viQueryf returning a NULL

[GoBlue]

I created an IVI Spectrum Analyzer driver using the Instrument Driver Development Wizard.  When it uses the viQueryf function, I get a NULL value returned. 

For example:

static ViStatus _VI_FUNC hp856xEAttrMarkerAmplitude_ReadCallback (ViSession vi,
                                                                            ViSession io,
                                                                            ViConstString channelName,
                                                                            ViAttr attributeId,
                                                                            ViReal64 *value)
{                
    ViStatus    error = VI_SUCCESS;
    ViChar      rdBuffer[BUFFER_SIZE];
    ViUInt32    retCnt;
   

    viCheckErr( viPrintf (io, "MKA?"));
    viCheckErr( viRead (io, rdBuffer, BUFFER_SIZE-1, &retCnt));  // This works!!
    
    viCheckErr (viQueryf (io, "MKA?", "%lf", &value));  //  This doesn't 
    
   
Error:
    return error;
}

If I use the NI-488.2 Communicator, the query function works fine.  I have LabWindows 7.1.1 and NI-Visa 3.3.1.  Does anyone have any ideas why the viQueryf function might not be working?

Thanks for your help!

[Mert_A.]

Hello,

Looks like you should be passing value into hp856xEAttrMarkerAmplitude_ReadCallback instead of &value.  Your read callback function takes a ViReal64* (value), but you pass the address of that (a ViReal64**) to viQueryf.  Instead of writing into value, it writes over value (not good, since it's writing 8 bytes to a 4-byte slot), making the number at value completely meaningless.

Hope this helps.

Mert A.
National Instruments

[GoBlue]

Thanks Mert!!  I trusted the wizard too much, and didn't look at what was really happening.  It was a long day, yeah that's it...

Thanks again.