From Newsgroup: comp.lang.c++

// Header:

#pragma once
#include <atomic>

struct benaphore
{
benaphore() noexcept = default;
benaphore( const benaphore & ) = delete;
void operator =( const benaphore & ) = delete;
void lock() noexcept;
void unlock() noexcept;
private:
std::atomic_uint32_t m_lockCnt = 0;
std::atomic_bool m_WakeUp;
void wait() noexcept;
void wakeUp() noexcept;
};

inline void benaphore::lock() noexcept
{
if( m_lockCnt.fetch_add( 1, std::memory_order_acquire ) ) [[unlikely]]
wait();
}

inline void benaphore::unlock() noexcept
{
if( m_lockCnt.fetch_sub( 1, std::memory_order_release ) > 1 ) [[unlikely]]
wakeUp();
}

// Cpp:

#include "benaphore.hpp"

void benaphore::wait() noexcept
{
using namespace std;
bool ref = m_WakeUp.load( memory_order_relaxed );
for( ; ; )
if( ref )
if( m_WakeUp.compare_exchange_strong( ref, false, memory_order_acquire, memory_order_relaxed ) )
return;
else;
else
{
m_WakeUp.wait( false, memory_order_relaxed );
ref = true;
}
}

void benaphore::wakeUp() noexcept
{
m_WakeUp.store( true, std::memory_order_release );
m_WakeUp.notify_one();
}
--- Synchronet 3.21f-Linux NewsLink 1.2

From Newsgroup: comp.lang.c++

Here's the final benapohre - with an templated option for recursive
access.

#pragma once
#include <atomic>
#include <semaphore>
#include <chrono>
#include <cassert>
#include "thread_id.hpp"

#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdangling-else"
#endif

template<bool Recursive = false>
struct benaphore
{
benaphore() noexcept = default;
benaphore( const benaphore & ) = delete;
void operator =( const benaphore & ) = delete;
void lock() noexcept;
void unlock() noexcept;
private:
struct non_recursive
{
std::atomic_uint32_t lockCnt = 0;
};
struct recursive : non_recursive
{
std::atomic<thread_id> threadId = thread_id();
unsigned recCount = 0;
};
using counter = std::conditional_t<Recursive, recursive, non_recursive>;
counter m_counter;
std::atomic_bool m_wakeUp = false;
};

template<bool Recursive>
void benaphore<Recursive>::lock() noexcept
{
using namespace std;
if constexpr( Recursive )
if( m_counter.threadId.load( memory_order_relaxed ) == thread_self() )
{
++m_counter.recCount;
return;
}
if( m_counter.lockCnt.fetch_add( 1, memory_order_acquire ) > 0 ) [[unlikely]]
for( bool ref; !m_wakeUp.compare_exchange_strong( ref = true, false,
memory_order_acquire, memory_order_relaxed ); )
m_wakeUp.wait( false, memory_order_relaxed );
if constexpr( Recursive )
{
m_counter.threadId.store( thread_self(), memory_order_relaxed );
m_counter.recCount = 0;
}
}

template<bool Recursive>
void benaphore<Recursive>::unlock() noexcept
{
using namespace std;
if constexpr( Recursive )
{
if( m_counter.threadId.load( memory_order_relaxed ) == thread_self()
&& m_counter.recCount )
{
--m_counter.recCount;
return;
}
m_counter.threadId.store( thread_id() );
m_counter.recCount = 0;
}
if( m_counter.lockCnt.fetch_sub( 1, memory_order_release ) > 1 ) [[unlikely]]
{
m_wakeUp.store( true, memory_order_release );
m_wakeUp.notify_one();
}
}

#if defined(TPL_TEST_INSTANTIATE)
template struct benaphore<true>;
#endif

template<bool Recursive = false, typename Clock = std::chrono::steady_clock> struct timed_benaphore
{
using time_point = Clock::time_point;
using duration = Clock::duration;
timed_benaphore() noexcept = default;
timed_benaphore( const timed_benaphore & ) = delete;
void operator =( const timed_benaphore & ) = delete;
void lock() noexcept;
bool try_lock_for( duration dur ) noexcept;
bool try_lock_until( time_point deadline ) noexcept;
void unlock() noexcept;
private:
static constexpr int32_t LockFlag = std::numeric_limits<int32_t>::min();
struct non_recursive
{
std::atomic_int32_t lckdWaiting = 0;
};
struct recursive : non_recursive
{
std::atomic<thread_id> threadId = thread_id();
unsigned recCount = 0;
};
using synch = std::conditional_t<Recursive, recursive, non_recursive>;
synch m_synch;
std::binary_semaphore m_wakeUp { 0 };
bool tryLock( auto calcDeadline ) noexcept;
};

template<bool Recursive, typename Clock>
inline void timed_benaphore<Recursive, Clock>::lock() noexcept
{
tryLock( []() noexcept { return time_point::max(); } );
}

template<bool Recursive, typename Clock>
bool timed_benaphore<Recursive, Clock>::try_lock_for( duration dur )
noexcept
{
return tryLock( [dur]() noexcept -> time_point
{
if( dur.count() < 0 )
return time_point::max();
time_point now = Clock::now();
if( dur > time_point::max() - now )
return time_point::max();
return now + dur;
} );
}

template<bool Recursive, typename Clock>
bool timed_benaphore<Recursive, Clock>::try_lock_until( time_point
deadline ) noexcept
{
return tryLock( [deadline]() noexcept { return deadline; } );
}

template<bool Recursive, typename Clock>
void timed_benaphore<Recursive, Clock>::unlock() noexcept
{
using namespace std;
if constexpr( Recursive )
{
if( m_synch.threadId.load( memory_order_relaxed ) == thread_self() &&
m_synch.recCount )
{
--m_synch.recCount;
return;
}
assert(m_synch.threadId == thread_self());
m_synch.threadId.store( thread_id() );
m_synch.recCount = 0;
}
assert(m_synch.lckdWaiting < 0);
if( (m_synch.lckdWaiting.fetch_and( ~LockFlag, memory_order_release ) &
~LockFlag) ) [[unlikely]]
m_wakeUp.release();
}

template<bool Recursive, typename Clock>
bool timed_benaphore<Recursive, Clock>::tryLock( auto calcDeadline )
noexcept
{
using namespace std;
using namespace chrono;
if constexpr( Recursive )
if( m_synch.threadId.load( memory_order_relaxed ) == thread_self() )
{
++m_synch.recCount;
return true;
}
bool waiter = false;
time_point deadline = time_point::min();
for( int32_t ref = m_synch.lckdWaiting.load( memory_order_relaxed ); ; )
{
if( ref >= 0 )
if( m_synch.lckdWaiting.compare_exchange_strong( ref, LockFlag | (ref
- waiter), memory_order_acquire, memory_order_relaxed ) )
break;
else
continue;
if( !waiter && !m_synch.lckdWaiting.compare_exchange_strong( ref, ref
+ 1, memory_order_relaxed, memory_order_relaxed ) )
continue;
waiter = true;
if( deadline.time_since_epoch().count() < 0 )
deadline = calcDeadline();
if( !m_wakeUp.try_acquire_until( deadline ) )
{
m_synch.lckdWaiting.fetch_sub( 1, memory_order_relaxed );
return false;
}
ref = m_synch.lckdWaiting.load( memory_order_relaxed );
}
if constexpr( Recursive )
{
m_synch.threadId.store( thread_self(), memory_order_relaxed );
m_synch.recCount = 0;
}
return true;
}

#if defined(__clang__)
#pragma clang diagnostic pop
#endif

---------

And the code includes a version with timeouts (see above).
--- Synchronet 3.21f-Linux NewsLink 1.2

From Newsgroup: comp.lang.c++

On 4/6/2026 10:24 AM, Bonita Montero wrote:

Here's the final benapohre - with an templated option for recursive
access.

[...]

fast path semaphores are fun:


Even in the post n case:
____________
void post_n(int n)
{
std::atomic_thread_fence(std::memory_order_release);
int old_count = m_count.fetch_add(n, std::memory_order_relaxed);

// If old_count was negative, there were waiters.
if (old_count < 0)
{
// Calculate how many actually need a signal.
// If we have 5 waiters (count == -5) and we post 10,
// we only signal 5.
int to_signal = std::min(-old_count, n);
m_semaphore.post_n(to_signal);
}
}
____________

Loopless, no CAS, just accounting.

--- Synchronet 3.21f-Linux NewsLink 1.2

From Newsgroup: comp.lang.c++

Am 07.04.2026 um 00:39 schrieb Chris M. Thomasson:

fast path semaphores are fun:
...
std::atomic_thread_fence(std::memory_order_release);

Semaphore posting has an included thread-fence, at least with
std::semaphore<N> and all operating system dependent semaphores.
But usually you have an atomic operation before you use a sema-
phore and you could apply a proper fence to this atomic op.

--- Synchronet 3.21f-Linux NewsLink 1.2