与 pthread 和互斥锁冲突

Question

我正在尝试做以下作业：

您将编写一个创建三个线程的程序。这些线程一次访问一个共享整数、缓冲区。缓冲区最初将设置为 0。每个线程都应在一条语句中打印其线程 ID、进程 ID 和缓冲区的当前值，然后将缓冲区递增 1。使用互斥锁来确保整个过程不被中断。让线程修改缓冲区 15 次。当每个线程完成后，它应该将其更改缓冲区的次数返回给主线程。

输出应如下所示：

TID: 3077897072, PID: 30656, Buffer: 0
TID: 3069504368, PID: 30656, Buffer: 1
TID: 3059014512, PID: 30656, Buffer: 2
TID: 3077897072, PID: 30656, Buffer: 3
TID: 3069504368, PID: 30656, Buffer: 4
TID: 3077897072, PID: 30656, Buffer: 5
TID: 3059014512, PID: 30656, Buffer: 6
TID: 3069504368, PID: 30656, Buffer: 7
TID: 3077897072, PID: 30656, Buffer: 8
TID: 3059014512, PID: 30656, Buffer: 9
TID: 3069504368, PID: 30656, Buffer: 10 
TID: 3077897072, PID: 30656, Buffer: 11 
TID: 3069504368, PID: 30656, Buffer: 12 
TID: 3059014512, PID: 30656, Buffer: 13 
TID: 3069504368, PID: 30656, Buffer: 14

TID 3077897072 worked on the buffer 5 times 
TID 3069504368 worked on the buffer 6 times 
TID 3059014512 worked on the buffer 4 times 

Total buffer accesses: 15

这是我为解决作业而编写的代码（C 语言）：

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

int buffer = 0;
const int NUM_MODIFICATIONS = 15;
pthread_mutex_t lock;

struct Solution {
    unsigned long TID;
    int mod;
};

void * thread_function() {

    struct Solution *sol = malloc(sizeof *sol);
    sol->TID = (unsigned long) pthread_self();
    sol->mod = 0;

    while(buffer < NUM_MODIFICATIONS) {

        pthread_mutex_lock(&lock);
        printf("TID: %lu, PID: %d, Buffer: %d\n", sol->TID, getpid(), buffer);
        buffer++; sol->mod++;
        pthread_mutex_unlock(&lock);

        sleep(1);
    }

    return sol;
}

int main() {

    pthread_t t1, t2, t3;
    pthread_mutex_init(&lock, NULL);

    pthread_create(&t1, NULL, thread_function, NULL);
    pthread_create(&t2, NULL, thread_function, NULL);
    pthread_create(&t3, NULL, thread_function, NULL);

    void * out_void1, * out_void2, * out_void3;
    struct Solution *out1, *out2, *out3;

    pthread_join(t1, &out_void1);
    pthread_join(t2, &out_void2);
    pthread_join(t3, &out_void3);

    out1 = out_void1;
    out2 = out_void2;
    out3 = out_void3;

    printf("TID %lu worked on the buffer %d times\n", out1->TID, out1->mod);
    printf("TID %lu worked on the buffer %d times\n", out2->TID, out2->mod);
    printf("TID %lu worked on the buffer %d times\n", out3->TID, out3->mod);

    printf("Total buffer accesses: %d\n", out1->mod + out2->mod + out3->mod);

    pthread_mutex_destroy(&lock);
}

起初，我没有包含

sleep(1)

指令，这使得程序无法按预期工作（缓冲区将达到 17，不知道为什么，因为我想说互斥锁已正确设置）。

我的直觉告诉我，线程执行之间的一些分离可能会解决这个问题，所以我在锁解锁后添加了

sleep(1)

指令。现在，它确实起作用了，但现在这个程序总是让每个线程在缓冲区上工作 5 次，而我无法让它工作，因此某些线程比其他线程工作得多一点或少一点。

任何有关如何解决此问题的帮助将不胜感激。

Answer 1

问题是您在互斥体创建的独占区域之外访问

buffer

。

while(buffer < NUM_MODIFICATIONS) {       // XXX WRONG
    pthread_mutex_lock(&lock);
    printf("TID: %lu, PID: %d, Buffer: %d\n", sol->TID, getpid(), buffer);
    buffer++; sol->mod++;
    pthread_mutex_unlock(&lock);
}