[Boost-users] [chrono] Initializing system_clock from microseconds

Subject: [Boost-users] [chrono] Initializing system_clock from microseconds
From: Kelly, Dan (Dan.Kelly_at_[hidden])
Date: 2013-06-11 08:22:07

I have been having some difficulty initializing a chrono::system_clock::time_point object.  Specifically,
I am trying to load a time stamp from a .pcap file using libpcap.  I have a structure, pcap_pkthdr,
which contains two fields:

struct timeval {
        long    tv_sec;         /* seconds */
        long    tv_usec;        /* and microseconds */
};

struct pcap_pkthdr {
       struct timeval ts;   /* time stamp */
       bpf_u_int32 caplen;  /* length of portion present */
       bpf_u_int32 len;     /* length this packet (off wire) */
};

Where tv_sec and tv_usec are seconds and microseconds respectively relative to an epoch
of Jan 1, 1970.  I can use system_clock::from_time_t() to initialize the time_point to an 64-bit integer
seconds (std:time_t).  The question is, how do I add the microseconds to this value?  My latest attempt
is:

void my_class::calculate_packet_statistics( const struct pcap_pkthdr *header ) {
   frame_interval_.intervalTimeStamp_ = system_clock::from_time_t( static_cast<time_t>( header->ts.tv_sec ) );
   frame_interval_.intervalTimeStamp_ += microseconds( header->ts.tv_usec ); // This is the problematic line
...
}

However, this appears to simply add tv_usec to the internal representation of the time_point.  The internal
representation appears to be in 100ns ticks with an epoch other than Jan 1, 1970. This is on a windows x64
build but the same code will have to work on Linux as well.

As a side note, I later do some date calculations using system_clock::to_time_t.  I lose my fractional seconds here
But since I am only working with dates, this is not a problem.

Dan Kelly
dan.kelly_at_[hidden]

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