[英]why does Pthread Signal from main, hangs the code?
下面給出了使用 pthread 創建示例應用程序的要求:
上述要求我已經嘗試使用 pthread 實現
代碼如下所示:
#include <pthread.h>
#include <stdio.h>
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;
int samples = 10;
int count = 0;
struct example
{
int i;
int a;
};
void *inc_x(void *x_void_ptr)
{
pthread_mutex_lock(&count_mutex);
printf("Thread is locked \n");
while(count < samples)
{
printf("inside While loop \n");
struct example *E2_ptr;
E2_ptr = (struct example *)x_void_ptr;
printf("inside thread count = %d\n",count);
E2_ptr->a = count;
E2_ptr->i = (count + 1);
count ++;
//pthread_cond_wait(&count_threshold_cv, &count_mutex);
}
pthread_mutex_unlock(&count_mutex);
printf ( "\n Test Successful for Thread\n");
pthread_exit(NULL);
}
int main()
{
int x = 100, y = 0,i = 0;
struct example *E1_ptr;
E1_ptr->a = 0;
E1_ptr->i = 0;
printf("Before\t E1_ptr->a = %d\t, E1_ptr->i = %d\n",E1_ptr->a,E1_ptr->i);
pthread_t inc_x_thread;
if(pthread_create(&inc_x_thread, NULL, inc_x, E1_ptr))
{
printf("Error creating thread\n");
}
if(pthread_join(inc_x_thread, NULL))
{
printf("Error joining thread\n");
}
for(i = 0; i<(samples-1); i++)
{
if(pthread_cond_signal(&count_threshold_cv))
{
printf("Error Signaling thread at sample = %d\n",i);
}
}
printf("after\t E1_ptr->a = %d\t, E1_ptr->i = %d\n",E1_ptr->a,E1_ptr->i);
pthread_mutex_destroy(&count_mutex);
pthread_cond_destroy(&count_threshold_cv);
pthread_exit (NULL);
return 0;
}
懷疑:
在上面的代碼中,線程正確地執行了它的功能並退出。 一旦應用了條件,即下面顯示的代碼將被取消注釋,
pthread_cond_wait(&count_threshold_cv, &count_mutex);
然后線程在 while 循環的第一次迭代后按預期停止。 信號由 main 生成,代碼如下所示:
for(i = 0; i<(samples-1); i++)
{
if(pthread_cond_signal(&count_threshold_cv))
{
printf("Error Signaling thread at sample = %d\n",i);
}
}
觀察到信號從未發送。
有人可以指導我,我哪里出錯了。 我是 Pthreads 的新手。
提前致謝。
count_mutex
和count_threshold_cv
沒有初始化,添加:
int main()
{
pthread_mutex_init(&count_mutex, NULL);
pthread_cond_init(&count_threshold_cv, NULL);
//...
E1_ptr
未初始化。 有很多方法可以解決:
您可以調用malloc
來分配內存:
struct example *E1_ptr = malloc(sizeof(struct example));
E1_ptr->a = 0;
E1_ptr->i = 0;
或持有指向局部變量的指針:
struct example ex;
struct example *E1_ptr = &ex; //malloc(sizeof(struct example));
E1_ptr->a = 0;
E1_ptr->i = 0;
或者
struct example ex;
ex.a = 0;
ex.i = 0;
然后用pthread_create(&inc_x_thread, NULL, inc_x, &ex)
創建線程
pthread_cond_signal
函數不等待。 如果一個線程被條件變量pthread_cond_signal
阻塞,則函數解除阻塞該線程,否則立即返回而不等待並且什么都不做。 因此,您的 for 循環會盡快執行 10 次迭代,而無需等待pthread_cond_wait
被調用。 因此可以將您的 for 循環重寫為無限循環,重復調用pthread_cond_signal
。
if(pthread_create(&inc_x_thread, NULL, inc_x, E1_ptr)) {
printf("Error creating thread\n");
}
while(1) { // INFINITE LOOP
if(pthread_cond_signal(&count_threshold_cv)) {
printf("Error Signaling thread at sample = %d\n",i);
}
if (taskDone) // testing global flag, if 1 break
break; // it means inc_x thread has ended
}
if(pthread_join(inc_x_thread, NULL)) { // it was pointed out in comment
printf("Error joining thread\n"); // you need to join at the end of main function
}
taskDone
是全局int ,默認值為 0。 在inc_x
函數中調用pthread_exit
之前,它被設置為1
。 設置/檢查taskDone
應該用一些同步機制包裝,例如通過添加新的互斥鎖或使用count_mutex
。
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