山东大学操作系统实验吸烟者问题

吸烟者问题是一个经典的同步问题，它涉及到三个进程：一个代表桌子，另外两个代表吸烟者。

桌子进程负责往桌子上放烟草、纸和火柴。每个吸烟者进程需要烟草、纸和火柴中的两个才能抽烟。当一个吸烟者进程拥有了两个必需的材料之后，它将抽烟并通知桌子进程继续放置烟草、纸和火柴。

以下是一种可能的实现：

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

pthread_mutex_t table_lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t match_cond = PTHREAD_COND_INITIALIZER;
pthread_cond_t paper_cond = PTHREAD_COND_INITIALIZER;
pthread_cond_t tobacco_cond = PTHREAD_COND_INITIALIZER;

int match_on_table = 0;
int paper_on_table = 0;
int tobacco_on_table = 0;

void *agent(void *arg) {
    while (1) {
        pthread_mutex_lock(&table_lock);
        while (match_on_table && paper_on_table && tobacco_on_table) {
            pthread_cond_wait(&match_cond, &table_lock);
            pthread_cond_wait(&paper_cond, &table_lock);
            pthread_cond_wait(&tobacco_cond, &table_lock);
        }

        int r = rand() % 3;
        if (r == 0) {
            match_on_table = 1;
            printf("agent: put match on table\n");
            pthread_cond_signal(&paper_cond);
            pthread_cond_signal(&tobacco_cond);
        } else if (r == 1) {
            paper_on_table = 1;
            printf("agent: put paper on table\n");
            pthread_cond_signal(&match_cond);
            pthread_cond_signal(&tobacco_cond);
        } else {
            tobacco_on_table = 1;
            printf("agent: put tobacco on table\n");
            pthread_cond_signal(&match_cond);
            pthread_cond_signal(&paper_cond);
        }

        pthread_mutex_unlock(&table_lock);
        sleep(1);
    }

    return NULL;
}

void *smoker_match(void *arg) {
    while (1) {
        pthread_mutex_lock(&table_lock);
        while (!paper_on_table || !tobacco_on_table) {
            pthread_cond_wait(&match_cond, &table_lock);
        }

        printf("smoker with match: start smoking\n");
        match_on_table = 0;
        paper_on_table = 0;
        tobacco_on_table = 0;

        pthread_mutex_unlock(&table_lock);
        sleep(1);
    }

    return NULL;
}

void *smoker_paper(void *arg) {
    while (1) {
        pthread_mutex_lock(&table_lock);
        while (!match_on_table || !tobacco_on_table) {
            pthread_cond_wait(&paper_cond, &table_lock);
        }

        printf("smoker with paper: start smoking\n");
        match_on_table = 0;
        paper_on_table = 0;
        tobacco_on_table = 0;

        pthread_mutex_unlock(&table_lock);
        sleep(1);
    }

    return NULL;
}

void *smoker_tobacco(void *arg) {
    while (1) {
        pthread_mutex_lock(&table_lock);
        while (!match_on_table || !paper_on_table) {
            pthread_cond_wait(&tobacco_cond, &table_lock);
        }

        printf("smoker with tobacco: start smoking\n");
        match_on_table = 0;
        paper_on_table = 0;
        tobacco_on_table = 0;

        pthread_mutex_unlock(&table_lock);
        sleep(1);
    }

    return NULL;
}

int main() {
    pthread_t agent_thread;
    pthread_t smoker_match_thread;
    pthread_t smoker_paper_thread;
    pthread_t smoker_tobacco_thread;

    pthread_create(&agent_thread, NULL, agent, NULL);
    pthread_create(&smoker_match_thread, NULL, smoker_match, NULL);
    pthread_create(&smoker_paper_thread, NULL, smoker_paper, NULL);
    pthread_create(&smoker_tobacco_thread, NULL, smoker_tobacco, NULL);

    pthread_join(agent_thread, NULL);
    pthread_join(smoker_match_thread, NULL);
    pthread_join(smoker_paper_thread, NULL);
    pthread_join(smoker_tobacco_thread, NULL);

    return 0;
}

在这个实现中，桌子进程负责放置材料，每个吸烟者进程会不断地等待桌子进程放置必需的材料。当桌子进程放置好两个必需的材料之后，对应的吸烟者进程会抽烟，并将桌子清空，通知桌子进程继续放置材料。这个实现使用了 pthread 库来实现线程同步。