g++ problem when attempting to cast a function pointer in an OpenGL GLU callback

g++ problem when attempting to cast a function pointer in an OpenGL GLU callback

[Day, Michael A AMRDEC/UAH]

I've got a program I'm attempting to port from Red Hat Enterprise Linux 3 to Cygwin (on Windows).
 
g++ -v in Cygwin reveals: gcc version 3.4.4
g++ -v in Red Hat reveals: gcc version 3.2.3-52
 
There's the background.  Here's the problem:
 
I'm attempting to cast a function pointer in the following C++ code:
 
------------------------------------------
/* ... includes and what not ... */
 
void tcbBegin(GLenum prim) { 
    cout << "BEGIN"; 
}
 
/* ... */
 
void tessellateTest() {
 //Allocate a new GLU tessellation object
 GLUtesselator *tess = gluNewTess();
 
 //Assigning callbacks
 
 //begin callback
 gluTessCallback(tess,GLU_TESS_BEGIN, (void(*)()) tcbBegin);    // THIS IS LINE 71 //
 
/* ... */
--------------------------------------
 
When compiling, this error happens in Cygwin:
test.cpp:72: error: invalid conversion from `void (*)()' to `void (*)()'
 
No error occurs in Red Hat.  The cast works just fine, and I've used this code with no problem for several months.
 
Not casting at all results in this error (on both Red Hat and Cygwin):
test.cpp:71: error: invalid conversion from `void (*)(GLenum)' to `void (*)()'
 
My compile line in Cygwin looks like:
$ g++ test.cpp -lglu32 -lopengl32
 
And in Red Hat it looks like:
$ g++ test.cpp -lGLU -lGL

Any idea whats going on?  It seems to me that the cast should be legal.  Have the rules changed in g++ between the two versions? I have already tried using a reinterpret_cast, but that didn't work.
 
Thanks,
 
Mike Day
Virtual Targets Center
https://modelexchange.army.mil/
 

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Re: g++ problem when attempting to cast a function pointer in an OpenGL GLU callback

[Brian Dessent]

"Day, Michael A AMRDEC/UAH" wrote:

> test.cpp:72: error: invalid conversion from `void (*)()' to `void (*)()'

Win32 API functions generally use the stdcall calling convention, not
the standard C one.  Your tcbBegin is not defined this way and this
could be what the compiler is trying to tell you, although the error
certainly isn't very clear.

If you look at glu.h:

GLAPI void APIENTRY gluTessCallback (GLUtesselator* tess, GLenum which,
_GLUfuncptr CallBackFunc);

where _GLUfuncptr is defined as:

typedef void (APIENTRY *_GLUfuncptr)();

where APIENTRY expands to "__attribute__ ((__stdcall__))".  So you'll
need to declare your callback with that attribute or else it will not
work.

Brian

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