Readers-Writer Problem Implementation
时间: 2024-01-04 11:02:36 浏览: 49
The Readers-Writer problem is a classic synchronization problem in computer science. In this problem, multiple threads need access to a shared resource. However, some of these threads only need to read from the resource, while others need to write to it. To ensure correctness and avoid data corruption, we need to implement a solution that allows concurrent reading but exclusive writing.
One common solution to this problem is to use a semaphore or a lock to coordinate access to the shared resource between readers and writers. Here's an implementation of the Readers-Writer problem using a mutex and a condition variable in C++:
```c++
#include <mutex>
#include <condition_variable>
class ReadersWriter {
public:
ReadersWriter() : readers_count(0), writer_active(false) {}
void start_read() {
std::unique_lock<std::mutex> lock(m);
while (writer_active) { // wait until no writer is active
cv_read.wait(lock);
}
++readers_count;
}
void end_read() {
std::unique_lock<std::mutex> lock(m);
--readers_count;
if (readers_count == 0) {
cv_write.notify_one(); // notify waiting writers
}
}
void start_write() {
std::unique_lock<std::mutex> lock(m);
while (writer_active || readers_count > 0) { // wait until no readers or writers are active
cv_write.wait(lock);
}
writer_active = true;
}
void end_write() {
std::unique_lock<std::mutex> lock(m);
writer_active = false;
cv_write.notify_one(); // notify waiting writers
cv_read.notify_all(); // notify waiting readers
}
private:
std::mutex m;
std::condition_variable cv_read;
std::condition_variable cv_write;
int readers_count;
bool writer_active;
};
```
In this implementation, we use a mutex `m` to ensure exclusive access to the shared variables `readers_count` and `writer_active`. We also use two condition variables `cv_read` and `cv_write` to coordinate access between readers and writers.
When a thread wants to start reading, it first acquires the lock `m` and then waits on the condition variable `cv_read` until there are no active writers. If there are active writers, the thread blocks until it's notified by a writer that it has finished writing. Once it's safe to read, the thread increases the `readers_count` and releases the lock `m`.
When a thread wants to end reading, it first acquires the lock `m` and then decreases the `readers_count`. If there are no more readers, the thread notifies waiting writers using the condition variable `cv_write` and releases the lock `m`.
When a thread wants to start writing, it first acquires the lock `m` and then waits on the condition variable `cv_write` until there are no active readers or writers. If there are active readers or writers, the thread blocks until it's notified by a reader or writer that it has finished accessing the shared resource. Once it's safe to write, the thread sets the `writer_active` flag to true and releases the lock `m`.
When a thread wants to end writing, it first acquires the lock `m` and then sets the `writer_active` flag to false. The thread then notifies waiting writers using the condition variable `cv_write` and waiting readers using the condition variable `cv_read`, and releases the lock `m`.
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