与线程同步混淆 [pthreads]

Question

我有一个问题线程很长一段时间。这段代码应该有一个工作线程在主线程打印出共享整数时递增共享整数的值。但是，我没有得到预期的输出。

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

pthread_mutex_t lock;
int shared_data = 0; //shared data


// Often shared data is more complex than just an int.
void* thread_function(void* arg)
{
    int i;
    for (i = 0; i < 10; ++i)
    {
        // Access the shared data here.
        pthread_mutex_lock(&lock);
        shared_data++;
        pthread_mutex_unlock(&lock);
    }

    return NULL;
}

int main(void)
{
    pthread_t thread;
    int i;
    void* exit_status;
    // Initialize the mutex before trying to use it.
    pthread_mutex_init(&lock, NULL);
    pthread_create(&thread, NULL, thread_function, NULL);
    // Try to use the shared data.
    for (i = 0; i < 10; ++i)
    {
        sleep(1);
        pthread_mutex_lock(&lock);
        printf ("\r for i= %d Shared integer 's value = %d\n", i, shared_data);
        pthread_mutex_unlock(&lock);
    }
    printf("\n");
    pthread_join(thread, &exit_status);
    // Clean up the mutex when we are finished with it.
    pthread_mutex_destroy(&lock);

    return 0;
}

这是我的期望：

for i=0 Shared Integer 's value = 0
for i=1 Shared Integer 's value = 1 
for i=3 Shared Integer 's value = 2
...
for i=10 Shared Integer 's value =10

但结果是：

for i=0 Shared Integer 's value = 0
for i=1 Shared Integer 's value = 10
for i=3 Shared Integer 's value = 10
...
for i=10 Shared Integer 's value =10

那我该如何解决呢？

Answer 1

主线程和您的工作线程同时运行。也就是说，如果没有额外的同步，让这些 for 循环完全一致几乎是不可能的。

您的输出正是您所期望的。产生线程所花费的时间允许主线程在另一个线程更改共享数据之前进行打印。然后，打印需要很长时间，以至于另一个线程完全完成其循环并将共享数据增加到 10，然后主线程才能进行第二次迭代。

在一个完美的世界里，这个使用条件变量的小技巧会让你得到你想要的： 编辑：条件变量对此是个坏主意。这是使用伪原子变量的工作版本，不包含 UB :)：

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

pthread_mutex_t want_incr_mut;
pthread_mutex_t done_incr_mut;
int want_incr = 0;
int done_incr = 0;
int shared_data = 0; //shared data

// Not using atomics, so...

void wait_for_want_increment()
{
    while (1)
    {
        pthread_mutex_lock(&want_incr_mut);
        if (want_incr)
        {
            pthread_mutex_unlock(&want_incr_mut);
            return;
        }
        pthread_mutex_unlock(&want_incr_mut);
    }
}

void wait_for_done_incrementing()
{
    while (1)
    {
        pthread_mutex_lock(&done_incr_mut);
        if (done_incr)
        {
            pthread_mutex_unlock(&done_incr_mut);
            return;
        }
        pthread_mutex_unlock(&done_incr_mut);
    }
}


void done_incrementing()
{
    pthread_mutex_lock(&done_incr_mut);

    done_incr = 1;
    pthread_mutex_lock(&want_incr_mut);
    want_incr = 0;
    pthread_mutex_unlock(&want_incr_mut);

    pthread_mutex_unlock(&done_incr_mut);
}

void want_increment()
{
    pthread_mutex_lock(&want_incr_mut);

    want_incr = 1;
    pthread_mutex_lock(&done_incr_mut);
    done_incr = 0;
    pthread_mutex_unlock(&done_incr_mut);

    pthread_mutex_unlock(&want_incr_mut);
}

// Often shared data is more complex than just an int.
void* thread_function(void* arg)
{
    int i;
    for (i = 0; i < 10; ++i)
    {
        wait_for_want_increment();
        // Access the shared data here.
        shared_data++;
        done_incrementing();
    }

    return NULL;
}

int main(void)
{
    pthread_t thread;
    int i;
    void* exit_status;

    // Initialize the mutex before trying to use it.
    pthread_mutex_init(&want_incr_mut, NULL);
    pthread_mutex_init(&done_incr_mut, NULL);
    pthread_create(&thread, NULL, thread_function, NULL);

    // Try to use the shared data.
    for (i = 0; i <= 10; ++i)
    {
        printf("\r for i= %d Shared integer 's value = %d\n", i, shared_data);
        if (i == 10) break;

        want_increment();
        wait_for_done_incrementing();
    }
    printf("\n");
    pthread_join(thread, &exit_status);
    // Clean up the mutexes when we are finished with them.
    pthread_mutex_destroy(&want_incr_mut);
    pthread_mutex_destroy(&done_incr_mut);

    return 0;
}

在这里，我们只是告诉工人我们想要一个增量，并等待他说他已经完成，然后再继续。同时，worker 等待我们想要一个增量，并在他完成时告诉我们。

我还将主循环更改为 10，因为我认为这是你想要的。

这是我的输出：

for i= 0 Shared integer 's value = 0
for i= 1 Shared integer 's value = 1
for i= 2 Shared integer 's value = 2
for i= 3 Shared integer 's value = 3
for i= 4 Shared integer 's value = 4
for i= 5 Shared integer 's value = 5
for i= 6 Shared integer 's value = 6
for i= 7 Shared integer 's value = 7
for i= 8 Shared integer 's value = 8
for i= 9 Shared integer 's value = 9
for i= 10 Shared integer 's value = 10