睡觉的理发师给僵局答案

【问题标题】：sleeping barber giving deadlock睡觉的理发师给僵局
【发布时间】：2012-08-28 12:53:08
【问题描述】：

我编写了睡眠理发师问题的代码，看起来很奇怪...... 代码如下..

#include<stdlib.h>
#include<stdio.h>
#include<pthread.h>

#define MAX_C 10
int a[MAX_C], head=0, tail=0, tmp, tb, tc, count=1;
pthread_mutex_t B;
double time_slot[]={0.125,0.5,0.75,1.00,1.25,1.50,1.75,2.00};

void wait(int a)
{
    clock_t g=clock();
    while(((float)(clock()-g))/CLOCKS_PER_SEC != time_slot[a]);
}

void barber()
{
    printf("barber started\n");
    while(1) {
        tmp=0;
        while(head==tail) {
            printf("b\n");
        }
        tb=rand()%8;
        printf("tail:%d\n", tail);
        tail=(tail+1)%MAX_C;
        wait(tb);
    }
}

void customer()
{       
    printf("customers started\n");
    while(1) {
        wait(rand()%8);
        while((head+1)%MAX_C == tail) {
            printf("c\n");
        }
        a[head]=count-1;
        printf("head:%d\n", head);
        head=(head+1)%MAX_C;
    }
}

int main(int argc, char* argv[])
{
    pthread_t b,c;

    pthread_mutex_init(&B, NULL);
    pthread_create(&c, NULL, (void*)&customer, NULL);
    pthread_create(&b, NULL, (void*)&barber, NULL);

    pthread_join(b, NULL);
    pthread_join(c, NULL);  
    exit(0);
}

问题在于，当缓冲区已满时……理发师正在等待客户……但客户根本没有执行！（既没有等待也没有填充缓冲区）……因为客户while 循环没有执行...

【问题讨论】：

你的线程函数原型应该是void* function (void*)
这不可能是死锁，因为你从来没有在我能看到的任何地方真正锁定你的互斥锁
它是否输入customer？ IE。第一个printf 语句是否被执行？您是否尝试过在调试器中运行它，在customer 中放置一个断点并单步执行以查看发生了什么？
您正在修改 2 个未同步的全局变量，这些变量用于控制循环逻辑。您的代码无法以有意义的方式进行预测。当您更改 head 和 tail 时，您或许应该考虑使用互斥锁来保护您的代码。当然，除非我在这里完全误解了某些东西......
@nitish712 （非常）不太可能发生，但是如果while 错过了正确的clock() 值，它将循环直到它换行（如果clock_t 是一个整数类型，如果@ 是世界末日987654331@ 是浮点类型）。使用< 只是更具防御性。

标签： c multithreading deadlock

【解决方案1】：

正如 Daneil Fischer 所说...while(((float)(clock()-g))/CLOCKS_PER_SEC != time_slot[a]); 有一个错误，我应该用

替换它

while(((float)(clock()-g))/CLOCKS_PER_SEC <= time_slot[a]);

但奇怪的是，时钟值的这种“丢失”仅在整个缓冲区被填满后才发生......

【讨论】：

【解决方案2】：

执行器使用 CPU 的信号量有问题

请查看您是否很好地使用了信号量。不要忘记#include

#include <unistd.h>
#include <stdlib.h>

#include <pthread.h>
#include <semaphore.h>

// The maximum number of customer threads.
#define MAX_CUSTOMERS 25
</b>

void *customer(void *number) {
    int num = *(int *)number;

    // Leave for the shop and take some random amount of
    // time to arrive.
    printf("Customer %d leaving for barber shop.\n", num);
    randwait(5);
    printf("Customer %d arrived at barber shop.\n", num);

    // Wait for space to open up in the waiting room...
    sem_wait(&waitingRoom);
    printf("Customer %d entering waiting room.\n", num);

    // Wait for the barber chair to become free.
    sem_wait(&barberChair);

    // The chair is free so give up your spot in the
    // waiting room.
    sem_post(&waitingRoom);

    // Wake up the barber...
    printf("Customer %d waking the barber.\n", num);
    sem_post(&barberPillow);

    // Wait for the barber to finish cutting your hair.
    sem_wait(&seatBelt);

    // Give up the chair.
    sem_post(&barberChair);
    printf("Customer %d leaving barber shop.\n", num);
}

void *barber(void *junk) {
    // While there are still customers to be serviced...
    // Our barber is omnicient and can tell if there are 
    // customers still on the way to his shop.
    while (!allDone) {

    // Sleep until someone arrives and wakes you..
    printf("The barber is sleeping\n");
    sem_wait(&barberPillow);

    // Skip this stuff at the end...
    if (!allDone) {

        // Take a random amount of time to cut the
        // customer's hair.
        printf("The barber is cutting hair\n");
        randwait(3);
        printf("The barber has finished cutting hair.\n");

        // Release the customer when done cutting...
        sem_post(&seatBelt);
    }
    else {
        printf("The barber is going home for the day.\n");
    }
    }
}

【讨论】：