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      1 /* Use a semaphore to implement mutual exclusion. */
      2 
      3 
      4 #include <assert.h>
      5 #include <stdio.h>     /* printf() */
      6 #include <pthread.h>
      7 #include <semaphore.h>
      8 #include <unistd.h>    /* sleep() */
      9 
     10 
     11 /* Local functions declarations. */
     12 
     13 static void* thread_func(void*);
     14 
     15 
     16 /* Local variables. */
     17 
     18 /* s_sem protects s_d3. */
     19 static sem_t s_sem;
     20 
     21 static double s_d1; /* accessed before thread creation and in the created */
     22                     /* thread (not a race). */
     23 static double s_d2; /* accessed in the created thread and after the join */
     24                     /* (not a race). */
     25 static double s_d3; /* accessed simultaneously from both threads (race). */
     26 static int    s_debug     = 0;
     27 static int    s_do_printf = 0;
     28 static int    s_do_mutual_exclusion = 0;
     29 
     30 
     31 /* Function definitions. */
     32 
     33 int main(int argc, char** argv)
     34 {
     35   int optchar;
     36   pthread_t threadid;
     37 
     38   while ((optchar = getopt(argc, argv, "dmp")) != EOF)
     39   {
     40     switch (optchar)
     41     {
     42     case 'd':
     43       s_debug = 1;
     44       break;
     45     case 'm':
     46       s_do_mutual_exclusion = 1;
     47       break;
     48     case 'p':
     49       s_do_printf = 1;
     50       break;
     51     default:
     52       assert(0);
     53     }
     54   }
     55 
     56   sem_init(&s_sem, 0, 1);
     57 
     58   /*
     59    * Switch to line-buffered mode, such that timing information can be
     60    * obtained for each printf() call with strace.
     61    */
     62   setlinebuf(stdout);
     63 
     64   if (s_debug)
     65   {
     66     printf("&s_d1 = %p; &s_d2 = %p; &s_d3 = %p\n", &s_d1, &s_d2, &s_d3);
     67   }
     68 
     69   s_d1 = 1;
     70   s_d3 = 3;
     71 
     72   pthread_create(&threadid, 0, thread_func, 0);
     73 
     74   sleep(1); /* Wait until thread_func() finished. */
     75 
     76   {
     77     if (s_do_mutual_exclusion) sem_wait(&s_sem);
     78     s_d3++;
     79     if (s_do_mutual_exclusion) sem_post(&s_sem);
     80   }
     81 
     82   /* Wait until the thread finished. */
     83   pthread_join(threadid, 0);
     84   if (s_do_printf) printf("s_d2 = %g (should be 2)\n", s_d2);
     85   if (s_do_printf) printf("s_d3 = %g (should be 5)\n", s_d3);
     86 
     87   sem_destroy(&s_sem);
     88 
     89   return 0;
     90 }
     91 
     92 static void* thread_func(void* thread_arg)
     93 {
     94   if (s_do_printf)
     95   {
     96     printf("s_d1 = %g (should be 1)\n", s_d1);
     97   }
     98   s_d2 = 2;
     99   {
    100     if (s_do_mutual_exclusion) sem_wait(&s_sem);
    101     s_d3++;
    102     if (s_do_mutual_exclusion) sem_post(&s_sem);
    103   }
    104   return 0;
    105 }
    106