编写C语言程序，模拟完成读者写者问题使用gcc编译并运行。

在C语言中，读者写者问题是操作系统中常见的同步问题，它涉及到多个线程如何共享数据资源，避免同时读写导致的数据混乱。这里我们可以使用互斥锁（mutex）来解决这个问题。以下是一个简单的C程序示例，它创建了两个线程，一个代表读者，另一个代表写者：

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h> // 使用信号量来控制并发

#define MAX_READERS 5
#define MAX_WRITERS 1
#define BUFFER_SIZE 10
char buffer[BUFFER_SIZE] = {0};
int readers_in = 0, writers_in = 0;

sem_t mutex; // 互斥锁
sem_t reader_sem; // 用于限制同时进入的读者数
sem_t writer_sem; // 用于限制同时进入的写者数

void* reader(void* arg) {
    while (readers_in < MAX_READERS && !writers_in) {
        sem_wait(&reader_sem); // 等待读者权限
        printf("Reader %d entered and read from buffer: ", pthread_self());
        for (int i = 0; i < BUFFER_SIZE; ++i) {
            if (buffer[i]) {
                printf("%c", buffer[i]);
                break;
            }
        }
        buffer[0] = 'R'; // 读取完成后更新缓冲区
        sem_post(&mutex); // 释放互斥锁
        sem_post(&reader_sem); // 允许下一个读者进来
        readers_in++;
    }
    return NULL;
}

void* writer(void* arg) {
    while (!writers_in || readers_in) {
        sem_wait(&writer_sem); // 等待写者权限
        printf("Writer %d entered and wrote to buffer: ", pthread_self());
        buffer[0] = 'W'; // 写入新的字符
        sem_post(&mutex); // 释放互斥锁
        sem_post(&writer_sem); // 允许下一个写者进来
        writers_in++;
    }
    return NULL;
}

int main() {
    pthread_t reader_thread, writer_thread;
    int rc;

    // 初始化信号量
    sem_init(&mutex, 0, 1);
    sem_init(&reader_sem, 0, MAX_READERS);
    sem_init(&writer_sem, 0, MAX_WRITERS);

    rc = pthread_create(&reader_thread, NULL, reader, NULL);
    if (rc) {
        perror("Error creating reader thread");
        exit(EXIT_FAILURE);
    }

    rc = pthread_create(&writer_thread, NULL, writer, NULL);
    if (rc) {
        perror("Error creating writer thread");
        exit(EXIT_FAILURE);
    }

    rc = pthread_join(reader_thread, NULL);
    if (rc) {
        perror("Error joining reader thread");
    }

    rc = pthread_join(writer_thread, NULL);
    if (rc) {
        perror("Error joining writer thread");
    }

    sem_destroy(&mutex);
    sem_destroy(&reader_sem);
    sem_destroy(&writer_sem);

    return 0;
}

要编译这个程序，你可以使用`gcc -lpthread program_name.c -o program_name`命令。然后，运行程序`./program_name`。