[Libav-user] Negative PTS when it is closer to wrap around at MPEG TS

[Leandro Moreira]

Hi there,

I'm using libav and I noticed that suddenly my video files (.ts) became to
show negative (start) PTS. I thought it was something wrong in my code but
then I tried to use ffprobe to see its contents:

# I ran this command for the files in sequence, therefore the start pts
from next is supposed to be the previous + duration * 90khz

$ ffprobe -loglevel panic -select_streams v -show_entries
"stream=start_pts,start_time,duration" <file.ts>

# 197562612960.ts
start_pts=8584117906
start_time=95379.087844
duration=3.999333

# 197562972960.ts
start_pts=8584477906 (it shows right! previous start_pts + 4 * 90000)
start_time=95383.087844
duration=3.999333

# 197563332960.ts
start_pts=-5096686 (it shows wrong! previous start_pts + 4 * 90000 =
8584837906 which is not bigger than 2^33-1 and even if it were it should
wrap around)
start_time=-56.629844
duration=3.999333

While I was trying to find what causes it, I found a function called
wrap_timestamp located at utils.c line 106, it seems that the value
-5096686 is obtained when you do the following calculation:  previous pts -
2^33.
(the line  return timestamp - (1ULL << st->pts_wrap_bits);)

Anyway, am I missing about the way PTS wraparound works? (for MPEG TS) or
is it a bug on this function?

PS: From all the players I tried, they played the video as if it didn't
have any error.

```/**
 * Wrap a given time stamp, if there is an indication for an overflow
 *
 * @param st stream
 * @param timestamp the time stamp to wrap
 * @return resulting time stamp
 */
static int64_t wrap_timestamp(const AVStream *st, int64_t timestamp)
{
    if (st->pts_wrap_behavior != AV_PTS_WRAP_IGNORE &&
        st->pts_wrap_reference != AV_NOPTS_VALUE && timestamp !=
AV_NOPTS_VALUE) {
        if (st->pts_wrap_behavior == AV_PTS_WRAP_ADD_OFFSET &&
            timestamp < st->pts_wrap_reference)
            return timestamp + (1ULL << st->pts_wrap_bits);
        else if (st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET &&
            timestamp >= st->pts_wrap_reference)
            return timestamp - (1ULL << st->pts_wrap_bits);
    }
    return timestamp;
}```

`utils.c` l:106
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