#include <signal.h>
#include <stdlib.h>
#include <sched.h>
#include <pthread.h>

int main(int argc, char *argv);
static int child_thread(void *data);
static sig_atomic_t counter;
static pthread_mutex_t clone_mutex;

int main(int argc, char *argv)
{
   char *stack[4];
   pid_t child[4];
   int status;

   pthread_mutex_init(&clone_mutex, NULL);
   stack[0] = malloc(4096);
   stack[1] = malloc(4096);
   stack[2] = malloc(4096);
   stack[3] = malloc(4096);
   child[0] = clone(child_thread, &stack[0][4096], CLONE_VM, NULL);
   child[1] = clone(child_thread, &stack[1][4096], CLONE_VM, NULL);
   child[2] = clone(child_thread, &stack[2][4096], CLONE_VM, NULL);
   child[3] = clone(child_thread, &stack[3][4096], CLONE_VM, NULL);
   waitpid(child[0], &status, 0);
   waitpid(child[1], &status, 0);
   waitpid(child[2], &status, 0);
   waitpid(child[3], &status, 0);
   pthread_mutex_destroy(&clone_mutex);
   printf("%d\n", counter);
   exit(0);
   return 0;
}

static int child_thread(void *data)
{
   pthread_mutex_lock(&clone_mutex);
   ++counter;
   pthread_mutex_unlock(&clone_mutex);
   return 0;
}