[FFmpeg-devel] [PATCH] lavf: add subfile protocol.

[Lukasz Marek]

> +static int64_t subfile_seek(URLContext *h, int64_t pos, int whence)
> +{
> +    SubfileContext *c = h->priv_data;
> +    int64_t new_pos = -1;
> +    int ret;
> +
> +    if (whence == AVSEEK_SIZE)
> +        return c->end - c->start;
> +    switch (whence) {
> +    case SEEK_SET:
> +        new_pos = c->start + pos;
> +        break;
> +    case SEEK_CUR:
> +        new_pos += pos;
> +        break;
> +    case SEEK_END:
> +        new_pos = c->end + c->pos;
> +        break;
> +    }
> +    if (new_pos < c->start)
> +        return AVERROR(EINVAL);

Note:
In file protocol lseek is used. If you pass SEEK_SET and n as new pos it 
always returns n and errno = 0. Even when seeking beyond end of the file 
or before beginning.  When you look at code file.c it will return 0 in 
case seeking to position before beginning. I simulated similar behavior 
in ftp.c and forgot about it in libssh.c where negative value is 
returned. http.c also return negative value (like lseek). In all these 
cases seeking is done anyway and this protocol may be inconsistent with 
them. Unfortunately ffmpeg doesn't state clearly what should happen in 
such case in docs.

Seeking to c->start may be more expected by users I think.

BTW, I used to send an email to list about that before and as I remember 
Michael responded to take file.c as reference.

> +    c->pos = new_pos;
> +    if ((ret = slave_seek(h)) < 0)
> +        return ret;
> +    return c->pos - c->start;
> +}

-- 
Best Regards,
Lukasz Marek

I may be drunk, Miss, but in the morning I will be sober and you will 
still be ugly. - Winston Churchill