C Pthread：可能的死鎖，線程1在線程2退出時掛起

我對C中的系統編程非常陌生，我使用pthreads創建兩個線程來遞增計數器並顯示值。如果計數器值爲偶數，則線程1遞增，如果計數器值爲奇數，則線程2遞增。C Pthread：可能的死鎖，線程1在線程2退出時掛起

pthread_mutex_t count_mutex  = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t condition_var = PTHREAD_COND_INITIALIZER; 

void *functionCount1(); 
void *functionCount2(); 
int count = 0; 
#define COUNT_DONE 2 

int main() 
{ 
    pthread_t thread1, thread2; 

    pthread_create(&thread1, NULL, &functionCount1, NULL); 
    pthread_create(&thread2, NULL, &functionCount2, NULL); 

    pthread_join(thread1, NULL); 
    pthread_join(thread2, NULL); 

    printf("Final count: %d\n",count); 

    exit(0); 
} 


void *functionCount1() 
{ 
    for(;;) 
    { 
     pthread_mutex_lock(&count_mutex); 

     pthread_cond_wait(&condition_var, &count_mutex); 
     count++; 
     printf("Counter value functionEven: %d\n",count); 

     pthread_mutex_unlock(&count_mutex); 

     if(count >= COUNT_DONE) { 
      printf("Thread 1: Exit condition reached.\n"); 
      return(NULL); 

     } 
    } 
} 

void *functionCount2() 
{ 
    for(;;) 
    { 
     pthread_mutex_lock(&count_mutex); 

     if(count % 2 == 0) 
     { 
      pthread_cond_signal(&condition_var); 
     } 
     else 
     { 
      count++; 
      printf("Counter value functionOdd: %d.\n",count); 
     } 

     pthread_mutex_unlock(&count_mutex); 

     if(count >= COUNT_DONE) { 
      printf("Thread 2: Exit condition reached.\n"); 
      return(NULL); 
     } 
    } 

}

從我可以看到，問題是線程2達到它首先存在狀態，而讓線程1掛。

Counter value functionEven: 1 
Counter value functionOdd: 2. 
Thread 2: Exit condition reached. 
(This is where it hangs, I have to press Ctrl-C to end)^C

當線程2達到其退出條件時，我試過另一個鎖，並且只有當線程1存在條件被調用時才解鎖它。但這似乎沒有做任何事情。

pthread_mutex_t mutex2 = PTHREAD_MUTEX_INITIALIZER; 

if(count >= COUNT_DONE) { 
    printf("Thread 1: Exit condition reached.\n"); 
    pthread_mutex_unlock(&mutex2); 
    return(NULL); 
} 

if(count >= COUNT_DONE) { 
    pthread_mutex_lock(&mutex2); 
    printf("Thread 2: Exit condition reached.\n"); 
    return(NULL); 
}

正確數量的最終解決方案。

以下答案幫了很大忙，解決了主要問題。

pthread_cond_wait(&condition_var, &count_mutex); 
if(count >= COUNT_DONE) { 
    return(NULL); 
} 
count++; 
printf("Counter value functionEven: %d\n",count);

來源

2017-03-18 Saad A

由於在兩個if語句中變量數不受保護，因此您有數據競爭。因此行爲是不確定的。我的意思是發佈一個完整的答案，但我不確定代碼的意圖是什麼。 – 2501

在退出functionCount2之前，您必須發信號給您的條件變量如下;

if(count >= COUNT_DONE) { 
     printf("Thread 2: Exit condition reached.\n"); 
     pthread_cond_signal(&condition_var); 
     return(NULL); 
    }

來源

2017-03-18 19:33:13

C Pthread：可能的死鎖，線程1在線程2退出時掛起

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