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3531227 Mar 16, 2015
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//---------------------------------------------------------
// For conditions of distribution and use, see
// https://github.com/preshing/cpp11-on-multicore/blob/master/LICENSE
//---------------------------------------------------------
#ifndef __CPP11OM_SEMAPHORE_H__
#define __CPP11OM_SEMAPHORE_H__
#include <atomic>
#include <cassert>
#if defined(_WIN32)
//---------------------------------------------------------
// Semaphore (Windows)
//---------------------------------------------------------
#include <windows.h>
#undef min
#undef max
class Semaphore
{
private:
HANDLE m_hSema;
Semaphore(const Semaphore& other) = delete;
Semaphore& operator=(const Semaphore& other) = delete;
public:
Semaphore(int initialCount = 0)
{
assert(initialCount >= 0);
m_hSema = CreateSemaphore(NULL, initialCount, MAXLONG, NULL);
}
~Semaphore()
{
CloseHandle(m_hSema);
}
void wait()
{
WaitForSingleObject(m_hSema, INFINITE);
}
void signal(int count = 1)
{
ReleaseSemaphore(m_hSema, count, NULL);
}
};
#elif defined(__MACH__)
//---------------------------------------------------------
// Semaphore (Apple iOS and OSX)
// Can't use POSIX semaphores due to http://lists.apple.com/archives/darwin-kernel/2009/Apr/msg00010.html
//---------------------------------------------------------
#include <mach/mach.h>
class Semaphore
{
private:
semaphore_t m_sema;
Semaphore(const Semaphore& other) = delete;
Semaphore& operator=(const Semaphore& other) = delete;
public:
Semaphore(int initialCount = 0)
{
assert(initialCount >= 0);
semaphore_create(mach_task_self(), &m_sema, SYNC_POLICY_FIFO, initialCount);
}
~Semaphore()
{
semaphore_destroy(mach_task_self(), m_sema);
}
void wait()
{
semaphore_wait(m_sema);
}
void signal()
{
semaphore_signal(m_sema);
}
void signal(int count)
{
while (count-- > 0)
{
semaphore_signal(m_sema);
}
}
};
#elif defined(__unix__)
//---------------------------------------------------------
// Semaphore (POSIX, Linux)
//---------------------------------------------------------
#include <semaphore.h>
class Semaphore
{
private:
sem_t m_sema;
Semaphore(const Semaphore& other) = delete;
Semaphore& operator=(const Semaphore& other) = delete;
public:
Semaphore(int initialCount = 0)
{
assert(initialCount >= 0);
sem_init(&m_sema, 0, initialCount);
}
~Semaphore()
{
sem_destroy(&m_sema);
}
void wait()
{
// http://stackoverflow.com/questions/2013181/gdb-causes-sem-wait-to-fail-with-eintr-error
int rc;
do
{
rc = sem_wait(&m_sema);
}
while (rc == -1 && errno == EINTR);
}
void signal()
{
sem_post(&m_sema);
}
void signal(int count)
{
while (count-- > 0)
{
sem_post(&m_sema);
}
}
};
#else
#error Unsupported platform!
#endif
//---------------------------------------------------------
// LightweightSemaphore
//---------------------------------------------------------
class LightweightSemaphore
{
private:
std::atomic<int> m_count;
Semaphore m_sema;
void waitWithPartialSpinning()
{
int oldCount;
// Is there a better way to set the initial spin count?
// If we lower it to 1000, testBenaphore becomes 15x slower on my Core i7-5930K Windows PC,
// as threads start hitting the kernel semaphore.
int spin = 10000;
while (spin--)
{
oldCount = m_count.load(std::memory_order_relaxed);
if ((oldCount > 0) && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire))
return;
std::atomic_signal_fence(std::memory_order_acquire); // Prevent the compiler from collapsing the loop.
}
oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
if (oldCount <= 0)
{
m_sema.wait();
}
}
public:
LightweightSemaphore(int initialCount = 0) : m_count(initialCount)
{
assert(initialCount >= 0);
}
bool tryWait()
{
int oldCount = m_count.load(std::memory_order_relaxed);
return (oldCount > 0 && m_count.compare_exchange_strong(oldCount, oldCount - 1, std::memory_order_acquire));
}
void wait()
{
if (!tryWait())
waitWithPartialSpinning();
}
void signal(int count = 1)
{
int oldCount = m_count.fetch_add(count, std::memory_order_release);
int toRelease = -oldCount < count ? -oldCount : count;
if (toRelease > 0)
{
m_sema.signal(toRelease);
}
}
};
typedef LightweightSemaphore DefaultSemaphoreType;
#endif // __CPP11OM_SEMAPHORE_H__