deskflow/src/lib/mt/CondVar.h

/*
 * Deskflow -- mouse and keyboard sharing utility
 * SPDX-FileCopyrightText: (C) 2012 - 2016 Symless Ltd.
 * SPDX-FileCopyrightText: (C) 2002 Chris Schoeneman
 * SPDX-License-Identifier: GPL-2.0-only WITH LicenseRef-OpenSSL-Exception
 */

#pragma once

#include "mt/Mutex.h"

class Stopwatch;

//! Generic condition variable
/*!
This class provides functionality common to all condition variables
but doesn't provide the actual variable storage.  A condition variable
is a multiprocessing primitive that can be waited on.  Every condition
variable has an associated mutex.
*/
class CondVarBase
{
public:
  /*!
  \c mutex must not be nullptr.  All condition variables have an
  associated mutex.  The mutex needn't be unique to one condition
  variable.
  */
  explicit CondVarBase(Mutex *mutex);
  ~CondVarBase();

  //! @name manipulators
  //@{

  //! Lock the condition variable's mutex
  /*!
  Lock the condition variable's mutex.  The condition variable should
  be locked before reading or writing it.  It must be locked for a
  call to wait().  Locks are not recursive;  locking a locked mutex
  will deadlock the thread.
  */
  void lock() const;

  //! Unlock the condition variable's mutex
  void unlock() const;

  //! Signal the condition variable
  /*!
  Wake up one waiting thread, if there are any.  Which thread gets
  woken is undefined.
  */
  void signal();

  //! Signal the condition variable
  /*!
  Wake up all waiting threads, if any.
  */
  void broadcast();

  //@}
  //! @name accessors
  //@{

  //! Wait on the condition variable
  /*!
  Wait on the condition variable.  If \c timeout < 0 then wait until
  signalled, otherwise up to \c timeout seconds or until signalled,
  whichever comes first.    Returns true if the object was signalled
  during the wait, false otherwise.

  The proper way to wait for a condition is:
  \code
  cv.lock();
  while (cv-expr) {
      cv.wait();
  }
  cv.unlock();
  \endcode
  where \c cv-expr involves the value of \c cv and is false when the
  condition is satisfied.

  (cancellation point)
  */
  bool wait(double timeout = -1.0) const;

  //! Wait on the condition variable
  /*!
  Same as \c wait(double) but use \c timer to compare against \c timeout.
  Since clients normally wait on condition variables in a loop, clients
  can use this to avoid recalculating \c timeout on each iteration.
  Passing a stopwatch with a negative \c timeout is pointless (it will
  never time out) but permitted.

  (cancellation point)
  */
  bool wait(Stopwatch &timer, double timeout) const;

  //! Get the mutex
  /*!
  Get the mutex passed to the c'tor.
  */
  Mutex *getMutex() const;

  //@}

private:
  // not implemented
  CondVarBase(const CondVarBase &);
  CondVarBase &operator=(const CondVarBase &);

private:
  Mutex *m_mutex;
  ArchCond m_cond;
};

//! Condition variable
/*!
A condition variable with storage for type \c T.
*/
template <class T> class CondVar : public CondVarBase
{
public:
  //! Initialize using \c value
  CondVar(Mutex *mutex, const T &value);
  //! Initialize using another condition variable's value
  CondVar(const CondVar &);
  ~CondVar();

  //! @name manipulators
  //@{

  //! Assigns the value of \c cv to this
  /*!
  Set the variable's value.  The condition variable should be locked
  before calling this method.
  */
  CondVar &operator=(const CondVar &cv);

  //! Assigns \c value to this
  /*!
  Set the variable's value.  The condition variable should be locked
  before calling this method.
  */
  CondVar &operator=(const T &v);

  //@}
  //! @name accessors
  //@{

  //! Get the variable's value
  /*!
  Get the variable's value.  The condition variable should be locked
  before calling this method.
  */
  operator const volatile T &() const;

  //@}

private:
  volatile T m_data;
};

template <class T> inline CondVar<T>::CondVar(Mutex *mutex, const T &data) : CondVarBase(mutex), m_data(data)
{
  // do nothing
}

template <class T> inline CondVar<T>::CondVar(const CondVar &cv) : CondVarBase(cv.getMutex()), m_data(cv.m_data)
{
  // do nothing
}

template <class T> inline CondVar<T>::~CondVar() = default;

template <class T> inline CondVar<T> &CondVar<T>::operator=(const CondVar<T> &cv)
{
  m_data = cv.m_data;
  return *this;
}

template <class T> inline CondVar<T> &CondVar<T>::operator=(const T &data)
{
  m_data = data;
  return *this;
}

template <class T> inline CondVar<T>::operator const volatile T &() const
{
  return m_data;
}