请用c++设计一个程序1、 A data set is shared among several concurrent processes: Readers – only read the data set; they do not perform any updates. Writers – can both read and write. 2、 Problem – allow multiple readers (at most 8) to read at the same time. Only one single writer can access the shared data at the same time.
时间: 2024-03-20 17:40:39 浏览: 14
好的,这是一个经典的读写锁问题,可以用C++中的互斥锁和条件变量来实现。以下是一个可能的解决方案:
```c++
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;
const int MAX_READERS = 8;
class ReadWriteLock {
private:
mutex mtx;
condition_variable cv;
int readers = 0;
bool writer = false;
public:
void readLock() {
unique_lock<mutex> lock(mtx);
cv.wait(lock, [this] { return !writer; });
readers++;
}
void readUnlock() {
unique_lock<mutex> lock(mtx);
readers--;
if (readers == 0) {
cv.notify_one();
}
}
void writeLock() {
unique_lock<mutex> lock(mtx);
cv.wait(lock, [this] { return readers == 0 && !writer; });
writer = true;
}
void writeUnlock() {
unique_lock<mutex> lock(mtx);
writer = false;
cv.notify_all();
}
};
void reader(int id, ReadWriteLock& rwLock) {
while (true) {
// read data
rwLock.readLock();
cout << "Reader " << id << " is reading data." << endl;
rwLock.readUnlock();
// sleep for a while
this_thread::sleep_for(chrono::milliseconds(1000));
}
}
void writer(int id, ReadWriteLock& rwLock) {
while (true) {
// write data
rwLock.writeLock();
cout << "Writer " << id << " is writing data." << endl;
rwLock.writeUnlock();
// sleep for a while
this_thread::sleep_for(chrono::milliseconds(2000));
}
}
int main() {
ReadWriteLock rwLock;
// create readers
thread readers[MAX_READERS];
for (int i = 0; i < MAX_READERS; i++) {
readers[i] = thread(reader, i+1, ref(rwLock));
}
// create writers
thread writers[1];
for (int i = 0; i < 1; i++) {
writers[i] = thread(writer, i+1, ref(rwLock));
}
// join threads
for (int i = 0; i < MAX_READERS; i++) {
readers[i].join();
}
for (int i = 0; i < 1; i++) {
writers[i].join();
}
return 0;
}
```
在这个程序中,ReadWriteLock是一个读写锁类,包含了readLock、readUnlock、writeLock和writeUnlock四个方法。readers和writer变量分别表示当前正在读取数据的读者数量和是否有写者正在修改数据。在readLock和writeLock方法中,使用了条件变量cv来进行线程的等待和唤醒。在readUnlock和writeUnlock方法中,使用了notify_one和notify_all方法来唤醒等待的线程。
程序中创建了8个读者线程和1个写者线程,并且它们会不断地读取和写入数据。读者在读取数据时,使用readLock方法来获取读锁,读取完毕后使用readUnlock方法来释放读锁。写者在写入数据时,使用writeLock方法来获取写锁,写完毕后使用writeUnlock方法来释放写锁。由于读锁可以被多个线程同时获取,所以可以允许多个读者同时读取数据。而写锁只能被一个线程获取,所以必须等待所有读者都释放读锁后才能获取写锁。
这个程序可以有效地保证多个读者可以同时读取数据,而写者必须等待所有读者都读取完毕并释放读锁后才能修改数据。