使用 pthread 创建示例应用程序的要求如下:
- 使用 Pthread 从 main 创建单个线程
- 在线程内部,互斥锁被锁定,计数器对值进行计数,同时 while 循环递增,While 设置为最大计数 10。
- while 循环结束后,互斥锁被解锁。
我尝试使用 pthread 实现上述要求
代码如下:
#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;
}
疑问:
在上面的代码中,线程正常执行其函数并退出。 一旦条件被应用,即下面显示的代码被取消注释,
pthread_cond_wait(&count_threshold_cv, &count_mutex);
然后线程在 while 循环的第一次迭代后按预期停止。 该信号是通过下面所示的代码从 main 生成的:
for(i = 0; i<(samples-1); i++)
{
if(pthread_cond_signal(&count_threshold_cv))
{
printf("Error Signaling thread at sample = %d\n",i);
}
}
观察到信号从未发送。
有人可以指导我吗,我哪里出错了。我是 Pthreads 的新手。
提前致谢。
最佳答案
count_mutex
和count_threshold_cv
未初始化,添加:
int main()
{
pthread_mutex_init(&count_mutex, NULL);
pthread_cond_init(&count_threshold_cv, NULL);
//...
<小时/>
E1_ptr
未初始化。
解决方法有很多种:
您可以调用malloc
来分配内存:
struct example *E1_ptr = malloc(sizeof(struct example));
E1_ptr->a = 0;
E1_ptr->i = 0;
或保存指向局部变量的指针:
struct example ex;
struct example *E1_ptr = &ex; //malloc(sizeof(struct example));
E1_ptr->a = 0;
E1_ptr->i = 0;
或
struct example ex;
ex.a = 0;
ex.i = 0;
然后使用pthread_create(&inc_x_thread, NULL, inc_x, &ex)创建线程
pthread_cond_signal
函数不等待。如果一个线程被条件变量阻塞,pthread_cond_signal
函数会解除对该线程的阻塞,否则立即返回而不等待并且不执行任何操作。因此,具有 10 次迭代的 for 循环会尽快执行,而无需等待 pthread_cond_wait
被调用。
因此可以将 for 循环重写为无限循环,重复调用 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,默认值为 0。在 inc_x
函数中调用 pthread_exit
之前,它被设置为 1
。设置/检查 taskDone
应该用某种同步机制包装,例如通过添加新的互斥体或使用 count_mutex
。
关于c - 为什么来自 main 的 Pthread 信号会挂起代码?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/53239149/