[C++ 常見問題] 線程的同步 (synchronized) : join

前言 : 
The threads library provides three synchronization mechanisms: 

• mutexes - Mutual exclusion lock: Block access to variables by other threads. This enforces exclusive access by a thread to a variable or set of variables.
• joins - Make a thread wait till others are complete (terminated).
• condition variables - data type pthread_cond_t

這裡將會對 Join 使用作簡單使用介紹 : 

Join 簡介 : 
A join is performed when one wants to wait for a thread to finish. A thread calling routine may launch multiple threads then wait for them to finish to get the results. One wait for the completion of the threads with a join. When a joinable thread terminates, its memory resources (thread descriptor and stack) are not deallocated until another thread performs pthread_join on it. Therefore, pthread_join must be called once for each joinable thread created to avoid memory leaks. 

範例代碼 : 
* thread_sample.h 代碼 : 

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1. #include   
2. #include   
3. #include   
4.   
5.   
6. /* 
7. * Thread Synchronization : Joins 
8. */  
9. void threadSampleTest3(bool b);  

* thread_sample.cpp 代碼 : 

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1. #include "thread_sample.h"  
2.   
3. pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;  
4. int counter = 0;  
5. #define NTHREADS 10  
6.   
7. void *thread_function(void *);  
8. void threadSampleTest3(bool b) {  
9.     if(b) {  
10.         pthread_t thread_id[NTHREADS];  
11.         for(int i=0; i
12.             pthread_create(&thread_id[i], NULL, &thread_function, NULL);  
13.         }  
14.         for(int i=0; i
15.             pthread_join(thread_id[i], NULL);  
16.         }  
17.         printf("Final count value=%d\n",counter);  
18.     }  
19. }  
20. void *thread_function(void *dummyPtr){  
21.    printf("Thread number %ld\n", pthread_self());  
22.    pthread_mutex_lock( &mutex1 );  
23.    counter++;  
24.    pthread_mutex_unlock( &mutex1 );  
25.    return NULL;  
26. }  

執行結果 : 

Thread number 3625464
...(中間省略)...
Thread number 3627208
Final count value=10

補充說明 : 
* 函式 pthread_join(3) 用法 
* Linux Tutorial : POSIX thread coding