Starting a method in another thread C++

Question

I'm having trouble finding out how to run a method in a seperate thread in C++ (using Visual C++ 2008), I've tried a number of ways to do this but none of them so far have been successful.

I'm very new to C++ but a fairly experienced programmer in Java, but have been given a task to fix some bugs in an old C++ application. The program uses an object called 'Mpeg' to control packetising and depackitising an Mpeg file. After setting up an Mpeg object properly, mpeg.Depacketise needs to be called, which then runs the method DepacketiseInputFile().

I've tried to make DepacketiseInputFile() run in a seperate thread by both using _beginthread and the System::Threading::Thread object using

Thread^ th = gcnew Thread(gcnew ThreadStart(DepacketiseInputFile));

however this returns the errors

using &Mpeg::Depacketise gives the error

when using _beginthread the code I tried was

However with this I constantly had trouble getting the arguments correct, with errors like

cropping up.

Is there any simple way to do this that anyone can reccomend? I've spent a few days playing around with this but seem to be getting nowhere :(

Any help would be greatly appreciated.

Cheers.

`gcnew` is not C++, is C++/CLI. Don't let the name confuse you, its **not** C++. — K-ballo, Oct 17 '11 at 14:39
That was something I tried briefly today, didn't seem to have much promise so thanks for letting me know, any advice on using _beginthread? — marasmuse, Oct 17 '11 at 14:42
My advise, if this is real C++, would be to use `boost::thread` — K-ballo, Oct 17 '11 at 14:49

score 1 · Answer 1 · answered Oct 17 '11 at 14:43

1

Change

_beginthread(DepacketiseInputFile(), (unsigned)0, (void *)NULL);

to

_beginthread(DepacketiseInputFile, (unsigned)0, (void *)NULL);

You wanna pass the address of the function to run (DepacketiseInputFile) and not its return value of that function (which is what you get from DepacketiseInputFile()).

I'm assuming DepacketiseInputFile is declared as void DepacketiseInputFile(void*), and is not a non-static member function of some class. Otherwise, the types won't match even when you do remove the brackets.

answered Oct 17 '11 at 14:43

Eran

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ok, tried that and I get `code` Packetiser Depacketiser\Mpeg.cpp(61) : error C3867: 'Mpeg::DepacketiseInputFile': function call missing argument list; use '&Mpeg::DepacketiseInputFile' to create a pointer to member – marasmuse Oct 17 '11 at 14:52
Trying to use '&Mpeg::DepackitiseInputFile' gives the error `\Packetiser Depacketiser\Mpeg.cpp(61) : error C2665: '_beginthread' : none of the 2 overloads could convert all the argument types C:\Program Files\Microsoft Visual Studio 9.0\VC\include\process.h(54): could be 'uintptr_t _beginthread(void (__cdecl *)(void *),unsigned int,void *)' C:\Program Files\Microsoft Visual Studio 9.0\VC\include\process.h(65): or 'uintptr_t _beginthread(void (__clrcall *)(void *),unsigned int,void *)' while trying to match the argument list '(void (__thiscall Mpeg::* )(void *), unsigned int, void *)'` – marasmuse Oct 17 '11 at 14:55
It's declared as `void Mpeg::DepacketiseInputFile(void* pointer)` I can't make it static though. – marasmuse Oct 17 '11 at 15:22
You can't use a non-static member function (`__thiscall`) with `_beginthread`. If you can't make it static, let `_beginthread` launch an intermediate function (static member or free-standing), and pass a pointer to the `Mpeg` object as the last argument. Then, in that function, cast the `void*` parameter back into an `Mpeg*`, and invoke `DepacketiseInputFile` on that object. – Eran Oct 17 '11 at 19:20

Lou Franco · Accepted Answer · 2011-10-17T15:22:32.603

What kind of type is Mpeg? What kind of method is DepacketiseInputFile?

If it's a regular unmanaged, C++ class, then use _beginthread, but you have to make DepacketiseInputFile a static. It cannot take a member function.

Also, don't call DepacketiseInputFile with DepacketiseInputFile(), pass it in with

  &Mpeg::DepacketiseInputFile

You should use the void* you get to pass it to pass in a pointer to the Mpeg object (and then cast it back).

If you want to use ThreadStart, then Mpeg needs to be a managed class.

EDIT: If you want to make DepacketiseInputFile, but it needs to access the object, then you use the void* argument to pass in a pointer.

So in the .h:

 void DepacketiseInputFileMember();
 static void DepacketiseInputFile(void *thisObj);

Your code goes in DepacketiseInputFileMember(), and write DepacketiseInputFile like this:

 void Mpeg::DepacketiseInputFile(void *thisObj)
 {
     Mpeg* mpeg = reinterpret_cast<Mpeg*>(thisObj);
     mpeg->DepacketiseInputFileMember();
 }

When you call _beginthread, use this

 _beginnthread(&Mpeg::DepacketiseInputFile, (unsigned)0, anMpegObjectPointer);

where anMpegObjectPointer is a pointer to an object of type Mpeg. You have to make sure the lifetime of the object is longer than it would be needed in the thread.

Forgive my syntax, I am writing this in a textarea, not Visual Studio

I don't seem to be able to make DepackitiseInputFile static as this causes tons of errors with the method. — marasmuse, Oct 17 '11 at 14:56

Starting a method in another thread C++

2 Answers2