Re: [Mingw-users] asm in MSC and in GCC

[<eg...@of...>]

On Sun, Jan 07, 2001 at 02:04:01PM -0500, Soren Andersen wrote:
> I am trying to compile the Perl 5 module Win32::API
> (http://dada.perl.it/) on WinNT4 sp6 using MinGW - gcc-2.95.2 and
> having a rough go of it with the xs code. The compilation fails
> because this code has an undeclared identifier: 

OK, now I understand.  :)

This is actually a somewhat valid use of inline assembler.  The deal is 
that this module's purpose is to call arbitrary Win32 functions with a 
programmatically constructed argument list.  The core of the module is
simply a loop which walks an array of parameters and uses _asm { push ... }
to put arguments on the stack before calling the API function in question.

The AbstractCallback() snippet that had us so mystified is actually a stub.
It looks like the implementor never got around to callback support (which
would be pretty tricky, given that the callback could have an arbitrary
signature), so instead they just have a function which prints out some random
value (which might be useful, given the calling conventions) and returns
(possibly to crash, but you're not supposed to use it anyway).  There might
as well have been "assert(false)" in there, except that <prejudice> Perl
hackers don't believe in reliability </prejudice>.

It should be possible to port this code, though it would take someone who
is pretty wizzy with GCC asm() and Win32 calling conventions.  It may be
that something similar happens in the guts of w32api -- I've never looked
there.  The biggest issue is making sure that the stack frame you're building
isn't clobbered halfway through by GCC; I'd recommend refactoring the code to
construct the entire frame in a char[] buffer in C, then entering one final
asm() statement to adjust the stack pointer, call the actual function, and
clean up.

The entire module is < 400 lines of code, though, so we're not talking
about heroic effort here.

Dan