none/tests/pth_cvsimple.c

   1 /********************************************************
   2  * An example source module to accompany...
   3  *
   4  * "Using POSIX Threads: Programming with Pthreads"
   5  *     by Brad nichols, Dick Buttlar, Jackie Farrell
   6  *     O'Reilly & Associates, Inc.
   7  *
   8  ********************************************************
   9  *
  10  * cvsimple.c
  11  *
  12  * Demonstrates pthread condvars.
  13  *
  14  */
  15 #include <unistd.h>
  16 #include <stdio.h>
  17 #include <pthread.h>
  18
  19 #define NUM_THREADS  3
  20 #define TCOUNT 10
  21 #define COUNT_THRES 12
  22
  23 int     condvar_was_hit = 0;
  24 int     count = 0;
  25 int     thread_ids[3] = {0,1,2};
  26 pthread_mutex_t count_lock=PTHREAD_MUTEX_INITIALIZER;
  27 pthread_cond_t count_hit_threshold=PTHREAD_COND_INITIALIZER;
  28
  29 void *inc_count(void *null)
  30 {
  31   int i=0;
  32
  33   for (i=0; i<TCOUNT; i++) {
  34     pthread_mutex_lock(&count_lock);
  35     count++;
  36     printf("inc_counter(): count = %d, unlocking mutex\n", count);
  37     if (count == COUNT_THRES) {
  38       printf("hit threshold!\n");
  39       pthread_cond_signal(&count_hit_threshold);
  40     }
  41     pthread_mutex_unlock(&count_lock);
  42   }
  43
  44   return(NULL);
  45 }
  46
  47 void *watch_count(void *null)
  48 {
  49   pthread_mutex_lock(&count_lock);
  50
  51   while (count < COUNT_THRES) {
  52     pthread_cond_wait(&count_hit_threshold, &count_lock);
  53     condvar_was_hit = 1;
  54   }
  55
  56   pthread_mutex_unlock(&count_lock);
  57
  58   return(NULL);
  59 }
  60
  61 extern int
  62 main(void)
  63 {
  64   int       i;
  65   pthread_t threads[3];
  66
  67   pthread_create(&threads[0], NULL, watch_count, NULL);
  68   sleep(1);
  69   pthread_create(&threads[1], NULL, inc_count,   NULL);
  70   pthread_create(&threads[2], NULL, inc_count,   NULL);
  71
  72   for (i = 0; i < NUM_THREADS; i++) {
  73     pthread_join(threads[i], NULL);
  74   }
  75
  76   // Nb: it's not certain that we'll hit here.  It's possible that the two
  77   // inc_count threads could fully run before watch_count begins, and so
  78   // pthread_cond_wait() is never called.  Or, we could get a spurious
  79   // wake-up in watch_count().  Nonetheless, it's very likely that things
  80   // will work out as expected, since we're starting watch_count() first.
  81   // (Also since the sleep() call was added after watch_count()!)
  82   if (condvar_was_hit == 1)
  83     printf("condvar was hit!\n");
  84   else if (condvar_was_hit > 1)
  85     printf("condvar was multi-hit...\n");
  86   else
  87     printf("condvar was missed...\n");
  88
  89   return 0;
  90 }
  91
  92