Thanks for the response Filipe, I'll let you know how it goes. I only went into the header file because example code provided by NI ... paste ...
iStatus = GPCTR_Read_Buffer(iDevice, ulGpctrNum, ND_READ_MARK, ulReadOffset, ulNumPtsToRead, ulTimeOut, &ulNumPtsRead, pulReadBuf);
showed use of 8 parameters - in agreement with the header file and in disagreement with the on-line help file function documentation.
Tom