理解 C 中的信号量

Question

我已经成功地在 C 中实现了互斥锁和条件变量锁，并尝试使用信号量来实现相同的目标。下面是我的代码，但是执行时，输出总是24而不是12。请帮助我理解如何使用信号量。

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>

#define NUM_THREADS  2
#define TCOUNT 12
#define COUNT_LIMIT 12

int count = 0;
sem_t sem;

void *dum(void *t_id){
    long id = long(t_id);
    for (int i = 0; i < TCOUNT; i++) {
        sem_wait(&sem);
        count++;
        sem_post(&sem);
        printf("In Dum(). Count :: %d. Thread :: %ld \n", count, id);
    }
    pthread_exit(NULL);
}

int main (int argc, char *argv[]) {
    int i;
    long t1=1, t2=2;
    sem_init(&sem, 0, 1);
    pthread_t threads[NUM_THREADS];
    pthread_create(&threads[1], NULL, dum, (void *)t1);
    pthread_create(&threads[2], NULL, dum, (void *) t2);
    for (i=1; i<=2; i++) {
        pthread_join(threads[i], NULL);
    }
    printf ("Main(): Waited on %d  threads. Done.\n", NUM_THREADS);
    printf ("Count should be 12. Actual Count: %d. \n", count);
    pthread_exit(NULL);
}

输出：

In Dum(). Count :: 1. Thread :: 1 
In Dum(). Count :: 2. Thread :: 1 
In Dum(). Count :: 3. Thread :: 1 
In Dum(). Count :: 4. Thread :: 1 
In Dum(). Count :: 5. Thread :: 1 
In Dum(). Count :: 6. Thread :: 1 
In Dum(). Count :: 7. Thread :: 1 
In Dum(). Count :: 8. Thread :: 1 
In Dum(). Count :: 9. Thread :: 1 
In Dum(). Count :: 10. Thread :: 1 
In Dum(). Count :: 11. Thread :: 1 
In Dum(). Count :: 13. Thread :: 1 
In Dum(). Count :: 13. Thread :: 2 
In Dum(). Count :: 14. Thread :: 2 
In Dum(). Count :: 15. Thread :: 2 
In Dum(). Count :: 16. Thread :: 2 
In Dum(). Count :: 17. Thread :: 2 
In Dum(). Count :: 18. Thread :: 2 
In Dum(). Count :: 19. Thread :: 2 
In Dum(). Count :: 20. Thread :: 2 
In Dum(). Count :: 21. Thread :: 2 
In Dum(). Count :: 22. Thread :: 2 
In Dum(). Count :: 23. Thread :: 2 
In Dum(). Count :: 24. Thread :: 2 
Main(): Waited on 2  threads. Done.
Count should be 12. Actual Count: 24. 

Answer 1

正如 kcraigie 所说，您对线程数组的索引错误。另外最好通过指针传递 t1 和 t2：

void *dum(void *t_id)
{
    long id = *(long*)t_id;

和

pthread_create(&threads[0], NULL, dum, (void *) &t1);
pthread_create(&threads[1], NULL, dum, (void *) &t2);
for (i=0; i<2; i++) 
{
    pthread_join(threads[i], NULL);
}

同样在你的线程中你锁定信号量，增加计数并释放信号量，然后你打印它。无法保证您打印出的值将是您增加后的 count 值。另一个线程可能也已经这样做了。将 printf 放入信号量锁中：

sem_wait(&sem);
count++;
printf("In Dum(). Count :: %d. Thread :: %ld \n", count, id);
sem_post(&sem);

但否则数到 24 似乎是正确的行为。您有两个循环分布在 2 个线程中，每个线程迭代 12 次，加一计数。它们中的一个先完成循环然后另一个完成的事实可能是因为首先启动的线程在主线程有机会启动线程 2 之前在一个时间片内完成了所有工作。