Stack alignmennt in call from C++ to FPC

Hi,

I have recently ported a Delphi application to Lazarus/FPC (Win32/x86). This application makes use of a DLL implemented in MS Visual C++ 2010.

The calls from FPC code to C++ code work fine. However this DLL also has a callback
system for notifications. When the DLL calls a FPC callback function a crash occurs inside the DLL with a 'Datatype misalignment' exception (1).

Some references (eg (2)) claim that MSVC aligns the stack at 4-byte boundaries while GCC aligns
at 16-byte boundaries. I have found no confirmation on those claims.

With GCC I could use __attribute__((force_align_arg_pointer)) in the callback declaration.
Or even -mstackrealign. However I can't find a similar option with FPC.

Any idea?

Thanks for any help.

Here some code excerpts:

{ FPC }
type
 TMyCallback = procedure(s : PChar); stdcall;
...
procedure RegisterCallBack(callback: TMyCallback); stdcall; external
'MyDll.dll';
...
procedure Callback(s : PChar); stdcall;
begin
{ use s }
end;
...
RegisterCallBack(Callback);
...
// C++
typedef void (__stdcall *Callback_t)(const char* s);
extern "C" {
__declspec(dllexport) void RegisterCallback(Callback_t p);
}

...

static Callback_t callback = 0;

void RegisterCallback(Callback_t p)
{
callback(p);
}
...
// at some point the callback is called;
if (callback)
callback("hello world"); // <== crashes here

Josue Andrade Gomes wrote

Some references (eg (2)) claim that MSVC aligns the stack at 4-byte boundaries while GCC aligns at 16-byte boundaries. I have found no confirmation on those claims.

Stack alignment is not a compiler property, but a platform property. The stack alignment on Win32 is 4 bytes. On Linux/i386 it used to 4 bytes, but there has been an unofficial switch to 16 bytes (which, in all fairness, was pushed by gcc for SSE/performance reasons).

I don't immediately see what the problem is in your case. Usually it's a mismatch of calling conventions, but that seems to be fine in the code you posted.

Jonas

Thanks for your answer. Anyway the problem seems to be something else. I have written a small test but could not reproduce the problem. I'll continue investigation.

If someone is interested my small test can be found at www.josuegomes.com/fp-msvc.zip
It contains a DLL built with MSVC and two Pascal applications that load the DLL, pass the callback function
and call it.

Thanks again for your help. If I find something of interest I'll post it here.

regards,
josue

Josue Andrade Gomes wrote

If someone is interested my small test can be found at www.josuegomes.com/fp-msvc.zip

The link is now fixed. Sorry.

josue

On Thu, Jun 6, 2013 at 11:09 AM, Josue Andrade Gomes wrote:

I reproduced the problem with a small Lazarus application. The crash occurs inside fpc_popaddrstack.

0x40b750 <fpc_popaddrstack+48> jmp 0x40b759 <fpc_popaddrstack+5 0x40b752 <fpc_popaddrstack+50> mov (%ebx),%eax

0x40b754 <fpc_popaddrstack+52> mov 0x4(%eax),%eax

0x40b757 <fpc_popaddrstack+55> mov %eax,(%ebx) 0x40b759 <fpc_popaddrstack+57> mov (%esp),%ebx 0x40b75c <fpc_popaddrstack+60> add $0x4,%esp 0x40b75f <fpc_popaddrstack+63> ret

I also found this http://comments.gmane.org/gmane.comp.ide.lazarus.general/68243

Could be related? Any ideas?

Sourcecode for my tests can be found at: www.josuegomes.com/fp-msvc.zip

regards,
josue

Initializing the thread system with a dummy thread as suggested by http://wiki.freepascal.org/Multithreaded_Application_Tutorial has (apparently) fixed the problem.

Thanks all for the support.