// remember to   CC -lpthread

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

int threads = 0;

struct threadinfo
{ int name, randomiser, delay;
  pthread_t id;
  pthread_mutex_t * semaphore; };

void * background(void * a)
{ threads += 1;
  threadinfo * info = (threadinfo *)a;
  for (int i=0; i<info->randomiser; i+=1)
    random();
  for (int i=1; i<20; i+=1)
  { pthread_mutex_lock(info->semaphore);
    printf("%c%d ", info->name, i);
    fflush(stdout);
    if (info->name!='A' || i%2==1)
      pthread_mutex_unlock(info->semaphore);
    usleep(random() % info->delay); }
  threads -= 1;
  return NULL; }

void main()
{ pthread_mutex_t sem;
  pthread_mutex_init(& sem, NULL);
  threadinfo a, b;
  a.name='A'; a.randomiser=7; a.delay = 1000000; a.semaphore = & sem;
  b.name='B'; b.randomiser=3; b.delay =  333333; b.semaphore = & sem;
  pthread_create(& a.id, NULL, background, & a);
  pthread_create(& b.id, NULL, background, & b);
  void * x;
  pthread_join(a.id, & x);
  pthread_join(b.id, & x);
  printf("done\n"); }