semaphore.hpp

Go to the documentation of this file.

// SPDX-FileCopyrightText: Deutsches Elektronen-Synchrotron DESY, MSK, ChimeraTK Project <chimeratk-support@desy.de>
// SPDX-License-Identifier: LGPL-3.0-or-later
#pragma once
 
#ifdef __linux__
#  include <features.h>
#endif
 
#include <cassert>
#include <system_error>
 
/*
 * Two implementations: one is based on the posix semaphore (sem_wait etc.), the
 * other is pure C++ 11. The posix implementation is considered to be more
 * efficient, so we use the pure C++ 11 implementation only as a fallback.
 */
 
#if _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
/**********************************************************************************************************************
 * POSIX-based implementation follows
 *********************************************************************************************************************/
 
#  include <semaphore.h>
 
namespace cppext {
 
  class semaphore {
   public:
    semaphore() {
      // create new semaphore which is not shared it between processes
      int ret = sem_init(&sem, 0, 0);
      if(ret != 0) throw std::system_error(std::error_code(errno, std::generic_category()));
    }
 
    ~semaphore() { sem_destroy(&sem); }
 
    void unlock() {
      int ret = sem_post(&sem);
      if(ret != 0) throw std::system_error(std::error_code(errno, std::generic_category()));
      // safety check against misuse
      int value;
      ret = sem_getvalue(&sem, &value);
      if(ret != 0 || value > 1) throw std::system_error(std::error_code(errno, std::generic_category()));
    }
 
    bool is_ready() {
      int value;
      int ret = sem_getvalue(&sem, &value);
      if(ret != 0) throw std::system_error(std::error_code(errno, std::generic_category()));
      return value > 0;
    }
 
    void wait_and_reset() {
      int ret = sem_wait(&sem);
      if(ret != 0) throw std::system_error(std::error_code(errno, std::generic_category()));
    }
 
    bool is_ready_and_reset() {
      int ret = sem_trywait(&sem);
      if(ret != 0) {
        if(errno != EAGAIN) throw std::system_error(std::error_code(errno, std::generic_category()));
        return false;
      }
      return true;
    }
 
   private:
    // the POSIX semaphore object
    sem_t sem;
  };
 
} // namespace cppext
 
#else
 
/**********************************************************************************************************************
 * Pure C++ 11 implementation follows
 *********************************************************************************************************************/
 
#  include <condition_variable>
#  include <mutex>
 
namespace cppext {
 
  class semaphore {
   public:
    semaphore() : _count(1) {}
 
    void unlock() {
      std::unique_lock<decltype(_mutex)> lock(_mutex);
      assert(_count > 0);
      --_count;
      if(_count == 0) _condition.notify_one();
    }
 
    bool is_ready() {
      std::unique_lock<decltype(_mutex)> lock(_mutex);
      return _count == 0;
    }
 
    void wait_and_reset() {
      std::unique_lock<decltype(_mutex)> lock(_mutex);
      while(_count > 0) _condition.wait(lock);
      _count = 1;
    }
 
    bool is_ready_and_reset() {
      std::unique_lock<decltype(_mutex)> lock(_mutex);
      if(_count == 0) {
        _count = 1;
        return true;
      }
      return false;
    }
 
   private:
    // mutex required for the condition variable to work
    std::mutex _mutex;
 
    // condition variable used for notification
    std::condition_variable _condition;
 
    // semaphore counter
    size_t _count;
  };
 
} // namespace cppext
 
#endif // POSIX feature switch