apueSection11.5关于thread cleanup handler的一个例子不太理解

zhuhefang2006

Figure 11.5 shows how to use thread cleanup handlers. Although the example is somewhat contrived, it illustrates the mechanics involved. Note that although we never intend to pass a nonzero argument to the thread start-up routines, we still need to match calls to pthread_cleanup_pop with the calls to pthread_cleanup_push; otherwise, the program might not compile.
不太明白红色标识部分的意思
结合实例，不知道为什么在thr_fn1()和thr_fn2()中arg为非空的时候，就不需要调用pthread_cleanup_pop()

实例代码如下

1. #include <stdlib.h>
   
2. #include <stdio.h>
   
3. 
   
4. void cleanup(void *arg)
   
5. {
   
6.         printf("cleanup: %s\n", (char *)arg);
   
7. }
   
8. 
   
9. void *thr_fn1(void *arg)
   
10. {
    
11.         printf("thrad 1 start\n");
    
12.         pthread_cleanup_push(cleanup, "thread 1 first handler");
    
13.         pthread_cleanup_push(cleanup, "thread 1 second handler");
    
14.         printf("thread 1 push complete\n");
    
15.        
    
16.         if (arg)
    
17.                 return ((void *)1);  
    
18.         pthread_cleanup_pop(0);
    
19.         pthread_cleanup_pop(0);
    
20.         return ((void *)1);
    
21. }
    
22. 
    
23. void *thr_fn2(void *arg)
    
24. {
    
25.         printf("thread 2 start\n");
    
26.         pthread_cleanup_push(cleanup, "thread 2 first handler");
    
27.         pthread_cleanup_push(cleanup, "thread 2 second handler");
    
28.         printf("thread 2 push complete\n");
    
29.        
    
30.         if (arg)
    
31.                 pthread_exit((void *)2);  //为什么arg为非空的时候，就不需要调用pthread_cleanup_pop()
    
32.         pthread_cleanup_pop(0);
    
33.         pthread_cleanup_pop(0);
    
34.         pthread_exit((void *)2);
    
35. }
    
36. 
    
37. int main(void)
    
38. {
    
39.         int err;
    
40.         pthread_t tid1, tid2;
    
41.         void *tret;
    
42. 
    
43. //        err = pthread_create(&tid1, NULL, thr_fn1, (void *)1);
    
44.         err = pthread_create(&tid1, NULL, thr_fn1, NULL);
    
45.         if (err != 0)
    
46.         {
    
47.                 printf("can't create thread 1:%s\n", strerror(errno));
    
48.                 exit(0);
    
49.         }
    
50. 
    
51.         err = pthread_create(&tid2, NULL, thr_fn2, (void *)2);
    
52. //    err = pthread_create(&tid2, NULL, thr_fn2, NULL);
    
53.         if (err != 0)
    
54.         {
    
55.                 printf("can't create thread 2:%s\n", strerror(errno));
    
56.                 exit(0);
    
57.         }
    
58. 
    
59.         err = pthread_join(tid1, &tret);
    
60.         if (err != 0)
    
61.         {
    
62.                 printf("can't join with thread 1:%s\n", strerror(errno));
    
63.                 exit(0);
    
64.         }
    
65.         printf("thread 1 exit code %d\n", (int)tret);
    
66. 
    
67.         err = pthread_join(tid2, &tret);
    
68.         if (err != 0)
    
69.         {
    
70.                 printf("can't join with thread 2:%s\n", strerror(errno));
    
71.                 exit(0);
    
72.         }
    
73.         printf("thread 2 exit code %d\n", (int)tret);
    
74.         exit(0);
    
75. }

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[ 本帖最后由 zhuhefang2006 于 2009-1-13 16:04 编辑 ]