ffi library port to Windows (MSCV_X86_WIN32): notes

Hi Etienne,

I just finished to compile on MSVC the C part of the ffi library. So, 
the next step will be translating the win32.s (assembly for GCC) for MSVC
During this initial part of the work, I noticed the following things:


warings
--------
I'm working at warning level 4 and MSVC issues a few warnings which I'm 
gonna fix later on.


config.h
--------
I'm porting an "instance" of the ffi library, based on a successful 
build on Cigwin. By that I mean that I'm working on a source tree and a 
config.h file, generated by ./configure on my PC. That means that some 
macros of the config.h file might change on another PC.


alloca() function limitations
--------------------------
The ffi library makes use of the function: alloca(); I attach here what 
MSVC says about it:

*_alloca* allocates /size/ bytes from the program stack. The allocated 
space is automatically freed when the calling function exits. Therefore, 
do not pass the pointer value returned by *_alloca* as an argument to 
free <_crt_free.htm>.

There are restrictions to explicitly calling _*alloca* in an exception 
handler (EH). EH routines that run on x86-class processors operate in 
their own memory “frame”: They perform their tasks in memory space that 
is not based on the current location of the stack pointer of the 
enclosing function. The most common implementations include Windows NT 
structured exception handling (SEH) and C++ catch clause expressions. 
Therefore, explicitly calling _*alloca* in any of the following 
scenarios results in program failure during the return to the calling EH 
routine:

    * Windows NT SEH exception filter expression: __*except* ( *alloca()* )

    * Windows NT SEH final exception handler: __*finally* { *alloca()* }

    * C++ EH catch clause expression

However, _*alloca* can be called directly from within an EH routine or 
from an application-supplied callback that gets invoked by one of the EH 
scenarios listed above


Enrico