Pthread冻结标准输出

假设c++程序中有两个线程(pthread):

• main thread
• child thread

程序的功能很简单:

1. 将两个线程绑定到两个不同的内核。
2. 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)。
3. child thread正在执行一些使用100% CPU的繁重任务。
4. child thread创建后，main thread正在尝试调用printf()

问题是，一旦创建了子线程，它就会冻结标准输出，并且不再在控制台上打印任何内容。但是，当程序退出时，所有消息突然显示在控制台中。下面是一个.cpp文件来演示这个效果:

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("Exitn");
        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("Exitn");
        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:

sudo ./main

那么您应该看到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)...

即使过了一个小时，你也不会看到任何输出。但是当按下Ctrl+C时。您将看到所有消息输出:

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的，因为如果你注释掉while循环中的两行(sleep和printf)，你可以看到它也使用了100%的CPU。但ht

我在这里错过了什么?