在GDB中运行程序与从命令行运行程序产生不同的结果

Question

作为学校作业的一部分，我正在解决理发师的睡眠问题 。 当我使用GDB运行它时，它可以正常工作，但是如果我从命令行运行它，它会卡住。 这是代码：

#include <stdio.h>
#include <semaphore.h>
#include <pthread.h>
#include <stdlib.h>
#define MAX 5

int nr_clients, open;
sem_t sem_open1, sem_clients, sem_chair, sem_number;

void *Barber(){
    sem_wait(&sem_open1);
    printf("Barber: Opening the shop\n");
    open = 1;
    sem_post(&sem_open1);
    while(1){
        printf("Barber: Sleeping while there is no clients\n");
        sem_wait(&sem_clients);
        sem_wait(&sem_number);
        if(nr_clients > 0){
            sem_post(&sem_number);
            printf("Barber: starting to work on a client\n");
            sem_post(&sem_chair);
            sleep(1);
            sem_wait(&sem_number);
            printf("Barber: Client finished\n");
            nr_clients--;
            sem_post(&sem_number);
        } else {
            printf("Barber: Closing the shop\n");
            break;
        }
    }
}


void *Client(void *x){
    int i = *((int*)x);
    printf("%8sClient(%d): I want a haircut\n", "", i);
    sem_wait(&sem_open1);
    sem_wait(&sem_number);
    if(open == 1 && nr_clients < MAX){
        sem_post(&sem_open1);   
        printf("%8sClient(%d): entering the shop\n", "", i);
        nr_clients++;
        sem_post(&sem_number);
        sem_post(&sem_clients);
        sem_wait(&sem_chair);
        printf("%8sClient(%d): barber is working on me\n", "", i);
    } else {
        printf("No more room\n");
    }
}



int main(int argc, char const *argv[])
{
    if(sem_init(&sem_open1,0,1) == -1 || sem_init(&sem_clients,0,0) == -1
        || sem_init(&sem_chair,0,0) == -1 || sem_init(&sem_number,0,1) == -1){
        printf("ERROR!\n");
        return 1;
    }

    int nr_threads = 5+1;

    pthread_t thr_id[nr_threads];

    if(pthread_create(&thr_id[0], NULL, Barber, NULL) != 0) {
        printf("ERROR!\n");
        exit(1);
    }

    int numbers[nr_threads-1];

    for (int i = 0; i < nr_threads-1; ++i)
    {
        numbers[i] = i;
    }

    for (int i = 1; i < nr_threads; ++i)
    {
        if(pthread_create(&thr_id[i], NULL, Client, &numbers[i-1]) != 0){
            printf("ERROR!\n");
            exit(1);
        }
    }




    sleep(10);

    sem_wait(&sem_open1);
    open = 0;
    sem_post(&sem_open1);

    sem_post(&sem_clients);

    for (int i = 0; i < nr_threads; ++i)
    {
        pthread_join(thr_id[i], NULL);
    }
}

我这样编译它：

gcc -g sleeping_barber.c -o barber -lpthread

然后只需在命令行中启动它即可。 命令行输出：

        Client(4): I want a haircut
No more room
        Client(3): I want a haircut
        Client(2): I want a haircut
        Client(1): I want a haircut
        Client(0): I want a haircut
^C

它只是卡在那里。

在GDB中，我这样运行它：

(gdb) file barber
Reading symbols from barber...done.
(gdb) r
Starting program: /home/marko/Desktop/barber 

GDB输出：

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff77f0700 (LWP 2808)]
Barber: Opening the shop
Barber: Sleeping while there is no clients
[New Thread 0x7ffff6fef700 (LWP 2809)]
        Client(0): I want a haircut
        Client(0): entering the shop
Barber: starting to work on a client
        Client(0): barber is working on me
[New Thread 0x7ffff67ee700 (LWP 2810)]
[Thread 0x7ffff6fef700 (LWP 2809) exited]
        Client(1): I want a haircut
        Client(1): entering the shop
[New Thread 0x7ffff5fed700 (LWP 2811)]
        Client(2): I want a haircut
        Client(2): entering the shop
[New Thread 0x7ffff57ec700 (LWP 2812)]
        Client(3): I want a haircut
        Client(3): entering the shop
[New Thread 0x7ffff4feb700 (LWP 2813)]
        Client(4): I want a haircut
        Client(4): entering the shop
Barber: Client finished
Barber: Sleeping while there is no clients
Barber: starting to work on a client
        Client(1): barber is working on me
[Thread 0x7ffff67ee700 (LWP 2810) exited]
Barber: Client finished
Barber: Sleeping while there is no clients
Barber: starting to work on a client
        Client(2): barber is working on me
[Thread 0x7ffff5fed700 (LWP 2811) exited]
Barber: Client finished
Barber: Sleeping while there is no clients
Barber: starting to work on a client
        Client(3): barber is working on me
[Thread 0x7ffff57ec700 (LWP 2812) exited]
Barber: Client finished
Barber: Sleeping while there is no clients
Barber: starting to work on a client
        Client(4): barber is working on me
[Thread 0x7ffff4feb700 (LWP 2813) exited]
Barber: Client finished
Barber: Sleeping while there is no clients
Barber: Closing the shop
[Thread 0x7ffff77f0700 (LWP 2808) exited]
[Inferior 1 (process 2804) exited normally]

按照我们的作业说明进行工作。

信息：

操作系统-> xubuntu-16.04.1

调试-> GNU gdb（Ubuntu 7.11.1-0ubuntu1〜16.04）7.11.1

Answer 1

您的代码中有多个问题。

如果客户不能进入理发店，则永远不要释放信号sem_open1和sem_client 。 这就是为什么您的线程明显等待sem_open1的原因。 您可以扣除这一点，因为“我要理发”之后没有更多输出。

但是，如果我们从头开始，则会看到当您的第一个线程进入车间时， open必须为0，因为您的条件if(open == 1 && nr_clients < MAX)为假。 由于尚无人增加nr_clients ，因此它必须是open部分。

此外，您可能只看您的输出：理发师尚未打开商店。 如果商店在第一次尝试关闭时，您可能会添加一些逻辑以重试。

出现问题的原因是您假定执行线程的特定顺序。 从输出中的线程号可以很容易地看到，这不是您期望的。 您根本不能依靠任何命令。

为了确保您的商店开张，您需要在启动其他线程之前给理发师线程一个运行的机会。 在创建理发师后添加一些sleep() 。

最后...根据我写的内容，您可能会得出结论，那就是所有关于时间的问题。 如果您是在调试器中运行程序，也可以在没有调试器的情况下运行程序，则计时可能会大不相同。