tag offset question

[jmfriedt]

I am trying to get an accurate timing of a given sample (1-PPS
transition). To do so, a marker is injected in the AD9831 datastream in
the B210 FPGA. When this tag is detected in the UHD source block, a tag
is created in the stream:

    for (item=0;item<noutput_items;item++)  // check all samples
       {if (ch==0)                          // check if channel 0 
          {if (in[item]==_tag_pps_int)      // check marker detection
           {add_item_tag(0,                 // Port number 
            nitems_written(0) + item,       // Offset
            pmt::string_to_symbol("1PPS"),  // Key
            pmt::from_double(3.14159));     // Value 
 ...

Displaying the item number when the marker is detected, I get things
like 444331/524288 or 364347/524288 (item/noutput_items) which seems
realistic.

Downstream the flowchart, after many other processing blocks, I check
for the tag in the window of samples that were just transferred
  std::vector<gr::tag_t> PPS_tag;
  get_tags_in_window(PPS_tag,0,
         0,             // abs_start,
         noutput_items, // uint64_t abs_end);
         pmt::string_to_symbol("1PPS")) ;
  if (PPS_tag.size() != 0)
     { printf("\n** PPS_tag received: %ld (%ld/%d):
          message=%.9f",PPS_tag.size(), noutput_items,
          to_double(PPS_tag[0].value));
...
and there I *always get* PPS_tag.offset=1.

If on the emitter side I create two tags separated by 42 i.e.
add_item_tag(0,                
            nitems_written(0) + item,       
            pmt::string_to_symbol("1PPS"),  
            pmt::from_double(3.14159)); 
add_item_tag(0,                
            nitems_written(0) + item + 42,    
            pmt::string_to_symbol("1PPS"), 
            pmt::from_double(3.14159)); 
then the receiver indeed detects PPS_tag.size() = 2 but still
both PPS_tag[0].offset and PPS_tag[1].offset are equal to 1 !
I am obviously misunderstanding the behviour of tag.offset: can anyone
enlighten me on how to identify the index of the sample carrying the
tag ?

Thanks, JM