#include <unistd.h>
#include <assert.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>

#define CHILDREN 5

/* This is just a proof of concept.  In a production implementation
 * you would have proper error checking and loop on EINTR (among many
 * other improvements). */

static void create_lock(int l[2]) { 
  assert(pipe(l) >= 0);
  assert(write(l[1], "", 1) == 1);
}

static void destroy_lock(int l[2]) {
  assert(close(l[0]) >= 0);
  assert(close(l[1]) >= 0);
}

static void lock(int l[2]) {
  char c;
  assert(read(l[0], &c, 1) == 1);
}

static void unlock(int l[2]) {
  assert(write(l[1], "", 1) == 1);
}

int main() {
  int l[2];
  pid_t p[CHILDREN];
  create_lock(l);
  for(int n = 0; n < CHILDREN; ++n) {
    if(!(p[n] = fork())) {
      lock(l);
      printf("child %d has lock\n", n);
      sleep(1);
      printf("child %d releasing...\n", n);
      unlock(l);
      _exit(0);
    }
  }
  for(int n = 0; n < CHILDREN; ++n)
    while(waitpid(p[n], 0, 0) < 0)
      ;
  return 0;
}