读者写者问题(写者优先)c语言代码
时间: 2023-09-29 17:02:56 浏览: 202
以下是一个简单的读者写者问题(写者优先)的C语言代码,其中写者具有优先权:
```
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#define NUM_READERS 5
#define NUM_WRITERS 2
sem_t reader_mutex, writer_mutex, read_mutex;
int readers = 0, writers = 0;
void *reader(void *arg) {
int id = *(int *)arg;
while (1) {
sem_wait(&reader_mutex);
readers++;
if (readers == 1) {
sem_wait(&write_mutex);
}
sem_post(&reader_mutex);
sem_wait(&read_mutex);
printf("Reader %d is reading...\n", id);
sem_post(&read_mutex);
sem_wait(&reader_mutex);
readers--;
if (readers == 0) {
sem_post(&write_mutex);
}
sem_post(&reader_mutex);
sleep(rand() % 3);
}
}
void *writer(void *arg) {
int id = *(int *)arg;
while (1) {
sem_wait(&writer_mutex);
writers++;
if (writers == 1) {
sem_wait(&read_mutex);
}
sem_post(&writer_mutex);
sem_wait(&write_mutex);
printf("Writer %d is writing...\n", id);
sem_post(&write_mutex);
sem_wait(&writer_mutex);
writers--;
if (writers == 0) {
sem_post(&read_mutex);
}
sem_post(&writer_mutex);
sleep(rand() % 3);
}
}
int main() {
pthread_t readers[NUM_READERS], writers[NUM_WRITERS];
int reader_ids[NUM_READERS], writer_ids[NUM_WRITERS];
int i;
sem_init(&reader_mutex, 0, 1);
sem_init(&writer_mutex, 0, 1);
sem_init(&read_mutex, 0, 1);
for (i = 0; i < NUM_READERS; i++) {
reader_ids[i] = i;
pthread_create(&readers[i], NULL, reader, &reader_ids[i]);
}
for (i = 0; i < NUM_WRITERS; i++) {
writer_ids[i] = i;
pthread_create(&writers[i], NULL, writer, &writer_ids[i]);
}
for (i = 0; i < NUM_READERS; i++) {
pthread_join(readers[i], NULL);
}
for (i = 0; i < NUM_WRITERS; i++) {
pthread_join(writers[i], NULL);
}
sem_destroy(&reader_mutex);
sem_destroy(&writer_mutex);
sem_destroy(&read_mutex);
return 0;
}
```
在此代码中,我们使用了三个信号量:reader_mutex,writer_mutex和read_mutex。reader_mutex保护读者计数器,writer_mutex保护写者计数器,read_mutex保护读操作。
读者线程执行以下步骤:
1. 获取reader_mutex,增加读者计数器
2. 如果是第一个读者,则获取write_mutex,以防止写者进入
3. 释放reader_mutex
4. 获取read_mutex,执行读操作
5. 释放read_mutex
6. 获取reader_mutex,减少读者计数器
7. 如果是最后一个读者,则释放write_mutex
写者线程执行以下步骤:
1. 获取writer_mutex,增加写者计数器
2. 如果是第一个写者,则获取read_mutex,以防止读者进入
3. 释放writer_mutex
4. 获取write_mutex,执行写操作
5. 释放write_mutex
6. 获取writer_mutex,减少写者计数器
7. 如果是最后一个写者,则释放read_mutex
通过使用这些信号量和计数器,我们确保了写者具有优先权,以避免读者饥饿,并确保读者和写者能够安全地访问共享资源。