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
main(int argc,char ** argv)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
thread_func(void * thread_arg)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