OpenEuphoria

Calling Conventions

• Posted by Chris Bensler <bensler at nt.net> Feb 10, 2007
• 465 views

It is my understanding that cdecl functions require the caller to cleanup the
stack, whereas the others are required to cleanup the stack themselves?

I'm working on some code for the various calling conventions using cprintf() as
one of my test functions which is a variadic function and thus is sure to be
cdecl. Another function I'm using is GetVersion() from the WINAPI which I'm quite
positive is stdcall.

The asm I've create seems to work fine except that I had thought that the
instruction "add esp, ebx" would have to be removed for stdcalls. Strangely, it
works for both cdecl and stdcall if the instruction is present but crashes for
both if it isn't.

I also thought that popa would restore the stack, meaning that the add esp, ebx
instruction was redundant, but the call to GetVerison() doesn't work without it.
Matt's fptr.e asm code doesn't do any explicit stack cleanup either.

Here is the asm:

constant STDC_ASM = get_asm(""
  & "pusha "

  & "mov ecx, ARGC " -- argument count must be in ecx for movsd
  -- get byte size of array
  -- ebx = ecx * 4
  & "mov ebx, ecx "
  & "shl ebx, 1 " -- ecx *= 2 -- can't shift by 2, instead of shifting 2x?
  & "shl ebx, 1 " -- ecx *= 2
  
  & "sub esp, ebx " -- offset stack pointer

  & "cld "           -- clear direction flag
  & "mov edi, esp "  -- movsd dst
  & "mov esi, ARGV " -- movsd src
  & "rep movsd "  -- poke4

  & "call near dword ptr [FUNC] " -- call the function
  & "add esp, ebx "

  & "mov ebx, RETV "
  & "mov [ebx], eax " -- return value
  & "add ebx, 4 "
  & "mov [ebx], edx " -- double

  & "popa "
  & "ret "
)

Chris Bensler
~ The difference between ordinary and extraordinary is that little extra ~
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