我正在开发一个程序,该程序处理多个线程访问、存入和从有界缓冲区容器中取出的问题。我注意到线程的一些主要问题,并怀疑我的缓冲区在某处部分或根本不正确。
为了确保我知道我在做什么,我希望检查一下我的缓冲区代码。该类使用了我在其他地方实现的信号量,我假设它现在可以工作(如果没有,我很快就会弄清楚!)我添加了试图解释我的推理的注释。
首先,.h 文件:
#ifndef BOUNDED_BUFFER_H
#define BOUNDED_BUFFER_H
#include "Semaphore.H"
#include <string>
#include <vector>
using namespace std;
class Item{ //supposed to eventually be more extensible...seems silly with just a string for now
public:
Item(string _content){content = _content;}
string GetContent() {return content;}
private:
};
class BoundedBuffer{
public:
BoundedBuffer();
void Deposit(Item* _item);
Item* Retrieve();
int GetNumItems() {return count;}
vector<Item*> GetBuffer() {return buffer;}
void SetSize(int _size){
capacity = _size;
buffer.reserve(_size); //or do I need to call "resize"
}
private:
int capacity;
vector<Item*> buffer; //I originally wanted to use an array but had trouble with
//initilization/sizing, etc.
int nextin;
int nextout;
int count;
Semaphore notfull; //wait on this one before depositing an item
Semaphore notempty; //wait on this one before retrieving an item
};
#endif
接下来是.cpp:
#include "BoundedBuffer.H"
#include <iostream>
using namespace std;
BoundedBuffer::BoundedBuffer(){
notfull.SetValue(0);
notempty.SetValue(0);
nextin = 0;
nextout = 0;
count = 0;
}
void BoundedBuffer::Deposit(Item* _item){
if(count == capacity){
notfull.P(); //Cannot deposit into full buffer; wait
}
buffer[nextin] = _item;
nextin = (nextin + 1) % capacity; //wrap-around
count += 1;
notempty.V(); //Signal that retrieval is safe
}
Item* BoundedBuffer::Retrieve(){
if(count == 0){
notempty.P(); //cannot take from empty buffer; wait
}
Item* x = buffer[nextout];
nextout = (nextout + 1) % capacity;
buffer.pop_back(); //or a different erase methodology?
count -= 1;
notfull.V(); //Signal that deposit is safe
return x;
}
我认为问题可能是由于我选择一个向量作为底层容器(或者更确切地说,错误使用它),或者可能需要更多的安全阻塞机制(互斥锁等?)事情,有人可以提供一些反馈吗?
这是一个非常常见的问题(关于如何正确执行多线程队列)。我以前见过的最好的答案是这个堆栈溢出问题 and 这个网站。这些答案适用于无界队列,因此我将在此处扩展并显示有界队列的答案。
您需要使用互斥体保护您的存款和检索功能,并使用条件变量来进行唤醒。
#include <mutex>
#include <condition_variable>
std::mutex the_mutex;
std::condition_variable the_notfull_cvar;
std::condition_variable the_notempty_cvar;
...
void BoundedBuffer::Deposit(Item* _item){
std::unique_lock<std::mutex> lock(the_mutex);
while ( /* buffer is full */ ){
/* simultaneously wait and release the mutex */
the_notfull_cvar.wait(lock);
/* the mutex is reaquired at this point */
}
/* buffer has space and we own the mutex: insert the item */
...
/* tell anyone waiting on an empty buffer that they can wake up. */
the_notempty_cvar.notify_all();
}
Item* BoundedBuffer::Retrieve(){
std::unique_lock<std::mutex> lock(the_mutex);
while ( /* buffer is empty */ ){
/* simultaneously wait and release the mutex */
the_notempty_cvar.wait(lock);
/* the mutex is reaquired at this point */
}
/* buffer has something in it and we own the mutex: get the item */
...
/* tell anyone waiting on a full buffer that they can wake up. */
the_notfull_cvar.notify_all();
return x;
}
您的 GetNumItems()、GetBuffer() 和 SetSize() 函数也需要使用 unique_locks 进行保护。
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