[FFmpeg-devel] [PATCH][RFC] ffmpeg: remove access to private FILE struct members on Windows

[Hendrik Leppkes]

On Mon, Aug 3, 2015 at 9:30 PM, Nicolas George <george at nsup.org> wrote:
> Le sextidi 16 thermidor, an CCXXIII, Michael Niedermayer a écrit :
>> i also found this comment in a patch in my inbox:
>>
>> +  /* When using Standard C input functions, also check if there
>> +   is anything in the buffer. After a call to such functions,
>> +   the input waiting in the pipe will be copied to the buffer,
>> +   and the call to PeekNamedPipe can indicate no input available.
>> +   Setting stdin to unbuffered was not enough, IIRC */
>> +  if (stdin->_cnt > 0)
>> +  {
>> +    char ch;
>> +    //Read it
>> +    read(0, &ch, 1);
>> +    return ch;
>> +  }
>>
>> This seems to explain what the code was intended to do
>
> That was my guess.
>
> This is ugly, and should be removed anyways. It is not possible to mix stdio
> and low-level reading like that, period.
>
> Fortunately, ffmpeg.c only uses stdin at one single place, a few lines below
> the corresponding code:
>
>         }else
>             if(scanf("%d", &debug)!=1)
>                 fprintf(stderr,"error parsing debug value\n");
>
> It needs to be changed to read a line with the low-level function
> (read_key()), exactly like 40 above for the 'c' case. I can not brew a patch
> right now, sorry.
>
> I suspect this would not work:
>
> printf 'd42\nC ... fontfile ...\n' | ffmpeg ...
>
> because the C will end up in the stdio buffer.
>
>

Good catch. This command doesn't work with or without the hunk this
patch removes however.
A simpler command without drawtext would be:

printf 'd0\n?' | ffmpeg -i ...

If you see help output, it works. Using another command instead of d0
works fine, like a C command followed by a \n?
It also doesn't work on Linux, so thats a more fundamental problem.

Since this fixes a build failure, unless we can find a use-case where
this hunk actually does make a difference, I would still rather remove
it.
If it causes any regression in the future, which we didn't think of
now, I'll be here to work on a fix.

- Hendrik