drd/tests/sem_as_mutex.c

/* Use a semaphore to implement mutual exclusion. */


#include <assert.h>
#include <stdio.h>     /* printf() */
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>    /* sleep() */


/* Local functions declarations. */

static void* thread_func(void*);


/* Local variables. */

/* s_sem protects s_d3. */
static sem_t s_sem;

static double s_d1; /* accessed before thread creation and in the created */
                    /* thread (not a race). */
static double s_d2; /* accessed in the created thread and after the join */
                    /* (not a race). */
static double s_d3; /* accessed simultaneously from both threads (race). */
static int    s_debug     = 0;
static int    s_do_printf = 0;
static int    s_do_mutual_exclusion = 0;


/* Function definitions. */

int main(int argc, char** argv)
{
  int optchar;
  pthread_t threadid;

  while ((optchar = getopt(argc, argv, "dmp")) != EOF)
  {
    switch (optchar)
    {
    case 'd':
      s_debug = 1;
      break;
    case 'm':
      s_do_mutual_exclusion = 1;
      break;
    case 'p':
      s_do_printf = 1;
      break;
    default:
      assert(0);
    }
  }

  sem_init(&s_sem, 0, 1);

  /*
   * Switch to line-buffered mode, such that timing information can be
   * obtained for each printf() call with strace.
   */
  setlinebuf(stdout);

  if (s_debug)
  {
    printf("&s_d1 = %p; &s_d2 = %p; &s_d3 = %p\n", &s_d1, &s_d2, &s_d3);
  }

  s_d1 = 1;
  s_d3 = 3;

  pthread_create(&threadid, 0, thread_func, 0);

  sleep(1); /* Wait until thread_func() finished. */

  {
    if (s_do_mutual_exclusion) sem_wait(&s_sem);
    s_d3++;
    if (s_do_mutual_exclusion) sem_post(&s_sem);
  }

  /* Wait until the thread finished. */
  pthread_join(threadid, 0);
  if (s_do_printf) printf("s_d2 = %g (should be 2)\n", s_d2);
  if (s_do_printf) printf("s_d3 = %g (should be 5)\n", s_d3);

  sem_destroy(&s_sem);

  return 0;
}

static void* thread_func(void* thread_arg)
{
  if (s_do_printf)
  {
    printf("s_d1 = %g (should be 1)\n", s_d1);
  }
  s_d2 = 2;
  {
    if (s_do_mutual_exclusion) sem_wait(&s_sem);
    s_d3++;
    if (s_do_mutual_exclusion) sem_post(&s_sem);
  }
  return 0;
}