Programming with POSIX® Threads

24 			pthread_testcancel ();
25 	}
26
27 /*
28 * Cancellation cleanup handler for the contractor thread. It
29 * will cancel and detach each worker in the team.
30 */
31 void cleanup (void *arg)
32 {
33 	team_t *team = (team_t *)arg;
34 	int count, status;
35
36 	for (count = team->join_i; count < THREADS; count++) {
37 		status = pthread_cancel (team->workers[count]);
38 		if (status != 0)
39 			err_abort (status, 'Cancel worker');
40
41 		status = pthread_detach(team->workers[count]);
42 		if (status != 0)
43 			err_abort (status, 'Detach worker');
44 		printf ('Cleanup: canceled %d
', count);
45 	}
46 }
47
48 /*
49 * Thread start routine for the contractor. It creates a team of
50 * worker threads, and then joins with them. When canceled, the
51 * cleanup handler will cancel and detach the remaining threads.
52 */
53 void *thread_routine (void *arg)
54 {
53 	team_t team; /* team info */
56 	int count;
57 	void *result; /* Return status */
58 	int status;
59
60 	for (count = 0; count < THREADS; count++) {
61 		status = pthread_create (
62 			&team.workers[count], NULL, worker_routine, NULL);
63 		if (status != 0)
64 			err_abort (status, 'Create worker');
55 	}
66 	pthread_cleanup_push (cleanup, (void*)&team);
67
68 	for (team.join_i = 0; team.join_i < THREADS; team.join_i++) {
69 		status = pthread_join (team.workers[team.join_i], &result);
70 		if (status != 0)
71 			err_abort (status, 'Join worker');
72 	}
73
74 	pthread_cleanup_pop (0);
75 	return NULL;
76 }
77
78 int main (int argc, char *argv[])
79 {
80 	pthread_t thread_id;
81 	int status;
82
83 #ifdef sun
84 /*
85 * On Solaris 2.5, threads are not timesliced. To ensure
86 * that our threads can run concurrently, we need to
87 * increase the concurrency level to at least 2 plus THREADS
88 * (the number of workers).
89 */
90 	DPRINTF (('Setting concurrency level to %d
', THREADS+2));
91 	thr_setconcurrency (THREADS+2);
92 #endif
93 	status = pthread_create (&thread_id, NULL, thread_routine, NULL);
94 	if (status != 0)
95 		err_abort (status, 'Create team');
96 	sleep (5);
97 	printf ('Cancelling...
');
98 	status = pthread_cancel (thread_id);
99 	if (status != 0)
100 		err_abort (status, 'Cancel team');
101	status = pthread_join (thread_id, NULL);
102	if (status != 0)
103		err_abort (status, 'Join team');
104 }

5.4 Thread-specific data

No, I've made up my mind about it: if I'm Mabel, I'll stay down here. It'll be no use their putting their heads down and saying 'Come up again, dear!' I shall only look up and say 'Who am I, then? Tell me that first, and then, if I like being that person, I'll come up: if not, I'll stay down here till I'm somebody else.'

— Lewis Carroll,Alice's Adventures in Wonderland

When a function in a single threaded program needs to create private data that persists across calls to that function, the data can be allocated statically in memory. The name's scope can be limited to the function or file that uses it (static) or it can be made global (extern).

It is not quite that simple when you use threads. All threads within a process share the same address space,