c - 使用互斥锁按顺序同步线程

Question

我有这个 C 程序，它必须按 0-6 的顺序显示线程。我正在使用互斥体，但是当我尝试运行我的代码时，没有任何反应，也没有任何显示。另外，编译器没有显示错误

我使用了锁定和解锁互斥体，但我不确定我是否在正确的位置创建了它。任何建议和帮助表示赞赏。

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


void *text(void *arg);
long code[] = { 4, 6, 3, 1, 5, 0, 2 }; // Order in which to start         threads
int num = 0;

pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;


int main()
{
int i;
pthread_t tid[7];
// Initialize random number generator
time_t seconds;
time(&seconds);
srand((unsigned int) seconds);

int rc;



// Create our threads
for (i = 0; i < 7; i++)
{
    pthread_create(&tid[i], NULL, text, (void*)code[i]);

    for (i = 0; i < 7; i++)

    {  rc = pthread_mutex_lock(&a_mutex);

        for (i = 0; i < 7; i++)
        {
             rc = pthread_mutex_unlock(&a_mutex);
        }
    }

}
//join threads
for (i=0; i<7; i++)
{
    if (pthread_join(tid[i], NULL));
                }

rc = pthread_mutex_destroy(&a_mutex);

// Exit main
return 0;

}
void *text(void *arg)
{
long n = (long)arg;
int rand_sec = rand() % (3 - 1 + 1) + 1; // Random num seconds to     sleep
while (num != n) {} // Busy wait used to wait for our turn
num++; // Let next thread go
sleep(rand_sec); // Sleep for random amount of time
printf("This is thread %d.\n", n);
// Exit thread
pthread_exit(0);
}

Answer 1

找出问题的关键是问自己这个问题“我的 6 个线程之间共享的数据是什么？” That 是需要在锁定的互斥锁 block 中进行互斥锁保护(读取和写入)的变量。目前，您只是从单个主线程锁定和解锁互斥量，这实际上什么都不做。

你可能想要更接近这个的东西(虽然这可以大大简化 - 例如你可以完全删除 sleep ):

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

void *text(void *arg);
long code[] = { 4, 6, 3, 1, 5, 0, 2 }; // Order in which to start threads
int num = 0;

pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;

int main()
{
    int i;
    pthread_t tid[7];
    // Initialize random number generator
    time_t seconds;
    time(&seconds);
    srand((unsigned int) seconds);
    // Create our threads
    for (i = 0; i < 7; i++)
        pthread_create(&tid[i], NULL, text, (void*)code[i]);

    //join threads
    for (i=0; i<7; i++)
        if (pthread_join(tid[i], NULL));

    pthread_mutex_destroy(&a_mutex);

    // Exit main
    return 0;
}

void *text(void *arg)
{   
    long n = (long)arg;
    long localNum = -1;
    int rand_sec = rand() % (3 - 1 + 1) + 1; // Random num seconds to     sleep
    int rc;
    while ( localNum != n )
    {
        rc = pthread_mutex_lock(&a_mutex);
        localNum = num;
        rc = pthread_mutex_unlock(&a_mutex);
        sleep(rand_sec); // Sleep for random amount of time
    }

    printf("This is thread %d.\n", n);

    rc = pthread_mutex_lock(&a_mutex);
    num++; // Let next thread go
    rc = pthread_mutex_unlock(&a_mutex);

    pthread_exit(0);
}