多线程代码无法理解的结果

Question

我将开始一个将使用多线程的 C 编程项目。在开始项目之前，我已经写了一个练习代码。我的目的是了解互斥锁和线程是如何工作的。但是它不能正常工作。这是代码：

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

pthread_mutex_t myMutex;
char myStrings[100][30];
int i=0;

void *PrintThread1()
{
    printf("this is initial of FIRST Thread\n");
    for (int j=0; j<33; j++) {
        pthread_mutex_lock(&myMutex);
        strcpy(myStrings[i], "This is FIRST thread");
        i++;
        pthread_mutex_unlock(&myMutex);
    }
    pthread_exit(NULL);
}

void *PrintThread2()
{
    printf("this is initial of SECOND Thread\n");
    for (int j=0; j<33; j++) {
        pthread_mutex_lock(&myMutex);
        strcpy(myStrings[i], "This is SECOND thread");
        i++;
        pthread_mutex_unlock(&myMutex);
    }

    pthread_exit(NULL);

}

void *PrintThread3()
{
    printf("this is initial of THIRD Thread\n");
    for (int j=0; j<33; j++) {
        pthread_mutex_lock(&myMutex);
        strcpy(myStrings[i], "This is THIRD thread");
        i++;
        pthread_mutex_unlock(&myMutex);
    }

    pthread_exit(NULL);
}



int main(int argc, char *argv[])
{
    pthread_t firstThread, secondThread, thirdThread;
    //pthread_attr_t attr;
    pthread_mutex_init(&myMutex, NULL);
    //pthread_attr_init(&attr);
    //pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    int ft;
    ft = pthread_create(&firstThread, NULL, PrintThread1(), NULL);
    if (ft){
        printf("ERROR; return code from pthread_create() is %d\n", ft);
        exit(-1);
    }
    ft = pthread_create(&secondThread, NULL, PrintThread2(), NULL);
    if (ft){
        printf("ERROR; return code from pthread_create() is %d\n", ft);
        exit(-1);
    }
    ft = pthread_create(&thirdThread, NULL, PrintThread3(), NULL);
    if (ft){
        printf("ERROR; return code from pthread_create() is %d\n", ft);
        exit(-1);
    }

    pthread_join(firstThread, NULL);
    pthread_join(secondThread, NULL);
    pthread_join(thirdThread, NULL);

    pthread_mutex_destroy(&myMutex);

    for (int j=0;j<100; j++) {
        printf("String[%d] = %s\n",j,myStrings[j]);
    }
    printf("\n");

    pthread_exit(NULL);
    return -1;
}

当我执行这段代码时，我的结果是：

this is initial of FIRST Thread
Program ended with exit code: 0

我想不出错误。

Answer 1

ft = pthread_create(&firstThread, NULL, PrintThread1(), NULL);

应该阅读

ft = pthread_create(&firstThread, NULL, PrintThread1, NULL);

你的所有 pthread_create 电话也是如此。

要使用 pthread_create，您需要将线程启动例程的地址传递给它，您在 C 中通过编写函数名称 而没有 来完成函数调用括号。

如您所写，您调用预期的线程启动例程，在主线程上，并传递任何它returns 到 pthread_create 成为线程启动例程。但它从来没有 returns，因为它调用了 pthread_exit，这（因为 pthread_create 还没有被调用，所以只有一个线程）终止了整个程序。

不幸的是，你必须在编译器发现这个错误之前 way 发出警告，即使那样也不是很清楚问题是什么：

$ gcc -std=c99 -Wall -pedantic -pthread test.c
test.c: In function ‘main’:
test.c:60:45: warning: ISO C forbids passing argument 3 
  of ‘pthread_create’ between function pointer and ‘void *’ [-Wpedantic]
     ft = pthread_create(&firstThread, NULL, PrintThread1(), NULL);
                                             ^

没有-pedantic，无怨无悔

多线程代码无法理解的结果

Incomprehensible result of a multithread code

c

multithreading

mutex

pthreads