[FFmpeg-devel] [PATCH v2 1/2] avutil/wchar_filename, file_open: Support long file names on Windows

[Hendrik Leppkes]

On Mon, May 23, 2022 at 5:48 PM Soft Works <softworkz at hotmail.com> wrote:
>
>
>
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of nil-
> > admirari at mailo.com
> > Sent: Monday, May 23, 2022 5:36 PM
> > To: ffmpeg-devel at ffmpeg.org
> > Subject: Re: [FFmpeg-devel] [PATCH v2 1/2] avutil/wchar_filename,
> > file_open: Support long file names on Windows
> >
> > >> Not possible for stat precisely because of function and struct sharing
> > a
> > >> name.
> >
> > > That's exactly what is said above and before :-)
> >
> > My previous question was not answered, so I had to look up the answer
> > myself.
> >
> > > I'm actually wondering how does it even compile. All stat structs in
> > code
> > > become struct win32_stat, and all calls to stat become calls to
> > win32_stat,
> > > which in turn wraps _wstati64. But _wstati64 does not accept struct
> > win32_stat*,
> > > it accepts struct _stati64*. Content of these structs is probably
> > identical, but
> > > it should not matter: C is typed nominally, not structurally.
> >
> > Turns out C actually has a concept of compatible types:
> > https://en.cppreference.com/w/c/language/type.
> >
> > The problems is:
> > > they are both structure/union/enumeration types, and
> > > - (c99) if one is declared with a tag, the other must also be declared
> > with the same tag.
> > > ...
> > > If two declarations refer to the same object or function and do not use
> > compatible types,
> > > the behavior of the program is undefined.
> >
> > Your structure does not have the same tag as the CRT's one.
> > Are you sure you want to rely on undefined behaviour?
> >
> > I haven't compiled your code, but the following simple example:
> >
> > struct A { int a, b, c; };
> > struct B { int a, b, c; };
> > void printA(struct A *a);
> >
> > struct B b = { 1, 2, 3 };
> > printA(&b);
> >
> > generates a
> >
> > warning: passing argument 1 of ‘printA’ from incompatible pointer type [-
> > Wincompatible-pointer-types]
> >     |     printA(&b);
> >     |            ^~
> >     |            |
> >     |            struct B *
> > note: expected ‘struct A *’ but argument is of type ‘struct B *’
> >     | void printA(struct A *a)
> >
> > Are you sure you wanna add a couple of similar warnings to the project?
>
> This is not what's happening. No warnings, not even from clang diagnostics
> with -Weverything.
>
>
> > Needless to repeat, _USE_32BIT_TIME_T is not supported.
>
> I don't think it ever was. Have you been compiling and using ffmpeg
> successfully with this?
>

_USE_32BIT_TIME_T is actually the default for 32-bit mingw-w64 builds,
so its certainly a situation on windows that needs to work - and does
in master.

- Hendrik