Question

假设在C ++程序中有两个线程（pthread）：

main thread
child thread

该计划的作用很简单：

将两个线程绑定到两个不同的核心。
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正在执行一项使用100％CPU的繁重任务。
main thread在创建child thread后尝试调用printf（）。

问题是，一旦创建了子线程，它就会冻结stdout，并且不再在控制台上打印任何内容。但是，当程序退出时，控制台中会突然显示所有消息。下面是一个展示这种效果的.cpp文件：

main.cpp中：

#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;
}

你可以用：

g++ main.cpp -pthread -o main

然后用：

sudo ./main

然后你应该在输出以下内容后看到stdout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)...

即使一小时后你也不会再看到输出了。但是当Ctrl+C被按下时。您将看到所有消息：

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，因为如果你注释掉while循环中的两行（sleep和printf），你会发现它也使用了100％的CPU。但是，

我在这里缺少什么？

Answer 1

我不会声称自己是专家，但您似乎只有一个资源，stdout和两个尝试使用它的线程。我相信printf是线程安全的，但不是reentrent。我的第四个本能就是在访问printf和stdout时使用某种线程安全锁定，以确保一次只有一个线程调用它。

Pthread冻结stdout？

1 个答案: