How many processes will be created

Question

I have this piece of code and was wondering how many processes will be created. I am uncertain as I think because of the loop it will be 12 processes but also could be 8.

#include <unistd.h>
#include <sys/types.h>

int main() {
  pid_t childpid;
  int i;

  childpid = fork();

  for (i = 0; i < 3 && childpid == 0; i++) {

    if (childpid == -1) {
      perror("Failed to fork.");
      return 1;
    }

    fprintf(stderr, "A\n");

    childpid = fork();

    if (childpid == 0) {
      fprintf(stderr, "B\n");
      childpid = fork();
      fprintf(stderr, "C\n");
    }

  }

  return 0;
}

Answer 1

Yes, you can use aptitude to evaluate how many processes are created, but I let the program decide it for me.

The child and parent are synchronized using semaphore, and a pcount variable is used to keep track of the number of created processes. pcount is incremented whenever childpid is evaluated to zero. Below are the additions to your program.

#include <semaphore.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void)
{
    /* Initialize and setup a semaphore */

    sem_t* sema = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);

    if (sema == MAP_FAILED)
        exit(1);

    if (sem_init(sema, 1, 1) != 0)
        exit(1);

    /* Initialize pcount */

    int* pcount = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);

    if (pcount == MAP_FAILED)
        exit(1);

    *pcount = 1;

    printf("pcount = %d\n", *pcount);

    pid_t childpid;
    int i;

    childpid = fork();

    if (childpid == 0) {
        sem_wait(sema);
        *pcount = *pcount + 1;
        printf("pcount = %d\n", *pcount);
        sem_post(sema);
    }

    for (i = 0; i < 3 && childpid == 0; i++) {

        if (childpid == -1) {
            perror("Failed to fork.");
            return 1;
        }

        fprintf(stderr, "A\n");

        childpid = fork();

        if (childpid == 0) {

            sem_wait(sema);
            *pcount = *pcount + 1;
            printf("pcount = %d\n", *pcount);
            sem_post(sema);

            fprintf(stderr, "B\n");
            childpid = fork();

            if (childpid == 0) {
                sem_wait(sema);
                *pcount = *pcount + 1;
                printf("pcount = %d\n", *pcount);
                sem_post(sema);
            }

            fprintf(stderr, "C\n");
        }
    }

    return 0;
}

Terminal Session:

$ gcc SO.c -lpthread 
$ ./a.out
pcount = 1
pcount = 2
A
pcount = 3
B
C
pcount = 4
C
A
pcount = 5
B
C
pcount = 6
C
A
pcount = 7
B
C
pcount = 8
C

So yes, 8 is the answer. Easy-Peasy :)