Re: [libmicrohttpd] 0.9.62 doesn't build on Visual Studio

[Jonathan McDougall]

On 12/10/18, Tim Rühsen <address@hidden> wrote:
> On 12/10/18 2:27 PM, Christian Grothoff wrote:
>> On 12/10/18 9:53 AM, Tim Rühsen wrote:
>>> instead of keeping all projects at C89, you can also use Visual
>>> Studio with LLVM/Clang or GCC

0.9.62 does build with the clang toolset on Visual Studio after some
changes to the project file. It's not something most users will have
installed though.

Note also that VLAs are an _optional_ feature of C11 (see 6.7.6.2/4 and
6.10.8.3).

>> Well, we do know the max for all of the VLAs, so it would be trivial
>> to change the code to use that value instead.  However, for the sake
>> of keeping the stack shallow on embedded systems, I would prefer to
>> do this conditionally only if the C compiler doesn't support VLAs.

Just how many bytes are we talking about? VLAs aren't free either, and
the code generated to create the array might just be bigger than the
number of bytes you save on allocation, as well as being generally less
efficient than a simple array of constant size.

>> Does the W32-crowd know whether AC_C_VARARRAYS and the resulting
>> __STDC_NO_VLA__ works nicely with Visual Studio?

__STDC_NO_VLA__ is undefined in Visual Studio, regardless of the toolset
selected. As for AC_C_VARARRAYS, the configure script isn't used on
Windows at all. I guess __STDC_NO_VLA__ could be conditionally defined
in w32/common/MHD_config.h.

>> If so, we could
>>
>> #if __STDC_NO_VLA__
>> #define VLA_LEN_DIGEST(n) (32*2+1)
>> #else
>> #define VLA_LEN_DIGEST(n) (n)
>> #endif

Well, at this point I guess it becomes a matter of taste, but I'd be
really uncomfortable with having such a difference in code between two
platforms, especially since Windows is not exactly tested very often. If
you go that route, at least put a check for `n < 32*2+1` to avoid buffer
overflows.

> You could also add a configure check for alloca and use that function
> in case __STDC_NO_VLA__ is defined.

alloca() is not part of any standard, whether it's C, C++ or POSIX.
Visual Studio had an _alloca(), but it's deprecated. It has a weird
_malloca() that requires a weirder _freea().

alloca() also has a bunch of problems, like interfering with inlining
on gcc and undefined behaviour on failure. Basically every single
manpage I could find discourages its use.

I would personally go for constant size arrays, or malloc() if you
can't. Keep it simple and portable.

-- 
Jonathan McDougall