C - 使用 pthread 重现两个线程之间全局变量的同步问题

Question

我想重现使用两个 threads 时应该遇到的问题修改 global variable: solde 。 first thread增加 solde和 second减少它。

有源代码就更清楚了:

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

int solde = 250;

void *threadFunc(void *arg)
{
    if (strcmp(arg, "credit") == 0)
    {
        // increment critical ressource
        printf("[credit-start] solde = %d\n", solde);
        solde = solde + 10000;
        printf("[credit-end] solde = %d\n", solde);
    } else {
        // decrement critical ressource
        printf("[debit-start] solde = %d\n", solde);
        solde = solde - 30;
        printf("[debit-end] solde = %d\n", solde);
    }

    return NULL;
}

int main()
{
    // spawn a thread
    pthread_t threadCredit;
    pthread_t threadDebit;

    int i;
    for (i = 0; i < 2; i++)
    {
        if (i == 0)
            pthread_create(&threadCredit, NULL, &threadFunc, "credit");
        else
            pthread_create(&threadDebit, NULL, &threadFunc, "debit");
    }

    pthread_join(threadCredit, NULL);
    pthread_join(threadDebit, NULL);

    printf("[final] solde = %d\n", solde);
    return 0;
}

我有时得到的是:
[debit-start] solde = 250
[debit-end] solde = 220 (-30)
[credit-start] solde = 250 // <= I wouldnt get this with mutex
[credit-end] solde = 10220 (+10000)
[final] solde = 10220

有时:
[credit-start] solde = 250
[credit-end] solde = 10250 (+10000)
[debit-start] solde = 250 // <= I wouldnt get this with mutex
[debit-end] solde = 10220 (-30)
[final] solde = 10220

但是，我观察到 result即使不使用 mutex 也是正确的。我想要得到的是这样的错误结果(其中两个 threads 以初始值 250 开头):

[credit-start] solde = 250
[credit-end] solde = 10250 (+10000)
[debit-start] solde = 250
[debit-end] solde = 220 (-30)
[final] solde = 220

P.S:当我使用pthread时互斥体around the inner content of线程函数`，结果更有逻辑:

[debit-start] solde = 250
[debit-end] solde = 220
[credit-start] solde = 220
[credit-end] solde = 10220
[final] solde = 10220

或者:

[credit-start] solde = 250
[credit-end] solde = 10250
[debit-start] solde = 10250
[debit-end] solde = 10220
[final] solde = 10220

Answer 1

更改您的线程函数，以便它保留数据的本地副本，并让其他线程在获取副本后运行。这将突出显示您正在寻找的问题:

void *threadFunc(void *arg)
{
    int local_solde = solde;
    sleep(1);
    if (strcmp(arg, "credit") == 0)
    {
        // increment critical ressource
        printf("[credit-start] solde = %d\n", local_solde);
        solde = local_solde + 10000;
        printf("[credit-end] solde = %d\n", solde);
    } else {
        // decrement critical ressource
        printf("[debit-start] solde = %d\n", local_solde);
        solde = local_solde - 30;
        printf("[debit-end] solde = %d\n", solde);
    }

    return NULL;
}