pthread 中的参数传递错误 [英] Arguments were passed wrong in pthread

问题描述

我编写了一个代码来打印字符串:Thread 0"；到线程 4"使用 pthread.

I write a code to print out strings: "Thread 0" to "Thread 4" using pthread.

这是我的代码:

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

void *print_message_function(void* parameter) {
    long *i = (long *)parameter;
    printf("Thread %ld\n", *i);
    pthread_exit(0);
}

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    long i = 0;
    for (i = 0; i < 5; i++) {
        pthread_create(&threads[i], 0, print_message_function, (void *)&i);
    }
    
    pthread_exit(NULL);
}

但结果是:

Thread 2
Thread 3
Thread 3
Thread 4
Thread 5

或:

Thread 0
Thread 0
Thread 0
Thread 0
Thread 0

当我再次运行它时它改变了.所以我不知道为什么我传递的值是(2 到 5)或全部(0)或 .....(许多情况).我想我传递的参数是从 0 到 4.

It changed when I run it again. So I don't know why the values I passed are (2 to 5) or all (0) or ..... (many cases). I think my arguments I passed is from 0 to 4.

当我更改为新代码时:

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

void *print_message_function(void *parameter);

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    int i = 0;
    for (i = 0; i < 5; i++) {
        char *msg = (char*)malloc(sizeof(char));
        sprintf(msg, "Thread %d", i);
        pthread_create(&threads[i], 0, print_message_function, (void *)msg);
    }
}

void *print_message_function(void *parameter) {
    printf("%s\n", (char *)parameter);
}

结果是:

Thread 1
Thread 0
Thread 3
Thread 2
Thread 4
Thread 4

这意味着循环运行了 6 次！为什么?

It means the loop run 6 times! Why?

推荐答案

将案例 1 更改为:

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

void *print_message_function(void* parameter) {
    long i = (long)parameter;  // <<<
    printf("Thread %ld\n", i); // <<<
    pthread_exit(0);
}

int main(int argc, char *argv[]) {
    pthread_t threads[5];
    long i = 0;
    for (i = 0; i < 5; i++) {
        pthread_create(&threads[i], 0, print_message_function, (void *)i); // <<<
    }

    pthread_exit(NULL);
}

您之前看到不一致结果的原因是因为您将一个指针传递给每个线程，其中每个指针都指向同一个局部变量，然后您正在修改该变量.

The reason that you were seeing inconsistent results before was because you were passing a pointer to each thread where each pointer was pointing at the same local variable, which you were then modifying.

在情况 2 中，您仅 malloc 处理单个字符，然后尝试向其中写入字符串.应该很容易修复.

In Case 2 you are mallocing only a single char and then trying to write a string to it. It should be fairly easy to fix.

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