[FFmpeg-devel] [RFC][PATCH] Windows Television (WTV) file system handling

Sat Jan 22 11:20:20 CET 2011

On Thu, Jan 20, 2011 at 06:39:11PM -0500, Ronald S. Bultje wrote:
> Hi,
> 
> On Thu, Jan 20, 2011 at 6:31 PM, Peter Ross <pross at xvid.org> wrote:
> > On Thu, Jan 20, 2011 at 09:03:57AM -0500, Ronald S. Bultje wrote:
> >> Hi Peter,
> >>
> >> On Thu, Jan 20, 2011 at 12:19 AM, Peter Ross <pross at xvid.org> wrote:
> >> > On Sun, Jan 09, 2011 at 05:21:20PM +1100, Peter Ross wrote:
> >> >> 0002-add-AVFMT_NOGENERICSEEK-flag.patch
> >> >> * adds AVFMT_NOGENERICSEEK flag, which should be self explanatory.
> >> [..]
> >> > @@ -23,7 +23,7 @@
> >> >
> >> > ?#define LIBAVFORMAT_VERSION_MAJOR 52
> >> > ?#define LIBAVFORMAT_VERSION_MINOR 92
> >> > -#define LIBAVFORMAT_VERSION_MICRO ?0
> >> > +#define LIBAVFORMAT_VERSION_MICRO ?1
> >> >
> >> > ?#define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
> >> > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? LIBAVFORMAT_VERSION_MINOR, \
> >> > @@ -324,6 +324,7 @@ typedef struct AVFormatParameters {
> >> > ?#define AVFMT_VARIABLE_FPS ?0x0400 /**< Format allows variable fps. */
> >> > ?#define AVFMT_NODIMENSIONS ?0x0800 /**< Format does not need width/height */
> >> > ?#define AVFMT_NOSTREAMS ? ? 0x1000 /**< Format does not require any streams */
> >> > +#define AVFMT_NOGENERICSEEK 0x2000 /**< Do not perform generic read_seek */
> >> >
> >> > ?typedef struct AVOutputFormat {
> >> > ? ? ?const char *name;
> >> > diff --git a/libavformat/utils.c b/libavformat/utils.c
> >> > index 32067e9..365272b 100644
> >> > --- a/libavformat/utils.c
> >> > +++ b/libavformat/utils.c
> >> > @@ -1747,6 +1747,9 @@ int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int f
> >> > ? ? ? ? ?return 0;
> >> > ? ? ?}
> >> >
> >> > + ? ?if ((s->iformat->flags & AVFMT_NOGENERICSEEK))
> >> > + ? ? ? ?return -1;
> >> > +
> >> > ? ? ?if(s->iformat->read_timestamp)
> >> > ? ? ? ? ?return av_seek_frame_binary(s, stream_index, timestamp, flags);
> >> > ? ? ?else
> >>
> >> What does this do/fix?
> >
> > The demuxer sets AVIndexEntry->pos to positions relative to the start of a file
> > *within* the filesystem. If the demuxer's read_seek() fails, lavf trys to
> > perform a generic seek using the AVIndexEntry[]->pos values.
> 
> I don't really have a strong opinion on this, but I would say that
> _maybe_ this is abuse of AVIndexEntry[]->pos in a way that it was not
> designed for. What do others think?

Fair enough. The enclosed patch adds ff_add_index_entry() and ff_index_search()
for handling privately maintained indexes.

> >> >> 0001-make-unicode-string-reader-accessible-to-other-modu.patch
> >> >> * the metadata format uses unicode structures similar, but not identical,
> >> >> ? to ASF/DRV-MS.
> >> [..]
> >> > ?/**
> >> > + * Read a null terminated string
> >> > + */
> >> > +static void get_utf16z(ByteIOContext *pb, char *buf, int buf_size)
> >> > +{
> >> > + ? ?char* q = buf;
> >> > + ? ?int ch;
> >> > + ? ?while ((ch = get_le16(pb)))
> >> > + ? ? ? ?if (q - buf < buf_size - 1) *q++ = ch;
> >> > + ? ?*q = '\0';
> >> > +}
> >>
> >> Already exists in asfdec.c, probably, and likely mms uses that one
> >> also. Can you reuse that function? It looks ok in general, I think
> >> we'd want some files in this format in fate so this sort of stuff is
> >> tested.

There is ff_asf_get_str16_nolen() in asfdec, but it expects the caller to
know the string length in advance. We dont have that information in wtv.
get_utf16z() reads until it hits the null terminator.

> >> > +static const ff_asf_guid dir_entry_guid =
> >> > + ? ?{0x92,0xb7,0x74,0x91,0x59,0x70,0x70,0x44,0x88,0xdf,0x06,0x3b,0x82,0xcc,0x21,0x3d};
> >>
> >> Should it be in the asf header? Seems duplicated.
> >
> > Which part is duplicated?
> 
> You're right, I was asleep. This part is fine.
> 
> >> > +/**
> >> > + * @param[in] buf unicode buffer
> >> > + * @param buf_size buffer size (bytes)
> >> > + * @return 0 if equal
> >> > + */
> >> > +static int unicode_compare(const uint8_t *buf, int buf_size, const uint8_t *s)
> >>
> >> I'd be shocked if libc didn't provide some way of doing this, convert
> >> utf16->utf8 and then strcmp()? Also you don't check buf[i*2+1] ever, I
> >
> > I do not know of any. While there are wchar functions, one cannot rely on
> > sizeof(wchar_t)==2.
> 
> If you insist on comparing strings, all strings being compared here
> are always static strings. This allows for tricks, e.g. declaring them
> as wchar_t strings, which you apparently don't want to do, or just
> creating one manually by doing "s\0t\0r\0i\0n\0g"; </ugly>. The
> advantage of these approaches (easiest is really to look for a
> wchar-style thing that ensures 16-bit unicode formatting) is that you
> can use memcmp() instead of this slow compare function, and memcmp()
> is probably better-optimized. Also saves code.

You're right, that looks horrid. I have updated to use wcscmp().

-- Peter
(A907 E02F A6E5 0CD2 34CD 20D2 6760 79C5 AC40 DD6B)
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