pthread互斥锁在macOS

时间:2017-05-06 12:55:43

标签: c linux macos pthreads mutex

目前我正在学习Linux上的POSIX线程。以下示例计算整数数组中有多少3(int),它在CentOS 6.5上返回正确答案,但在macOS 10.12.4上返回错误答案。

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

#define thread_num 16
#define MB 1024 * 1024
int *array;
int length; //array length
int count;
int t; //number of thread
void *count3s_thread(void* id);

pthread_mutex_t myMutex;//create a mutex

int main()
{
    //initialize mutex
    if (pthread_mutex_init(&myMutex, NULL) != 0)
        printf("Mutex init failed!\n");
    int i;
    int tid[thread_num];
    pthread_t threads[thread_num];
    length = 64 * MB;
    array = malloc(length * 4);

    //initialize the array
    //when i is an odd number, array[i] = 4
    //when i is an even number, array[i] = 3
    for (i = 0; i < length; i++)
        array[i] = i % 2 ? 4 : 3;

    for (t = 0; t < thread_num; t++)
    {
        count = 0;
        tid[t]=t;
        int err = pthread_create(&(threads[t]), NULL, count3s_thread,&(tid[t]) );
        if (err)
        {
            printf("create thread error!\n");
            return 0;           
        }
    }
    for (t = 1; t < thread_num; t++)
        pthread_join(threads[t], NULL);
    printf("Total count= %d \n",count);

    pthread_mutex_destroy(&myMutex);

    return 0;
}

void *count3s_thread(void* id)
{
    //printf("id from count3s_thread = %d\n", *(int *)id);
    int length_per_thread = length / thread_num; //length of every thread
    int start = *(int *)id * length_per_thread;
    int i;
    for (i = start; i < start + length_per_thread; i++)
    {
        if (array[i] == 3)
        {
            pthread_mutex_lock(&myMutex);
            count++;
            pthread_mutex_unlock(&myMutex);
        }
    }
    return NULL;
}

答案应该是64 * 1024 * 1024/2 = 67,108,864 / 2 = 33,554,432。但macOS的答案永远是对的。我使用以下选项编译代码:

gcc mutex-demo.c -lpthread -o mutex-demo

我尝试解决this link之后的问题。但是,我在初始化pthread互斥锁时仍然得到了错误的答案。我错过了什么吗?

PS:我已经尝试了pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;pthread_mutex_init( &mutex, NULL)但是在macOS上运行的程序仍然会返回错误的答案。

高级谢谢!

1 个答案:

答案 0 :(得分:2)

count = 0;之前的行pthread_createcount设置为0,因此您在线程递增时重置count的值。删除此行。当主线程在线程开始计数之前完成pthread_create时,它可能适用于其他系统。

我在评论中已经提到过:

for (t = 1; t < thread_num; t++)
    pthread_join(threads[t], NULL);

应该是

for (t = 0; t < thread_num; t++)
     //  ^
    pthread_join(threads[t], NULL);

否则你不会等待第一个线程完成。