Thread: Processes - wait()

How is WEXITSTATUS(stat_val) obtaining the value 37? I know that WEXITSTATUS is used to obtain the child's exit code and the exit_code variable is assigned 37 in this program but i don't see how this WEXITSTATUS is working. Is the WEXITSTATUS function written to look through the child process code for an exit_code variable?

Code:

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
int main()
{

  pid_t pid;
  char *message;
  int n;
  int exit_code;
  printf(“fork program starting\n”);
  pid = fork();
  switch(pid)
  {
    case -1:
      perror(“fork failed”);
      exit(1);
    case 0:
      message = “This is the child”;
      n = 5;
      exit_code = 37;
      break;
    default:
      message = “This is the parent”;
      n = 3;
      exit_code = 0;
      break;
  }
  for(; n > 0; n--) {
    puts(message);
    sleep(1);
  }
  //This section of the program waits for the child process to finish.
  if (pid != 0) {
    int stat_val;
    pid_t child_pid;
    child_pid = wait(&stat_val);
    printf(“Child has finished: PID = %d\n”, child_pid);
    if(WIFEXITED(stat_val))
      printf(“Child exited with code %d\n”, WEXITSTATUS(stat_val));
    else
      printf(“Child terminated abnormally\n”);
    }
  exit(exit_code);
}

The value given in exit/return is returned to the operating system. It's not looking for specific variable names, because quite frankly your executable doesn't have any variable names in it.

The exit_code variable is a member of "struct process_state" and when the child process returns this field is populated if the variable is set.
But it is baffling that ps->exit_code is being filled in with a variable of the same name w/o any explicit assignments ie totally out of context.

Originally Posted by itCbitC

The exit_code variable is a member of "struct process_state" and when the child process returns this field is populated if the variable is set.
But it is baffling that ps->exit_code is being filled in with a variable of the same name w/o any explicit assignments ie totally out of context.

What about "int exit_code" and "exit(exit_code)"?

Originally Posted by tabstop

What about "int exit_code" and "exit(exit_code)"?

That's precisely my dilemma!
"exit_code" is just another variable. It is not part of "struct process_state" a struct that has not been defined anywhere. So when the child returns it sets "exit_code" to 37 and the macro WEXITSTATUS prints it out as the return status of the child. The exit(exit_code) is being used in the parent unrelated to the same name variable in the child.

[Update] I think my compiler is driving me nuts
Just a few minutes ago it was capturing the lower 8 bits of the exit_code variable in the "child" case label and now it seems to have a mind of its own and is consistently printing 120. Go figure!

Originally Posted by itCbitC

That's precisely my dilemma!
"exit_code" is just another variable. It is not part of "struct process_state" a struct that has not been defined anywhere. So when the child returns it sets "exit_code" to 37 and the macro WEXITSTATUS prints it out as the return status of the child. The exit(exit_code) is being used in the parent unrelated to the same name variable in the child.

[Update] I think my compiler is driving me nuts
Just a few minutes ago it was capturing the lower 8 bits of the exit_code variable in the "child" case label and now it seems to have a mind of its own and is consistently printing 120. Go figure!

Both the parent and child do exit(exit_code). And I was (and still am) unaware that WEXITSTATUS has anything to do with struct process_state. It might, I suppose, but this is what I've found on t'internet:

Originally Posted by man wait

WEXITSTATUS(status)
returns the exit status of the child. This consists of the least significant 16-8 bits of the status argument that the child specified in a call to exit() or _exit() or as the argument for a return statement in main(). This macro should only be employed if WIFEXITED returned true.