How can I call C functions with D function pointers as operands?

I tried this:
	// gl2psUserWritePNG & gl2psUserFlushPNG are static
	void function(png_struct*, ubyte*, uint) gl2psUserWritePNG_ptr
= &gl2psUserWritePNG;
	void function(png_struct*)
gl2psUserFlushPNG_ptr = &gl2psUserFlushPNG;

	png_set_write_fn(
		png_ptr,
		cast(void*)png,
		gl2psUserFlushPNG_ptr,
		gl2psUserFlushPNG_ptr
	);

but there are some errors:
 Error: cannot implicitly convert expression (gl2psUserWritePNG_ptr)
of type void function(png_struct*, ubyte*, uint) to voidC  function
(png_struct*, ubyte*, uint)

 Error: cannot implicitly convert expression (gl2psUserFlushPNG_ptr)
of type void function(png_struct*) to voidC  function(png_struct*)

"Zarathustra" <adam.chrapkowski@gmail.com> wrote in message news:fuvj7c$1hf7$1@digitalmars.com...
> How can I call C functions with D function pointers as operands?
>
> I tried this:
> // gl2psUserWritePNG & gl2psUserFlushPNG are static
> void function(png_struct*, ubyte*, uint) gl2psUserWritePNG_ptr
> = &gl2psUserWritePNG;
> void function(png_struct*)
> gl2psUserFlushPNG_ptr = &gl2psUserFlushPNG;
>
> png_set_write_fn(
> png_ptr,
> cast(void*)png,
> gl2psUserFlushPNG_ptr,
> gl2psUserFlushPNG_ptr
> );
>
> but there are some errors:
> Error: cannot implicitly convert expression (gl2psUserWritePNG_ptr)
> of type void function(png_struct*, ubyte*, uint) to voidC  function
> (png_struct*, ubyte*, uint)
>
> Error: cannot implicitly convert expression (gl2psUserFlushPNG_ptr)
> of type void function(png_struct*) to voidC  function(png_struct*)

You have to declare your callback functions as extern(C), like so:

extern(C) void gl2psUserFlushPNG(png_struct* s)
{
...
}

Same with the other one.