the wait4 function allows you to specify additional options about which processes to wait for.