pw_sync/public/pw_sync/binary_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/binary_semaphore_native.h"

namespace pw::sync {

// BinarySemaphore is a specialization of CountingSemaphore with an arbitrary
// token limit of 1. Note that that max() is >= 1, meaning it may be
// released up to max() times but only acquired once for those N releases.
// Implementations of BinarySemaphore are typically more efficient than the
// default implementation of CountingSemaphore. The entire API is thread safe
// but only a subset is IRQ safe.
//
// WARNING: In order to support global statically constructed BinarySemaphores,
// 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).
//
// The BinarySemaphore is initialized to being empty or having no tokens.
class BinarySemaphore {
 public:
  using native_handle_type = backend::NativeBinarySemaphoreHandle;

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

  // Atomically increments the internal counter by 1.
  // Any thread(s) waiting for the counter to be greater than 0, i.e.
  // blocked in acquire, will subsequently be unblocked.
  // This is thread and IRQ safe.
  //
  // There exists an overflow risk if one releases more than max() times
  // between acquires because many RTOS implementations internally
  // increment the counter past one where it is only cleared when acquired.
  //
  // Precondition: 1 <= max() - counter
  void release();

  // Decrements the internal counter to 0 or blocks indefinitely until it can.
  // This is thread safe, but not IRQ safe.
  void acquire();

  // Tries to decrement by the internal counter to 0 without blocking.
  // Returns true if the internal counter was reset successfully.
  // This is thread and IRQ safe.
  bool try_acquire() noexcept;

  // Tries to decrement the internal counter to 0. Blocks until the specified
  // timeout has elapsed or the counter was decremented to 0, 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 to 0. Blocks until the specified
  // deadline has been reached or the counter was decremented to 0, 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);

  static constexpr ptrdiff_t max() noexcept {
    return backend::kBinarySemaphoreMaxValue;
  }

  native_handle_type native_handle();

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

}  // namespace pw::sync

#include "pw_sync_backend/binary_semaphore_inline.h"

using pw_sync_BinarySemaphore = pw::sync::BinarySemaphore;

#else  // !defined(__cplusplus)

typedef struct pw_sync_BinarySemaphore pw_sync_BinarySemaphore;

#endif  // __cplusplus

PW_EXTERN_C_START

void pw_sync_BinarySemaphore_Release(pw_sync_BinarySemaphore* semaphore);
void pw_sync_BinarySemaphore_Acquire(pw_sync_BinarySemaphore* semaphore);
bool pw_sync_BinarySemaphore_TryAcquire(pw_sync_BinarySemaphore* semaphore);
bool pw_sync_BinarySemaphore_TryAcquireFor(
    pw_sync_BinarySemaphore* semaphore, pw_chrono_SystemClock_Duration timeout);
bool pw_sync_BinarySemaphore_TryAcquireUntil(
    pw_sync_BinarySemaphore* semaphore,
    pw_chrono_SystemClock_TimePoint deadline);
ptrdiff_t pw_sync_BinarySemaphore_Max(void);

PW_EXTERN_C_END