-pedantic warning and pthread_create

Question

I'm newbie in the ansi and iso world, i compiled my program with :

-asni -pedantic -std=c++11 -std=c++98

Now, i get the following warning:

 warning: converting from 'void (NetworkSocket::*)()' to 'void* (*)(void*)' [-pedantic]

for the following line:

if (pthread_create(this->threadArray, NULL, (void*(*)(void*)) &NetworkSocket::threadProcedure  , (void *)this) != 0)
        { /* error */      }

How can i pass pedantic warning?

First, drop two of the three: `-ansi -std=c++11 -std=c++98` because `-ansi` and `-std=c++98` are the same, and `std=c++11` conflicts with the other two. — rubenvb, Jul 28 '12 at 14:19
Good point, it's just a thread procedure. If for some reason you need instance member access in there and can't use the extra data parameter, `std::bind` is your best option. If not, go for the static function. — chris, Jul 28 '12 at 14:29
[A function pointer cast is a bug waiting to happen](http://blogs.msdn.com/b/oldnewthing/archive/2011/05/06/10161590.aspx). "There, that shut up the compiler. Those compiler guys are so stupid. They can't even figure out how to convert one function pointer to another. I bet they need help wiping their butts when they go to the bathroom." — Adam Rosenfield, Jul 28 '12 at 14:42
@chris, how do you pass a bind expression to `pthread_create`? — Jonathan Wakely, Jul 28 '12 at 15:06
@JonathanWakely, Good point. I forgot it can't take those. Guess if you need more than one piece of extra data, a pointer to a struct containing both? — chris, Jul 28 '12 at 15:08
possible duplicate of [pthread Function from a Class](http://stackoverflow.com/questions/1151582/pthread-function-from-a-class) — Jonathan Wakely, Jul 28 '12 at 15:09

Torsten Robitzki · Answer 1 · 2012-07-28T15:16:31.513

3

pthread_create is a C function expecting a C function taking a void pointer, and returning a void pointer. So you can use such a C function to dispatch the call to you member function, if you use the this pointer as thread argument:

extern "C" void* startThread( void* p )
{
    static_cast< NetworkSocket* >( p )->threadProcedure();

    return 0;
} 

if ( pthread_create( this->threadArray, 0, startThread, this ) )
   ...

edited Jul 28 '12 at 15:16

answered Jul 28 '12 at 14:27

Torsten Robitzki

3,041
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Using C-linkage (the `extern "C"` part) is unnecessary. Additionally it is possible to use a function inside the class, if it's a static function (with `void* (*fn)(void*)` as its signature). – Giel Jul 28 '12 at 17:57
2

@Giel: while a function without `extern "C"` linkage will work in almost all cases, strictly speaking it *is* necessary (C++ and C language ABIs need not be the same for free functions, though they nearly always are - similarly for static member functions). Even if you don't care for the language lawyer rationale, since there's no harm in having the `extern "C"` and it describes exactly what is intended, it should be used. Especially if you're interested in `-pedantic` warnings... – Michael Burr Jul 28 '12 at 20:46
1

@Giel: That is a common misconception. You can **not** use static member functions. Unfortunately a lot of people do and it `seems` to work. The problem is that The pthread library is a C library and does not understand anything about C++ so it can technically only call C functions. Calling C++ functions is non portable and they are just getting lucky `on that particular compiler` that the same calling convention is used for C function as for a C++ static member function. There is no requirement in the standard that this is true (and I know several platforms where it is not). – Martin York Jul 31 '12 at 14:31

-pedantic warning and pthread_create

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