[FFmpeg-devel] [PATCH 1/3] avformat/utils: function to get the formatted ntp time

Dixit, Vishwanath vdixit at akamai.com
Sun May 6 21:04:35 EEST 2018



On 4/28/18 6:38 AM, Michael Niedermayer wrote:
> On Fri, Apr 27, 2018 at 08:00:23AM +0000, Dixit, Vishwanath wrote:
>>
>>
>> On 4/27/18 5:15 AM, Michael Niedermayer wrote:
>>> On Thu, Apr 26, 2018 at 11:05:59AM +0000, Dixit, Vishwanath wrote:
>>>>
>>>>
>>>> On 4/26/18 1:04 AM, Michael Niedermayer wrote:
>>>>> On Tue, Apr 24, 2018 at 02:46:56PM +0530, vdixit at akamai.com wrote:
>>>>>> From: Vishwanath Dixit <vdixit at akamai.com>
>>>>>>
>>>>>> This utility function creates 64-bit NTP time format as per the RFC
>>>>>> 5905.
>>>>>> A simple explaination of 64-bit NTP time format is here
>>>>>> http://www.beaglesoft.com/Manual/page53.htm
>>>>>> ---
>>>>>>  libavformat/internal.h |  8 ++++++++
>>>>>>  libavformat/utils.c    | 20 ++++++++++++++++++++
>>>>>>  2 files changed, 28 insertions(+)
>>>>>>
>>>>>> diff --git a/libavformat/internal.h b/libavformat/internal.h
>>>>>> index 3582682..e9f758f 100644
>>>>>> --- a/libavformat/internal.h
>>>>>> +++ b/libavformat/internal.h
>>>>>> @@ -240,6 +240,14 @@ void ff_read_frame_flush(AVFormatContext *s);
>>>>>>  uint64_t ff_ntp_time(void);
>>>>>>  
>>>>>>  /**
>>>>>> + * Get the NTP time stamp formatted as per the RFC-5905.
>>>>>> + *
>>>>>> + * @param ntp_time NTP time in micro seconds (since NTP epoch)
>>>>>> + * @return the formatted NTP time stamp
>>>>>> + */
>>>>>> +uint64_t ff_time_ntp_format(uint64_t ntp_time);
>>>>>> +
>>>>>> +/**
>>>>>>   * Append the media-specific SDP fragment for the media stream c
>>>>>>   * to the buffer buff.
>>>>>>   *
>>>>>> diff --git a/libavformat/utils.c b/libavformat/utils.c
>>>>>> index c25eab4..b59d426 100644
>>>>>> --- a/libavformat/utils.c
>>>>>> +++ b/libavformat/utils.c
>>>>>> @@ -4637,6 +4637,26 @@ uint64_t ff_ntp_time(void)
>>>>>>      return (av_gettime() / 1000) * 1000 + NTP_OFFSET_US;
>>>>>>  }
>>>>>>  
>>>>>> +uint64_t ff_time_ntp_format(uint64_t ntp_time)
>>>>>> +{
>>>>>> +    uint64_t ntp_ts, frac_part;
>>>>>> +    uint32_t sec, usec;
>>>>>> +
>>>>>> +    //current ntp time in seconds and micro seconds
>>>>>
>>>>>> +    sec = ntp_time / 1000000;
>>>>>
>>>>> This can be truncated
>>>> Yes, but the truncated part is not getting discarded, as the following line is taking care of that. Please correct me if I have not understood what you have intended to say here.
>>>
>>> ok, correcting you then :)
>>> sec is 32bit
>>> not all values of "ntp_time / 1000000;" fit in 32bit
>>> there is no check for this it just produces a wrong result
>> Yes, you are right. But, I can use only the LS 32bits of the result in the following code. It is a limitation of 64-bit NTP time format and the issue is going to occur in 2036. Not sure how to handle it here. At most, I can print a warning message when the result gets truncated. Could you please suggest any alternative option?
>
> i cant make it work after 2036 as that seems what the design is limited at.
> But the error can be detected and handled
Yes, the error can be detected and one possible way I can think of handling that error, is to truncate LS bits instead of MS bits. But, this solution also will not be completely error free. Honestly, I am not able to find any useful ways of handling the error, apart from displaying a warning message. Request you to let me know if you have any solution in mind.
>
> [...]
>
>
>
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