<分区>
我在 C++11 中实现了读者/作者问题……我想知道它出了什么问题，因为这些事情我自己很难预测。
- 共享数据库:
- 当没有写者时，读者可以访问数据库
- 当没有读者或作者时，作者可以访问数据库
- 一次只有一个线程操作状态变量
该示例有 3 个读者和 1 个作者，但也使用 2 个或更多作者....
代码:
class ReadersWriters {
private:
int AR; // number of active readers
int WR; // number of waiting readers
int AW; // number of active writers
int WW; // number of waiting writers
mutex lock;
mutex m;
condition_variable okToRead;
condition_variable okToWrite;
int data_base_variable;
public:
ReadersWriters() : AR(0), WR(0), AW(0), WW(0), data_base_variable(0) {}
void read_lock() {
unique_lock<mutex> l(lock);
WR++; // no writers exist
// is it safe to read?
okToRead.wait(l, [this](){ return WW == 0; });
okToRead.wait(l, [this](){ return AW == 0; });
WR--; // no longer waiting
AR++; // now we are active
}
void read_unlock() {
unique_lock<mutex> l(lock);
AR--; // no longer active
if (AR == 0 && WW > 0) { // no other active readers
okToWrite.notify_one(); // wake up one writer
}
}
void write_lock() {
unique_lock<mutex> l(lock);
WW++; // no active user exist
// is it safe to write?
okToWrite.wait(l, [this](){ return AR == 0; });
okToWrite.wait(l, [this](){ return AW == 0; });
WW--; // no longer waiting
AW++; // no we are active
}
void write_unlock() {
unique_lock<mutex> l(lock);
AW--; // no longer active
if (WW > 0) { // give priority to writers
okToWrite.notify_one(); // wake up one writer
}
else if (WR > 0) { // otherwize, wake reader
okToRead.notify_all(); // wake all readers
}
}
void data_base_thread_write(unsigned int thread_id) {
for (int i = 0; i < 10; i++) {
write_lock();
data_base_variable++;
m.lock();
cout << "data_base_thread: " << thread_id << "...write: " << data_base_variable << endl;
m.unlock();
write_unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
}
void data_base_thread_read(unsigned int thread_id) {
for (int i = 0; i < 10; i++) {
read_lock();
m.lock();
cout << "data_base_thread: " << thread_id << "...read: " << data_base_variable << endl;
m.unlock();
read_unlock();
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
}
};
int main() {
// your code goes here
ReadersWriters rw;
thread w1(&ReadersWriters::data_base_thread_write, &rw, 0);
thread r1(&ReadersWriters::data_base_thread_read, &rw, 1);
thread r2(&ReadersWriters::data_base_thread_read, &rw, 2);
thread r3(&ReadersWriters::data_base_thread_read, &rw, 3);
w1.join();
r1.join();
r2.join();
r3.join();
cout << "\nThreads successfully completed..." << endl;
return 0;
}