Buffer problem when using mex file created with Math Interface Toolkit

[CDancer]

I have a vi which does simultaneous clocked AO and digital pattern output on a PCI-6221, with finite samples. It works perfectly in Labview.

I have created a mex file from it using math interface toolkit. This also works perfectly in Matlab, but only once! The second time I try to run the function, I get the error -200288 Attempted to write a sample beyond the final sample generated.

My vi is tidy - the sequence goes:

[first set up the timing and write the data to the buffers, then...]

DAQmx Start Task

DAQmx Wait Until Done

DAQmx Stop Task (not sure if strictly necessary, but can't do any harm)

DAQmx Clear task

So, if I am ending with a Clear Task,  why is at least one of the buffers and the pointer into the buffer being retained in Matlab between calls to the function?

Chris

[CDancer]

More information...

If I try to call the function a 3rd time in Matlab, Matlab hangs.

I tried setting the DAQmx Write properties Relative to and Offset to "First sample" and zero respectively, before the data is written to the buffers. I really thought this would fix the problem because there is no way it should be trying to write data after the end of the buffer if I have specifically told it to write at the start. It doesn't make any difference, I still get the same error.

I noticed that after I have used the function once in Matlab, I cannot run the corresponding vi in Labview because the hardware resource (ie. the PCI-6221) is in use by another program. I think this is a clue to what's going on - Matlab is not releasing the DAQ card after the function executes.

[CDancer]

OK, found the cause of the problem - I had not cleared the digital output task.