Pthread 冻结标准输出？

Question

假设 C++ 程序中有两个线程 (pthread)：

main thread
child thread

程序的作用很简单：

将两个线程绑定到两个不同的内核。
将两个线程的优先级设置为一些非常高的值（child thread 为 -99，main thread 为 -98）。
child thread 正在执行一些使用 100% CPU 的繁重任务。
main thread 在创建 child thread 后尝试调用 printf()。

问题是，一旦子线程被创建，它就会冻结标准输出并且控制台上不再打印任何内容。但是，当程序退出时，所有消息突然显示在控制台中。下面是演示此效果的 .cpp 文件：

main.cpp:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/mman.h>

bool EXIT = false;

void signal_handler(int signal){
    EXIT = true;
}

void *child_thread(void *x_args){
    printf("Setting child thread CPU affinity (Core #1)...\n");
    cpu_set_t cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(1, &cpuset);
    if (pthread_setaffinity_np(pthread_self(), sizeof(cpu_set_t), &cpuset)){
        perror("Cannot set child thread CPU affinity");
        printf("Exit\n");
        exit(1);
    }

    printf("Locking memory of child thread...\n");
    mlockall(MCL_CURRENT | MCL_FUTURE);

    printf("Setting child thread priority (-99)...\n");
    struct sched_param sched_param;
    sched_param.sched_priority = sched_get_priority_max(SCHED_FIFO)-1;
    if (sched_setscheduler(0, SCHED_FIFO, &sched_param)){
        perror("Cannot set child thread priority");
        printf("Exit\n");
        exit(1);  
    }

    printf("Entering while loop inside child thread...\n");
    while(!EXIT){}
    return NULL;
}

int main(){
    signal(SIGINT, signal_handler);

    pthread_t thread;

    printf("Setting main thread CPU affinity (Core #0)...\n");
    cpu_set_t cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(0, &cpuset);
    if (pthread_setaffinity_np(pthread_self(), sizeof(cpu_set_t), &cpuset)){
        perror("Cannot set main thread CPU affinity");
        printf("Exit.\n");
        exit(1);
    }

    printf("Locking memory of main thread...\n");
    mlockall(MCL_CURRENT | MCL_FUTURE);

    printf("Setting main thread priority (-98)...\n");
    struct sched_param sched_param;
    sched_param.sched_priority = sched_get_priority_max(SCHED_FIFO)-2;
    if (sched_setscheduler(0, SCHED_FIFO, &sched_param)){
        perror("Cannot set main thread priority");
        printf("Exit.\n");
        exit(1);
    }

    printf("Creating child thread...\n");
    if (pthread_create(&thread, NULL, child_thread, NULL)){
        perror("Cannot create child thread");
        printf("Exit.\n");
        exit(1);
    }

    printf("Entering while loop in main thread...\n");
    while(!EXIT){
        sleep(1);
        printf("I can't see this until I press Ctrl+C!\n");
    }
    pthread_join(thread, NULL);

    printf("Exit.\n");
    return 0;
}

你可以编译它：

g++ main.cpp -pthread -o main

然后运行它与：

sudo ./main

然后你应该看到 stdout 在输出以下内容后冻结：

Setting main thread CPU affinity (Core #0)...
Locking memory of main thread...
Setting main thread priority (-98)...
Creating child thread...
Entering while loop in main thread...
Setting child thread CPU affinity (Core #1)...

即使在一个小时后，您也看不到任何输出。但是当 Ctrl+C 被按下时。你会看到所有的消息都出来了：

I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!
I can't see this until I press Ctrl+C!

main thread 实际上是在后台运行ning 因为如果你注释掉 while 循环中的两行（sleep 和 printf），你可以看到它也在使用 100 CPU 的百分比。但是ht

我在这里错过了什么？

Answer 1

我不会声称自己是专家，但您似乎只有一个资源、stdout 和两个试图使用它的线程。我相信 printf 是线程安全的，但不是可重入的。我的直觉是在对 printf 和 stdout 的访问周围使用某种线程安全锁定，以确保一次只有一个线程调用它。

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