线程未按正确顺序打印 [英] Thread not printing out in correct order

问题描述

我对 C 中的线程相当陌生.对于这个程序，我需要声明一个线程，我将它传入一个 for 循环，该循环旨在从线程中打印出 printf.

I'm fairly new to threads in C. For this program I need to declare a thread which I pass in a for loop thats meant to print out the printfs from the thread.

我似乎无法按正确顺序打印.这是我的代码:

I can't seem to get it to print in correct order. Here's my code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define NUM_THREADS 16

void *thread(void *thread_id) {
    int id = *((int *) thread_id);
    printf("Hello from thread %d\n", id);
    return NULL;
}

int main() {
    pthread_t threads[NUM_THREADS];
    for (int i = 0; i < NUM_THREADS; i++) {
        int code = pthread_create(&threads[i], NULL, thread, &i);

        if (code != 0) {
            fprintf(stderr, "pthread_create failed!\n");
            return EXIT_FAILURE;
        }
    }
    return EXIT_SUCCESS;
}

//gcc -o main main.c -lpthread

推荐答案

这是理解多线程的经典例子.线程并发运行，由操作系统调度程序调度.当我们谈论并行运行时，没有正确顺序"这样的东西.

That's the classic example of understanding multi-threading. The threads are running concurrently, scheduled by OS scheduler. There is no such thing as "correct order" when we are talking about running in parallel.

此外，还有为 stdout 输出刷新缓冲区之类的东西.意思是，当您打印"某些内容时，并不会保证它会立即发生，而是会在达到某个缓冲区限制/超时后发生.

Also, there is such thing as buffers flushing for stdout output. Means, when you "printf" something, it is not promised it will happen immediately, but after reaching some buffer limit/timeout.

另外，如果你想以正确的顺序"完成工作，意味着等到第一个线程完成它的工作再开始下一个，考虑使用加入":http://man7.org/linux/man-pages/man3/pthread_join.3.html

Also, if you want to do the work in the "correct order", means wait until the first thread finishes it's work before staring next one, consider using "join": http://man7.org/linux/man-pages/man3/pthread_join.3.html

更新:在这种情况下，将指针传递给 thread_id 也是不正确的，因为线程可能会打印不属于他的 id(感谢 Kevin)

UPD: passing pointer to thread_id is also incorrect in this case, as a thread may print id that doesn't belong to him (thanks Kevin)

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