IlmThread.h

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//
// SPDX-License-Identifier: BSD-3-Clause
// Copyright (c) Contributors to the OpenEXR Project.
//

#ifndef INCLUDED_ILM_THREAD_H
#define INCLUDED_ILM_THREAD_H

//-----------------------------------------------------------------------------
//
//      class Thread
//
//      Class Thread is a portable interface to a system-dependent thread
//      primitive.  In order to make a thread actually do something useful,
//      you must derive a subclass from class Thread and implement the
//      run() function.  If the operating system supports threading then
//      the run() function will be executed int a new thread.
//
//      The actual creation of the thread is done by the start() routine
//      which then calls the run() function.  In general the start()
//      routine should be called from the constructor of the derived class.
//
//      The base-class thread destructor will join/destroy the thread.
//
//      IMPORTANT: Due to the mechanisms that encapsulate the low-level
//      threading primitives in a C++ class there is a race condition
//      with code resembling the following:
//
//          {
//              WorkerThread myThread;
//          } // myThread goes out of scope, is destroyed
//            // and the thread is joined
//
//      The race is between the parent thread joining the child thread
//      in the destructor of myThread, and the run() function in the
//      child thread.  If the destructor gets executed first then run()
//      will be called with an invalid "this" pointer.
//
//      This issue can be fixed by using a Semaphore to keep track of
//      whether the run() function has already been called.  You can
//      include a Semaphore member variable within your derived class
//      which you post() on in the run() function, and wait() on in the
//      destructor before the thread is joined.  Alternatively you could
//      do something like this:
//
//          Semaphore runStarted;
//
//          void WorkerThread::run ()
//          {
//              runStarted.post()
//              // do some work
//              ...
//          }
//
//          {
//              WorkerThread myThread;
//              runStarted.wait ();    // ensure that we have started
//                                     // the run function
//          } // myThread goes out of scope, is destroyed
//            // and the thread is joined
//
//-----------------------------------------------------------------------------

#include "IlmThreadConfig.h"
#include "IlmThreadExport.h"
#include "IlmThreadNamespace.h"

#if ILMTHREAD_THREADING_ENABLED
#    include <thread>
#endif

ILMTHREAD_INTERNAL_NAMESPACE_HEADER_ENTER

//
// Query function to determine if the current platform supports
// threads AND this library was compiled with threading enabled.
//

ILMTHREAD_EXPORT bool supportsThreads ();

class ILMTHREAD_EXPORT_TYPE Thread
{
public:
    ILMTHREAD_EXPORT Thread ();
    ILMTHREAD_EXPORT virtual ~Thread ();

    ILMTHREAD_EXPORT void start ();

    virtual void run () = 0;

    //
    // wait for thread to exit - must be called before deleting thread
    //
    ILMTHREAD_EXPORT void join ();
    ILMTHREAD_EXPORT bool joinable () const;

private:
#if ILMTHREAD_THREADING_ENABLED
    std::thread _thread;
#endif

    Thread& operator= (const Thread& t) = delete;
    Thread& operator= (Thread&& t) = delete;
    Thread (const Thread& t)       = delete;
    Thread (Thread&& t)            = delete;
};

ILMTHREAD_INTERNAL_NAMESPACE_HEADER_EXIT

#endif // INCLUDED_ILM_THREAD_H