Read acquisition start/end timestamp

[TSSkyhits]

When start/stop the measurement, a timestamp is set:
void MdfWriter::StartMeasurement(uint64_t start_time) {
write_state_ = WriteState::StartMeas;
start_time_ = start_time;
stop_time_ = 0; // Zero indicate not stopped
...
}
But when creating a reader of the mf4 data file, I can not find a API to get the start_time/stop_time directly. Is there a solution to read all these information?

[ihedvall]

@TSSkyhits
The problem of storing the start and stop time is a known problem area of the ASAM MDF. The absolute start time is stored in the header block and the time in each DG (measurement) block is relative to that time. There is no standard way to store the absolute stop time. This causes problems for external readers to get the stop time or measurement length as it have to be calculated.

The StartMeasurement(start time) function updates the header start time at the first measurement. Note that the relative sample time can be negative (see pre-trig time setting). But the MDF file support storage of more than one measurement i.e. more than one DG block, so what is the start time of the second measurement? Was the measurement done in parallel or sequential?

There are several approaches to solve the above problem.

1. Adding events (EV blocks) for the start and stop time but this quite messy to deal with.
2. Adding an extra CG group with a start and stop sample that including the absolute times. This is good as you also can store other interesting values as test result and initial set-point values.
3. Similar to [2] but the information can be stored in a MD block attached to the DG block after stopping the measurement.

The last solution is the simplest solution, at least for this MDF library but some other readers may not have the same MD comment (XML) support as this library. I have added some non-tested code below:

InitMeasurement();
StartMeasurement(start_time);
// Do the measurement
StopMeasurement(stop_time);

DgComment dg_comment;
dg_comment.Comment(measurement description);

MdProperty start;
start.Name("StartTime");
start.DataType(MdDataType::MdDateTime);
start.Value(start_time); // Convert ns since 1970 to a ISO date time string
dg_comment.AddProperty(start);

MdProperty stop;
stop.Name("StopTime");
stop.DataType(MdDataType::MdDateTime);
stop.Value(stop_time);
dg_comment.AddProperty(stop);

// Add more key/value pairs about the measurement

data_group->SetDgComment(dg_comment);

FinalizeMeasurement()

[ihedvall]

@TSSkyhits
The start and stop times for a measurement are normally wanted by an external application. My idea is to add this functionality to the library. The start and stop times are recorded by the MdfWriter but currently not stored in the file. As the MDF file may include many measurements, adding this information to a DG block is possible.

By adding the functionality to the FinalizeMeasurement() function. Basically it implements the above (previous message) function as the DG MD block comment rarely is used. Two new function that return the start and stop time for a DG block. These functions tries to find these times by analyzing the file. Plan A is to get the times from the DG MD block but if this fails, try the most common ways to find them as the above list.

Should this be an acceptable solution?