Question

I am working on some code to create a process that goes blocked and then ends, I have to be able to see the blocked state with ps.

I tried with this, but my C knowledge is not good. The code doesn’t print anything.

Here it is:

#include <stdio.h>
#include <stdlib.h> //exit();
#include <unistd.h> //sleep();

int main(int argc, char *argv[]) {
    createblocked();
}

int pid;
int i;
int estado;

void createblocked() {
   pid = fork();

   switch( pid ) {
      case -1: // pid -1 error ocurred
         perror("error\n");
      break;
      case 0: // pid 0 means its the child process
         sleep(); // we put the child to sleep so the parent will be blocked.
         printf("child sleeping...");
      break;
      default: // !=0 parent process
         // wait function puts parent to wait for the child
         // the child is sleeping so the parent will be blocked 
         wait( estado );
         printf("parent waiting...\n");
         printf("Child terminated.\n");
         break;
   }
   exit(0);
}

It should be easy because its only a little program that goes blocked, but I am walking in circles I think. Any advice?

Answer 1

sleep() takes a parameter: the number of seconds to sleep. When you omit it, it tends to return immediately.

Also wait() takes an int *, not an int.

try this:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    createblocked();
}

int pid;
int i;
int estado;

void createblocked() {
    pid = fork();

    switch(pid)
    {
        case -1: // pid -1 error ocurred
            perror("error\n");
            break;
        case 0: // pid 0 means its the child process
            printf("child sleeping...\n");
            sleep(500); // we put the child to sleep so the parent will be blocked.
            break;
        default: // !=0 parent process
            // wait function puts parent to wait for the child
            // thechild is sleeping so the parent will be blocked 
            printf("parent waiting...\n");
            wait(&estado);
            printf("Child terminated.\n");
            break;

    }
    exit(0);
}

note: I also moved the printf("parent waiting...\n") above the call to wait(), so you should see it before the parent blocks waiting on the child.

edit: Also, include <unistd.h>. While not strictly required in order for the program to work (on most systems), doing so will give you better compile-time error reporting for things like missing and/or incorrectly-typed function arguments.