[FFmpeg-devel] intent of 'lsf' variable in libavformat/mp3enc.c mp3_write_xing() ?
Jason Vas Dias
jason.vas.dias at synchronoss.com
Thu Jun 19 20:07:02 CEST 2014
Thanks for responding, Michael -
Actually, I checked out the GIT HEAD, then checked out the latest 'stable' tag 'n2.2.3'.
Sorry, my original mail should have mentioned the version I was trying to build.
The complete section of code I'm concerned with is :
linavformat/mp3enc.c , line 127:
<quote>
int bytes_needed, lsf;
const char *vendor = (codec->flags & CODEC_FLAG_BITEXACT) ? "Lavf" : LIBAVFORMAT_IDENT;
if (!s->pb->seekable || !mp3->write_xing)
return 0;
for (i = 0; i < FF_ARRAY_ELEMS(avpriv_mpa_freq_tab); i++) {
const uint16_t base_freq = avpriv_mpa_freq_tab[i];
if (codec->sample_rate == base_freq) ver = 0x3; // MPEG 1
else if (codec->sample_rate == base_freq / 2) ver = 0x2; // MPEG 2
else if (codec->sample_rate == base_freq / 4) ver = 0x0; // MPEG 2.5
else continue;
srate_idx = i;
break;
}
if (i == FF_ARRAY_ELEMS(avpriv_mpa_freq_tab)) {
av_log(s, AV_LOG_WARNING, "Unsupported sample rate, not writing Xing header.\n");
return -1;
}
switch (codec->channels) {
case 1: channels = MPA_MONO; break;
case 2: channels = MPA_STEREO; break;
default: av_log(s, AV_LOG_WARNING, "Unsupported number of channels, "
"not writing Xing header.\n");
return -1;
}
/* dummy MPEG audio header */
header = 0xffU << 24; // sync
header |= (0x7 << 5 | ver << 3 | 0x1 << 1 | 0x1) << 16; // sync/audio-version/layer 3/no crc*/
header |= (srate_idx << 2) << 8;
header |= channels << 6;
for (bitrate_idx = 1; bitrate_idx < 15; bitrate_idx++) {
int bit_rate = 1000 * avpriv_mpa_bitrate_tab[lsf][3 - 1][bitrate_idx];
</quote>
^- ends at line #164 .
I checked out the GIT HEAD and did a diff of this file between n2.2.3 and HEAD - no differences are shown.
I'd really just like to understand what the INTENT of this lsf variable is here -
should it be initialized to 0 or to mpah.lsf ?
Thanks & Regards,
Jason
On Thursday 19 June 2014 19:20:50 Michael Niedermayer wrote:
> On Thu, Jun 19, 2014 at 02:51:33PM +0100, Jason Vas Dias wrote:
> > Good day -
> > I am building ffmpeg, and fixing this compiler warning :
> >
> > libavformat/mp3enc.c:164: warning: 'lsf' may be used uninitialized in this
> > function
> >
> > The line in question and variable declaration is :
> >
> > line 127: int bytes_needed, lsf;
>
> line 127 is
> int bytes_needed;
>
> you are looking at a old version i assume
>
> [...]
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