[FFmpeg-devel] av_fopen_utf8 and cross-DLL CRT object sharing issue on Windows

[Martin Storsjö]

On Mon, 9 May 2022, Soft Works wrote:

>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
>> Martin Storsjö
>> Sent: Sunday, May 8, 2022 10:02 PM
>> To: FFmpeg development discussions and patches <ffmpeg-
>> devel at ffmpeg.org>
>> Subject: Re: [FFmpeg-devel] av_fopen_utf8 and cross-DLL CRT object
>> sharing issue on Windows
>>
>> On Sat, 7 May 2022, Soft Works wrote:
>>
>>>> This means that CRT objects (file descriptors from open(), FILE*
>>>> opened
>>>> with fopen/fdopen) mustn't be shared across DLLs; such an object
>> must
>>>> be
>>>> opened, accessed and closed within the same DLL.
>>>
>>> This only happens when you explicitly modify the build configuration
>>> to statically link to the CRT.
>>
>> No, this is not a custom build configuration. This is the build
>> configuration you get if you configure with "configure --enable-shared
>> --toolchain=msvc".
>
> Ok, then this is what needs to be fixed. When you configure for "shared",
> the exe and dll binaries need to be all compiled with /MD.

I disagree. Both (statically linked or dynamically linked CRT) are 
entirely valid configurations, and ffmpeg works fine (except for this 
particular, so far marginally used, function) in both those build 
configurations.

>> Also, another fairly common situation where the "different CRTs"
>> scenario
>> happens if you'd e.g. build the ffmpeg libraries as DLLs with mingw,
>> but
>> then link against those DLLs with a user application built with MSVC.
>
> AFAIK, it is possible to create DLLs with mingw/MSYS2 in a way that
> these can link to a specific version of the MS CRT, but that's just
> a side note.

Yes, that's true. (As a side note to this side note, I'm the one who added 
support for UCRT in mingw-w64 in the first place, so I do know a thing or 
two about that.)

But in short, yes it's possible to spend effort at making them use the 
same shared CRT, but it's also fairly common to use different CRTs.

// Martin