1 /* Use a semaphore to implement mutual exclusion. */
2
3 #include <assert.h>
4 #include <fcntl.h> /* O_CREAT */
5 #include <pthread.h>
6 #include <semaphore.h>
7 #include <stdio.h> /* printf() */
8 #include <stdlib.h> /* exit() */
9 #include <unistd.h> /* sleep() */
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 char semaphore_name[32];
38
39 while ((optchar = getopt(argc, argv, "dmp")) != EOF)
40 {
41 switch (optchar)
42 {
43 case 'd':
44 s_debug = 1;
45 break;
46 case 'm':
47 s_do_mutual_exclusion = 1;
48 break;
49 case 'p':
50 s_do_printf = 1;
51 break;
52 default:
53 assert(0);
54 }
55 }
56
57 /*
58 * Use the ipcs and ipcrm commands to clean up named semaphores left by
59 * aborted instances of this process.
60 */
61 snprintf(semaphore_name, sizeof(semaphore_name), "/drd-sem-open-test-%ld",
62 (long) getpid());
63 s_sem = sem_open(semaphore_name, O_CREAT | O_EXCL, 0600, 1);
64 if (s_sem == SEM_FAILED)
65 {
66 fprintf(stderr, "Failed to create a semaphore with name %s\n",
67 semaphore_name);
68 exit(1);
69 }
70
71 /*
72 * Switch to line-buffered mode, such that timing information can be
73 * obtained for each printf() call with strace.
74 */
75 setlinebuf(stdout);
76
77 if (s_debug)
78 {
79 printf("&s_d1 = %p; &s_d2 = %p; &s_d3 = %p\n", &s_d1, &s_d2, &s_d3);
80 }
81
82 s_d1 = 1;
83 s_d3 = 3;
84
85 pthread_create(&threadid, 0, thread_func, 0);
86
87 sleep(1); /* Wait until thread_func() finished. */
88
89 {
90 if (s_do_mutual_exclusion) sem_wait(s_sem);
91 s_d3++;
92 if (s_do_mutual_exclusion) sem_post(s_sem);
93 }
94
95 /* Wait until the thread finished. */
96 pthread_join(threadid, 0);
97 if (s_do_printf) printf("s_d2 = %g (should be 2)\n", s_d2);
98 if (s_do_printf) printf("s_d3 = %g (should be 5)\n", s_d3);
99
100 sem_close(s_sem);
101 sem_unlink(semaphore_name);
102
103 return 0;
104 }
105
thread_func(void * thread_arg)106 static void* thread_func(void* thread_arg)
107 {
108 if (s_do_printf)
109 {
110 printf("s_d1 = %g (should be 1)\n", s_d1);
111 }
112 s_d2 = 2;
113 {
114 if (s_do_mutual_exclusion) sem_wait(s_sem);
115 s_d3++;
116 if (s_do_mutual_exclusion) sem_post(s_sem);
117 }
118 return 0;
119 }
120