On Fri, 29 Nov 2024, Martin Storsjö wrote:
On Tue, 19 Nov 2024, Pali Rohár wrote:
C99 vscanf-family functions are natively available since msvcr120.
This change allows to use native UCRT and msvcr120 C99 vscanf-family
functions.
I think this commit message would require a much longer explanation about the
status quo of these functions - I didn't quite remember what the situation
was, and I felt surprised to read that we didn't use the native vscanf family
of functions in the UCRT.
From a brief digging around, I see that for e.g. vsscanf, we have:
- stdio/vsscanf2.S, providing vsscanf, redirecting to __ms_vsscanf
- stdio/vsscanf.c, providing __ms_vsscanf, calling __argtos
- stdio/scanf.S, providing __argtos, which converts from a va_list back to
parameters on the stack, and calls sscanf()
And this, because older msvcr*.dll doesn't provide v*scanf functions at all.
The stdio/scanf.S file has got quite good comments explaining parts of it.
But some parts of the comments there feel inaccurate (like "This is needed
because mingw-w64 uses msvcr100.dll" - that's not really the default
anywhere), so a patch to fix up inaccuracies in the comments there would be
nice.
And now we move this to the older msvcr* import libraries, because we
actually do have proper v*scanf in msvcr120 and UCRT.
So this seems reasonable, but the commit message needs an explanation that
explanins this, as it's not at all very clear.
(The surprise about this situation was what converted my reaction on this
patchset from "seems straightforward and probably good", to "wait, what? I
need to look into that"; having more such explanations in the commit messages
makes it easier to reason about the patch.)
Can you try to write that up? Otherwise I can try to have a look at writing
it tomorrow.
I rewrote this commit message into this bit:
crt: Move vscanf-family functions from libmingwex.a to individual CRT
import libraries
msvcr110 and earlier did not have any v*scanf functions. Because of
that, the mingw-w64-crt contained wrappers that implement the
v*scanf functions based on the non-variadic *scanf functions, by
reading the va_list object and re-passing them as regular parameters
to the *scanf functions.
The sequence involved for vsscanf is:
- stdio/vsscanf2.S, providing vsscanf, calling __ms_vsscanf
- stdio/vsscanf.c, providing __ms_vsscanf, calling __argtos
- stdio/scanf.S, providing __argtos, which converts from a
va_list back to regular parameters, and calls sscanf()
These functions were provided in libmingwex, making them available
and used with all CRT versions.
However, since msvcr120 (and in UCRT), the proper v*scanf functions
are available. Therefore, move this set of wrappers into the
individual CRT import libraries, allowing using the proper v*scanf
functions on msvcr120 and UCRT.
Does that sound ok?
// Martin
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