为什么sem_wait（）函数块，即使sem_t值为零？

Question

我正在尝试实现一个简单的解决方案来解决哲学家进餐问题（有五位哲学家），我的解决方案基于以下逻辑：

sem_t S[philosophers_number]
for each philosopher
{
    while(TRUE)
    {
        if(current philosopher number != last philosopher)
        {
            thinking()

            //i is number of current philosopher
            sem_wait(take_chopstick(S[(i+1) % philosophers_number])) // right chopstick
            sem_wait(take_chopstick(S[i])) // left chopstick

            eat()

            sem_post(put_chopstick(S[(i+1) % philosophers_number]))
            sem_post(put_chopstick(S[i]))
        }
        else
        {
            thinking()

            //i is number of current philosopher
            sem_wait(take_chopstick(S[i])) // left chopstick
            sem_wait(take_chopstick(S[(i+1) % philosophers_number])) // right chopstick

            eat()

            sem_post(put_chopstick(S[i]))          
            sem_post(put_chopstick(S[(i+1) % philosophers_number]))  
        }
}

每个哲学家首先思考的时间不到三秒钟

然后，如果右边有筷子，哲学家会拿走它，如果左边也有筷子，哲学家也会拿走它，开始吃不到三秒钟

然后哲学家会放下筷子，让其他哲学家也能使用

为了避免循环等待，对于最后一位哲学家，我将首先选择左边的筷子，然后选择右边的筷子，并继续相同的过程

以下是我基于此逻辑实现的代码：

#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
#include <stdlib.h>

#define THREADS 5
sem_t chopstick[THREADS];

void thinking(int ph_num)
{
    printf("philosopher %d is thinking\n", ph_num);
    int t = rand() % 3;
    sleep(t);// up to 3 secs thinking
}

void eat(int ph_num)
{
    printf("philosopher %d is eating\n", ph_num);
    int t = rand() % 3;
    sleep(t);// up to 3 secs eating
}

void *philosopher(void * ph_num )
{
    int num=(int)ph_num;
    while(1)
    {
        if(num < THREADS - 1)
        {
            thinking(num);
            
            //pick up right chopstick
            sem_wait(&chopstick[(num + 1) % THREADS]);

            //to make deadlocks absolutly happen, wait 1 sec then pickup left chopstick
            sleep(1);
            
            //pick up left chopstick
            sem_wait(&chopstick[num]);
        
            eat(num);   

            //put down right chopstick
            sem_post(&chopstick[(num + 1) % THREADS]);

            //put down left chopstick
            sem_post(&chopstick[num]);
        }

        else // last one pick left chopstick first, instead of right one to avoid cyclic wait
        {
                        thinking(num);

                        //pick up left chopstick
                        sem_wait(&chopstick[num]);

                        //to make deadlocks absolutly happen, wait 1 sec then pickup left chopstick 
                        sleep(1); 
                        
                        //pick up right chopstick
                        sem_wait(&chopstick[(num + 1) % THREADS]);

                        eat(num);

                        //put down left chopstick
                        sem_post(&chopstick[num]);

                        //put down right chopstick
                        sem_post(&chopstick[(num + 1) % THREADS]);
        }
    }

    pthread_exit((void *)num);
}

int main ()
{
    for(int i = 0; i < THREADS; i++)
    {
        sem_init(&chopstick[i],0,1);
    }

    pthread_t threads[THREADS];
    
    for(int i = 0; i < THREADS; i++)
        pthread_create(&threads[i], NULL, philosopher, (void *)i);
    
    for(int i = 0; i < THREADS; i++)
        pthread_join(threads[i],NULL);
    return 0;
}

但是在调试这段代码的过程中出现了一个问题，chomptick[i]在sem\u等待之前是0(

谁能帮我找出我的问题出在哪里？




Answer 1

你的实现是正确的，你的问题在于调试的方法。如果您使用gdb，您将仅在一个线程上停止，而其余线程将继续执行，因此在您检查信号量的时间和您进入下一行的时间之间，其他线程将继续执行执行并可以更改您检查过的值。

为了有效地调试线程，您需要确保只有当前观察到的线程被调度，其余的线程被阻塞。为此，您需要在线程上停止后更改调度器锁定。您可以将其设置为on或step，这取决于您希望线程完全停止，还是仅在单步操作期间停止（有关更多详细信息，请参阅帮助设置调度器锁定）。

锁定线程后，您可以使用info threads检查其余线程当时正在执行的操作。您可以使用线程

下面是将调度程序设置为步骤的示例。您可以看到，只有一个线程在next命令上运行。

(gdb) b 37
Breakpoint 1 at 0x1388: file test003.c, line 37.
(gdb) r
Starting program: /home/jordan/Development/tmptest/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff7d90700 (LWP 4002538)]
philosopher 0 is thinking
[New Thread 0x7ffff758f700 (LWP 4002539)]
philosopher 1 is thinking
[New Thread 0x7ffff6d8e700 (LWP 4002540)]
philosopher 2 is thinking
[2] picking 3
[New Thread 0x7ffff658d700 (LWP 4002541)]
[Switching to Thread 0x7ffff6d8e700 (LWP 4002540)]

Thread 4 "a.out" hit Breakpoint 1, philosopher (ph_num=0x2) at test003.c:37
37              sem_wait(&chopstick[(num + 1) % THREADS]);
(gdb) set scheduler-locking step
(gdb) info threads
  Id   Target Id                                   Frame
  1    Thread 0x7ffff7d91740 (LWP 4002534) "a.out" clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:78
  2    Thread 0x7ffff7d90700 (LWP 4002538) "a.out" 0x00007ffff7e743bf in __GI___clock_nanosleep (clock_id=clock_id@entry=0, flags=flags@entry=0,
    req=req@entry=0x7ffff7d8fe60, rem=rem@entry=0x7ffff7d8fe60) at ../sysdeps/unix/sysv/linux/clock_nanosleep.c:78
  3    Thread 0x7ffff758f700 (LWP 4002539) "a.out" 0x00007ffff7e743bf in __GI___clock_nanosleep (clock_id=clock_id@entry=0, flags=flags@entry=0,
    req=req@entry=0x7ffff758ee60, rem=rem@entry=0x7ffff758ee60) at ../sysdeps/unix/sysv/linux/clock_nanosleep.c:78
* 4    Thread 0x7ffff6d8e700 (LWP 4002540) "a.out" philosopher (ph_num=0x2) at test003.c:37
  5    Thread 0x7ffff658d700 (LWP 4002541) "a.out" clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:78
(gdb) n
38              printf("[%i] picked %i\n", num, (num + 1) % THREADS);
(gdb) info threads
  Id   Target Id                                   Frame
  1    Thread 0x7ffff7d91740 (LWP 4002534) "a.out" clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:78
  2    Thread 0x7ffff7d90700 (LWP 4002538) "a.out" 0x00007ffff7e743bf in __GI___clock_nanosleep (clock_id=clock_id@entry=0, flags=flags@entry=0,
    req=req@entry=0x7ffff7d8fe60, rem=rem@entry=0x7ffff7d8fe60) at ../sysdeps/unix/sysv/linux/clock_nanosleep.c:78
  3    Thread 0x7ffff758f700 (LWP 4002539) "a.out" 0x00007ffff7e743bf in __GI___clock_nanosleep (clock_id=clock_id@entry=0, flags=flags@entry=0,
    req=req@entry=0x7ffff758ee60, rem=rem@entry=0x7ffff758ee60) at ../sysdeps/unix/sysv/linux/clock_nanosleep.c:78
* 4    Thread 0x7ffff6d8e700 (LWP 4002540) "a.out" philosopher (ph_num=0x2) at test003.c:38
  5    Thread 0x7ffff658d700 (LWP 4002541) "a.out" clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:78

如您所见，在执行下一步之后，我仍然在同一个线程上，其他线程没有进展。

我已经修改了代码，以使发生的事情更加可见，下面是我使用的代码：

#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
#include <stdlib.h>

#define THREADS 5

sem_t chopstick[THREADS];

void thinking(int ph_num)
{
    printf("philosopher %d is thinking\n", ph_num);
    int t = rand() % 3;
    sleep(t);// up to 3 secs thinking
}

void eat(int ph_num)
{
    printf("philosopher %d is eating\n", ph_num);
    int t = rand() % 3;
    sleep(t);// up to 3 secs eating
}

void *philosopher(void * ph_num )
{
    int num=(int)ph_num;
    while(1)
    {
        if(num < THREADS - 1)
        {
            thinking(num);

            //pick up right chopstick
            printf("[%i] picking %i\n", num, (num + 1) % THREADS);
            sem_wait(&chopstick[(num + 1) % THREADS]);
            printf("[%i] picked %i\n", num, (num + 1) % THREADS);

            //to make deadlocks absolutly happen, wait 1 sec then pickup left chopstick
            //sleep(1);

            //pick up left chopstick
            printf("[%i] picking %i\n", num, num);
            sem_wait(&chopstick[num]);
            printf("[%i] picked %i\n", num, num);

            eat(num);

            //put down right chopstick
            printf("[%i] put %i\n", num, (num + 1) % THREADS);
            sem_post(&chopstick[(num + 1) % THREADS]);

            //put down left chopstick
            printf("[%i] put %i\n", num, num);
            sem_post(&chopstick[num]);
        }

        else // last one pick left chopstick first, instead of right one to avoid cyclic wait
        {
            thinking(num);

            //pick up left chopstick
            printf("[%i] picking %i\n", num, num);
            sem_wait(&chopstick[num]);
            printf("[%i] picked %i\n", num, num);

            //to make deadlocks absolutly happen, wait 1 sec then pickup left chopstick
            //sleep(1);

            //pick up right chopstick
            printf("[%i] picking %i\n", num, num+1);
            sem_wait(&chopstick[(num + 1) % THREADS]);
            printf("[%i] picked %i\n", num, num+1);

            eat(num);

            //put down left chopstick
            printf("[%i] put %i\n", num, num);
            sem_post(&chopstick[num]);

            //put down right chopstick
            printf("[%i] put %i\n", num, (num + 1) % THREADS);
            sem_post(&chopstick[(num + 1) % THREADS]);
        }
    }

    pthread_exit((void *)num);
}

int main ()
{
    for(int i = 0; i < THREADS; i++)
    {
        sem_init(&chopstick[i],0,1);
    }

    pthread_t threads[THREADS];

    for(int i = 0; i < THREADS; i++)
        pthread_create(&threads[i], NULL, philosopher, (void *)i);

    for(int i = 0; i < THREADS; i++)
        pthread_join(threads[i],NULL);
    return 0;
}