C语言多线程读写交替执行问题求助（附出错代码）

我想编写一个包含reader和writer两个线程的程序，两个线程各运行10次迭代，需要通过同步机制实现二者交替执行。仅使用mutex无法实现需求，改用条件变量后程序陷入无限等待状态，以下是我尝试的代码：

int my_global = 0;
int write_flag = 0;
pthread_mutex_t mutex;
pthread_cond_t mycond;

void *reader() {
    for(int j=0;j<10;j++){
        pthread_mutex_lock(&mutex);
        printf("\nReader - value of global is %d & write flag is %d and j is %d",my_global,write_flag,j);
        write_flag = 1;
        pthread_mutex_unlock(&mutex);
        pthread_cond_signal(&mycond);
        // sleep(1);
    }
}

void *writer(){
    for(int k=0;k<10;k++){
        pthread_mutex_lock(&mutex);
        printf("\nstarted writer");
        while(write_flag!=1){
            pthread_cond_wait(&mycond,&mutex);
        }
        write_flag = 0;
        my_global++;
        printf("\nwriter done and writeflag is %d and global is %d and k is %d",write_flag,my_global,k);
        pthread_mutex_unlock(&mutex);
    }
}

int main()
{
    pthread_t my_reader;
    pthread_t my_writer;
    pthread_cond_init(&mycond,NULL);
    pthread_mutex_init(&mutex,NULL);
    if (pthread_create(&my_reader, NULL, &reader, NULL) != 0) {
        perror("Failed to create thread");
    }
    if (pthread_create(&my_writer, NULL, &writer, NULL) != 0) {
        perror("Failed to create thread");
    }
    if (pthread_join(my_reader, NULL) != 0) {
        perror("Failed to join thread");
    }
    if (pthread_join(my_writer, NULL) != 0) {
        perror("Failed to join thread");
    }
    pthread_mutex_destroy(&mutex);
    pthread_cond_destroy(&mycond);
    return 0;
}

你的代码存在几个关键问题导致无限等待：

• 信号丢失：reader线程解锁后才发送条件变量信号，如果此时writer线程还未进入pthread_cond_wait，信号会直接丢失，后续writer进入等待后无法被唤醒。
• 单向通知：只有reader通知writer执行，writer完成后没有通知reader启动下一次迭代，导致reader跑完10次后writer可能还未完成全部迭代，或者出现执行顺序混乱。
• 状态逻辑模糊：write_flag仅标记writer是否可以执行，但没有标记reader的执行权限，无法形成双向交替的同步逻辑。

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

int my_global = 0;
// 0: 等待reader执行，1: 等待writer执行
int turn = 0;
pthread_mutex_t mutex;
pthread_cond_t cond;

void *reader() {
    for(int j=0;j<10;j++){
        pthread_mutex_lock(&mutex);
        // 循环检查状态，防止虚假唤醒
        while(turn != 0) {
            pthread_cond_wait(&cond, &mutex);
        }
        printf("\nReader - global value: %d, turn: %d, iteration: %d", my_global, turn, j);
        // 切换状态，让writer可以执行
        turn = 1;
        // 持有锁时发送信号，确保writer能接收到
        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}

void *writer(){
    for(int k=0;k<10;k++){
        pthread_mutex_lock(&mutex);
        while(turn != 1) {
            pthread_cond_wait(&cond, &mutex);
        }
        my_global++;
        printf("\nWriter done - turn: %d, global value: %d, iteration: %d", turn, my_global, k);
        // 切换状态，让reader可以执行
        turn = 0;
        pthread_cond_signal(&cond);
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}

int main()
{
    pthread_t my_reader;
    pthread_t my_writer;
    pthread_cond_init(&cond, NULL);
    pthread_mutex_init(&mutex, NULL);
    
    if (pthread_create(&my_reader, NULL, &reader, NULL) != 0) {
        perror("Failed to create reader thread");
    }
    if (pthread_create(&my_writer, NULL, &writer, NULL) != 0) {
        perror("Failed to create writer thread");
    }
    
    if (pthread_join(my_reader, NULL) != 0) {
        perror("Failed to join reader thread");
    }
    if (pthread_join(my_writer, NULL) != 0) {
        perror("Failed to join writer thread");
    }
    
    pthread_mutex_destroy(&mutex);
    pthread_cond_destroy(&cond);
    return 0;
}

• 明确状态变量：用turn变量清晰标记当前该哪个线程执行，避免原逻辑中状态模糊的问题。
• 双向通知机制：每个线程执行完任务后，切换状态并通知对方线程，形成交替执行的闭环。
• 规范条件变量使用：在持有锁的情况下发送信号，避免信号丢失；用while循环检查状态，处理条件变量的虚假唤醒问题。
• 优化输出逻辑：移除writer线程中提前打印的"started writer"，确保只有轮到自己执行时才输出操作信息，符合交替执行的预期。

内容的提问来源于stack exchange，提问作者Mughunth Srinivasan