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semaphore.h
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semaphore.h
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/*
* Implement a simple semaphore interface:
*
* wait()
* post()
* trywait()
*
* There are two implementations available:
*
* - "semaphore" is blocking version that is implemented based on mutex and
* condition variable.
*
* - "semaphore_lockfree" is lock-less version that is implemented based on
* c++ atomic CAS (compare and swap).
*
*/
#include <mutex>
#include <condition_variable>
#include <atomic>
#include <thread>
namespace std {
class semaphore_interface {
public:
virtual void wait(void) = 0;
virtual void post(void) = 0;
virtual bool trywait(void) = 0;
};
class semaphore : public semaphore_interface {
mutex m_mtx;
condition_variable m_cv;
volatile int m_val;
public:
semaphore(int m_val) : m_val(m_val) {}
virtual void wait(void) {
unique_lock<mutex> lck(m_mtx);
m_cv.wait(lck, [&](void)->bool {return m_val > 0;});
m_val--;
// lck will be unlocked after out of scope.
}
virtual void post(void) {
unique_lock<mutex> lck(m_mtx);
m_val++;
if (m_val > 0) m_cv.notify_all();
// lck will be unlocked after out of scope.
}
virtual bool trywait(void) {
unique_lock<mutex> lck(m_mtx);
if (m_val > 0) {
m_val--;
return true;
} else {
return false;
}
}
};
class semaphore_lockfree : public semaphore_interface {
atomic<int> m_val;
public:
semaphore_lockfree(int val) {m_val = val;}
virtual void wait(void) {
while (true) {
this_thread::yield();
int old_val = m_val;
if (old_val <= 0) {
continue;
}
if (m_val.compare_exchange_strong(old_val, old_val - 1)) {
break;
}
}
}
virtual void post(void) {
m_val++;
}
virtual bool trywait(void) {
int old_val = m_val;
if (old_val <= 0) {
return false;
}
if (m_val.compare_exchange_strong(old_val, old_val - 1)) {
return true;
}
return false;
}
};
}