pthread_cond_signal如何才能保证不被丢失

shiva_cu

1. #include <unistd.h>
   
2. #include <pthread.h>
   
3. #include <ostream>
   
4. #include <iostream>
   
5. 
   
6. using namespace std ;
   
7. 
   
8. void* routine1(void* p) ;
   
9. void* routine2(void* p) ;
   
10. void* routine3(void* p) ;
    
11. 
    
12. int flag = 1 ;
    
13. 
    
14. pthread_mutex_t mux ;
    
15. //static pthread_mutexattr_t muxattr = PTHREAD_MUTEX_TIMED_NP ;
    
16. static pthread_cond_t cond1 = PTHREAD_COND_INITIALIZER ;
    
17. static pthread_cond_t cond2 = PTHREAD_COND_INITIALIZER ;
    
18. static pthread_cond_t cond3 = PTHREAD_COND_INITIALIZER ;
    
19. 
    
20. int main(int argc, char* argv[])
    
21. {
    
22.     pthread_t id1, id2, id3 ;
    
23.     pthread_attr_t attr1, attr2, attr3 ;
    
24. 
    
25.     pthread_cond_init(&cond1, NULL) ;
    
26.     pthread_cond_init(&cond2, NULL) ;
    
27.     pthread_cond_init(&cond3, NULL) ;
    
28. 
    
29.     pthread_mutex_init(&mux, 0) ;
    
30. 
    
31.     pthread_attr_init(&attr1) ;
    
32.     pthread_attr_init(&attr2) ;
    
33.     pthread_attr_init(&attr3) ;
    
34. 
    
35.     pthread_attr_setdetachstate(&attr1, PTHREAD_CREATE_JOINABLE) ;
    
36.     pthread_attr_setdetachstate(&attr2, PTHREAD_CREATE_JOINABLE) ;
    
37.     pthread_attr_setdetachstate(&attr3, PTHREAD_CREATE_JOINABLE) ;
    
38. 
    
39.     pthread_attr_setstacksize(&attr1, 1024*100) ;
    
40.     pthread_attr_setstacksize(&attr2, 1024*100) ;
    
41.     pthread_attr_setstacksize(&attr3, 1024*100) ;
    
42. 
    
43.     pthread_create(&id1, &attr1, routine1, (void*)NULL) ;
    
44.     pthread_create(&id2, &attr2, routine2, (void*)NULL) ;
    
45.     pthread_create(&id3, &attr3, routine3, (void*)NULL) ;
    
46. 
    
47.     sleep(1) ;
    
48.     pthread_cond_signal(&cond1) ;
    
49. 
    
50.     pthread_join(id1, NULL) ;
    
51.     pthread_join(id2, NULL) ;
    
52.     pthread_join(id3, NULL) ;
    
53. 
    
54.     cout << "END\n" ;
    
55. 
    
56.     return 0 ;
    
57. }
    
58. 
    
59. void* routine1(void* p)
    
60. {
    
61.     for(int i=0; i<10; i++)
    
62.     {
    
63.         pthread_mutex_lock(&mux) ;
    
64.         //cout<< "A lock" << endl ;
    
65.         pthread_cond_wait(&cond1, &mux) ;
    
66.         cout << "A " << i << endl ;
    
67.         pthread_cond_signal(&cond2) ;
    
68.         pthread_mutex_unlock(&mux) ;
    
69.         //cout << "A unlock" << endl ;
    
70.     }
    
71.     cout << "thread1 is about to exit\n" ;
    
72. }
    
73. 
    
74. void* routine2(void* p)
    
75. {
    
76.     for(int i=0; i<10; i++)
    
77.     {
    
78.         pthread_mutex_lock(&mux) ;
    
79.         //cout<< "B lock" << endl ;
    
80.         pthread_cond_wait(&cond2, &mux) ;
    
81.         cout << "B " << i << endl ;
    
82.         pthread_cond_signal(&cond3) ;
    
83.         pthread_mutex_unlock(&mux) ;
    
84.         //cout << "B unlock" << endl ;
    
85.     }
    
86.     cout << "thread2 is about to exit\n" ;
    
87. }
    
88. 
    
89. void* routine3(void* p)
    
90. {
    
91.     for(int i=0; i<10; i++)
    
92.     {
    
93.         pthread_mutex_lock(&mux) ;
    
94.         //cout<< "C lock" << endl ;
    
95.         pthread_cond_wait(&cond3, &mux) ;
    
96.         cout << "C " << i << endl ;
    
97.         pthread_cond_signal(&cond1) ;
    
98.         pthread_mutex_unlock(&mux) ;
    
99.         //cout << "C unlock" << endl ;
    
100.     }
     
101.     cout << "thread3 is about to exit\n" ;
     
102. }
     

复制代码

3个线程A，B，C分别按照顺序打印0,1,2,3,4,5,6,7,8,9.

如下代码所示，但是由于线程的执行先后顺序是无序的，所以pthread_cond_signal有可能在一个线程未pthread_cond_wait之前就发出了。这种情况一般如何解决？

rqzrqh

pthread_mutex_lock(&mux);
while (条件)
    pthread_cond_wait(&cond, &mux);
pthread_mutex_unlock(&mux);

sonicling

pthread_cond_signal是边沿触发的，你的需求是要像Windows的SetEvent一样水平触发，需要一个额外的变量来记录这个信号。

rqzrqh

在唤醒对方时修改条件
在进入睡眠前和被唤醒后判断条件

唤醒只是唤醒阻塞在条件变量所对应的线程链表上的线程，如果没有线程在该链表上阻塞等待，唤醒就不会起作用。