理发师问题的描述：一个理发店接待室有n张椅子，工作室有1张椅子；没有顾客时，理发师睡觉；第一个顾客来到时，必须将理发师唤醒；顾客来时如果还有空座的话，他就坐在一个座位上等待；如果顾客来时没有空座位了，他就离开，不理发了；当理发师处理完所有顾客，而又没有新顾客来时，他又开始睡觉。C语言实现

好的，以下是C语言实现。假设理发店接待室有5张椅子。

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

#define N_SEATS 5  // 接待室座位数
#define WORK_CHAIR 1  // 理发师工作室椅子数

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond_customers = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond_barber = PTHREAD_COND_INITIALIZER;

int waiting_customers = 0;
int barber_sleeping = 0;

void *customer(void *arg);
void *barber(void *arg);

int main()
{
    pthread_t barber_thread, customer_thread[20];
    int i, id[20];

    pthread_create(&barber_thread, NULL, barber, NULL);

    for (i = 0; i < 20; i++) {
        id[i] = i;
        pthread_create(&customer_thread[i], NULL, customer, &id[i]);
        sleep(1);
    }

    for (i = 0; i < 20; i++) {
        pthread_join(customer_thread[i], NULL);
    }

    pthread_cancel(barber_thread);
    pthread_join(barber_thread, NULL);

    return 0;
}

void *customer(void *arg)
{
    int id = *(int *)arg;

    pthread_mutex_lock(&mutex);
    printf("Customer %d comes.\n", id);

    while (waiting_customers >= N_SEATS) {
        printf("No seats available for customer %d. The customer leaves.\n", id);
        pthread_mutex_unlock(&mutex);
        return NULL;
    }

    waiting_customers++;
    printf("Customer %d takes a seat. Waiting customers: %d.\n", id, waiting_customers);
    pthread_cond_signal(&cond_customers);  // 唤醒理发师

    while (!barber_sleeping) {
        printf("Customer %d is waiting for the barber.\n", id);
        pthread_cond_wait(&cond_barber, &mutex);  // 等待理发师完成理发
    }

    waiting_customers--;
    barber_sleeping = 0;
    printf("Customer %d is getting a haircut.\n", id);
    pthread_mutex_unlock(&mutex);
    return NULL;
}

void *barber(void *arg)
{
    while (1) {
        pthread_mutex_lock(&mutex);
        while (waiting_customers == 0) {
            printf("The barber is sleeping.\n");
            barber_sleeping = 1;
            pthread_cond_wait(&cond_customers, &mutex);  // 等待顾客到来
        }

        printf("The barber starts to work. Waiting customers: %d.\n", waiting_customers);
        waiting_customers--;
        barber_sleeping = 0;
        pthread_cond_signal(&cond_barber);  // 唤醒顾客
        pthread_mutex_unlock(&mutex);
        sleep(3);  // 理发师完成理发需要一定时间
    }
}

上述代码中，使用了两个条件变量`cond_customers`和`cond_barber`，分别用于唤醒理发师和顾客。当顾客到来时，顾客线程调用`pthread_mutex_lock`锁定互斥量`mutex`，判断是否有空座位。如果没有，顾客离开；否则，顾客将等待理发师完成理发。当理发师完成理发后，顾客线程解锁互斥量`mutex`，跳出循环。当理发师睡眠时，理发师线程调用`pthread_cond_wait`等待条件变量`cond_customers`。当有顾客到来时，理发师被唤醒，理发师线程解锁互斥量`mutex`，跳出循环。然后理发师开始工作，完成后重新进入循环等待下一个顾客的到来。