pw_sync/public/pw_sync/counting_semaphore.h

// Copyright 2020 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once

#include <stdbool.h>
#include <stddef.h>

#include "pw_chrono/system_clock.h"
#include "pw_preprocessor/util.h"

#ifdef __cplusplus

#include "pw_sync_backend/counting_semaphore_native.h"

namespace pw::sync {

/// The `CountingSemaphore` is a synchronization primitive that can be used for
/// counting events and/or resource management where receiver(s) can block on
/// acquire until notifier(s) signal by invoking release.
/// Note that unlike Mutexes, priority inheritance is not used by semaphores
/// meaning semaphores are subject to unbounded priority inversions.
/// Pigweed does not recommend semaphores for mutual exclusion. The entire API
/// is thread safe but only a subset is IRQ safe.
///
/// @rst
/// .. WARNING::
///    In order to support global statically constructed ``CountingSemaphores``
///    the user and/or backend MUST ensure that any initialization required in
///    your environment is done prior to the creation and/or initialization of
///    the native synchronization primitives (e.g. kernel initialization).
/// @endrst
///
/// The `CountingSemaphore` is initialized to being empty or having no tokens.
class CountingSemaphore {
 public:
  using native_handle_type = backend::NativeCountingSemaphoreHandle;

  CountingSemaphore();
  ~CountingSemaphore();
  CountingSemaphore(const CountingSemaphore&) = delete;
  CountingSemaphore(CountingSemaphore&&) = delete;
  CountingSemaphore& operator=(const CountingSemaphore&) = delete;
  CountingSemaphore& operator=(CountingSemaphore&&) = delete;

  /// Atomically increments the internal counter by the value of update.
  /// Any thread(s) waiting for the counter to be greater than 0, i.e. blocked
  /// in acquire, will subsequently be unblocked.
  /// This is IRQ safe.
  ///
  /// @b Precondition: update >= 0
  ///
  /// @b Precondition: update <= max() - counter
  void release(ptrdiff_t update = 1);

  /// Decrements the internal counter by 1 or blocks indefinitely until it can.
  ///
  /// This is thread safe, but not IRQ safe.
  void acquire();

  /// Tries to decrement by the internal counter by 1 without blocking.
  /// Returns true if the internal counter was decremented successfully.
  ///
  /// This is IRQ safe.
  bool try_acquire() noexcept;

  /// Tries to decrement the internal counter by 1. Blocks until the specified
  /// timeout has elapsed or the counter was decremented by 1, whichever comes
  /// first.
  ///
  /// Returns true if the internal counter was decremented successfully.
  /// This is thread safe, but not IRQ safe.
  bool try_acquire_for(chrono::SystemClock::duration timeout);

  /// Tries to decrement the internal counter by 1. Blocks until the specified
  /// deadline has been reached or the counter was decremented by 1, whichever
  /// comes first.
  ///
  /// Returns true if the internal counter was decremented successfully.
  ///
  /// This is thread safe, but not IRQ safe.
  bool try_acquire_until(chrono::SystemClock::time_point deadline);

  /// Returns the internal counter's maximum possible value.
  static constexpr ptrdiff_t max() noexcept {
    return backend::kCountingSemaphoreMaxValue;
  }

  native_handle_type native_handle();

 private:
  /// This may be a wrapper around a native type with additional members.
  backend::NativeCountingSemaphore native_type_;
};

}  // namespace pw::sync

#include "pw_sync_backend/counting_semaphore_inline.h"

using pw_sync_CountingSemaphore = pw::sync::CountingSemaphore;

#else  // !defined(__cplusplus)

typedef struct pw_sync_CountingSemaphore pw_sync_CountingSemaphore;

#endif  // __cplusplus

PW_EXTERN_C_START

void pw_sync_CountingSemaphore_Release(pw_sync_CountingSemaphore* semaphore);
void pw_sync_CountingSemaphore_ReleaseNum(pw_sync_CountingSemaphore* semaphore,
                                          ptrdiff_t update);
void pw_sync_CountingSemaphore_Acquire(pw_sync_CountingSemaphore* semaphore);
bool pw_sync_CountingSemaphore_TryAcquire(pw_sync_CountingSemaphore* semaphore);
bool pw_sync_CountingSemaphore_TryAcquireFor(
    pw_sync_CountingSemaphore* semaphore,
    pw_chrono_SystemClock_Duration timeout);
bool pw_sync_CountingSemaphore_TryAcquireUntil(
    pw_sync_CountingSemaphore* semaphore,
    pw_chrono_SystemClock_TimePoint deadline);
ptrdiff_t pw_sync_CountingSemaphore_Max(void);

PW_EXTERN_C_END