ContGen-IntClk - DoneCallback

[hn_sofec]

I am following this example code, ContGen-IntClk.c.

#include <NIDAQmx.h>
#include <stdio.h>
#include <math.h>

#define     DAQmxErrChk (functionCall)      if( DAQmxFailed(error = (functionCall)) ) goto Error; else
#define     PI                              3.1415926535       
#define     DAQmxFailed(error)              ((error)<0)  

int32 CVICALLBACK DoneCallback (TaskHandle taskHandle, int32 status, void *callbackData);

int main(void)
{
    int32           error               = 0;
    TaskHandle      taskHandle          = 0;
    float64         data        [1000];
    char            errBuff     [2048]  = {'\0'};
    int             i                   = 0;

    for (; i < 1000; i++)       data [i] = 9.95 * sin( (double) i * 2.0 * PI / 1000.0);

    /*********************************************/
    // DAQmx Configure Code
    /*********************************************/
    DAQmxErrChk (DAQmxCreateTask            ("",&taskHandle)                                                    );
    DAQmxErrChk (DAQmxCreateAOVoltageChan   (taskHandle,"Dev1/ao0","",-10.0,10.0,DAQmx_Val_Volts,NULL)          );
    DAQmxErrChk (DAQmxCfgSampClkTiming      (taskHandle,"",1000.0,DAQmx_Val_Rising,DAQmx_Val_ContSamps,1000)    );
    DAQmxErrChk (DAQmxRegisterDoneEvent     (taskHandle,0,DoneCallback,NULL)                                    );

    /*********************************************/
    // DAQmx Write Code
    /*********************************************/
    DAQmxErrChk (DAQmxWriteAnalogF64        (taskHandle,1000,0,10.0,DAQmx_Val_GroupByChannel,data,NULL,NULL)    );

    /*********************************************/
    // DAQmx Start Code
    /*********************************************/
    DAQmxErrChk (DAQmxStartTask(taskHandle));

    printf      ("Generating voltage continuously. Press Enter to interrupt\n");
    getchar     ();

    Error:

    if ( DAQmxFailed (error) )
    DAQmxGetExtendedErrorInfo (errBuff,2048);

    if ( taskHandle != 0 )
    {
        /*********************************************/
        // DAQmx Stop Code
        /*********************************************/
        DAQmxStopTask   (taskHandle);
        DAQmxClearTask  (taskHandle);
    }

    if ( DAQmxFailed(error) )
    printf  ("DAQmx Error: %s\n",errBuff);

    printf  ("End of program, press Enter key to quit\n");
    getchar ();

    return 0;
}

int32 CVICALLBACK DoneCallback (TaskHandle taskHandle, int32 status, void *callbackData)
{
    int32   error           = 0;
    char    errBuff [2048]  = {'\0'};

    // Check to see if an error stopped the task.
    DAQmxErrChk (status);

    Error:
    if( DAQmxFailed (error) ) 
    {
        DAQmxGetExtendedErrorInfo   (errBuff,2048);
        DAQmxClearTask              (taskHandle);
        printf                      ("DAQmx Error: %s\n", errBuff);
    }

    return 0;
}

I would like to ask what generates the call to DoneCallback() function.  Thanks.

[Wolfgang · Accepted solution]

That's easy to find out yourself

Search the code for this function name and you will see that this callback is due to DAQmxRegisterDoneEvent, so this callback function will be executed when the acquisition task stops.

[hn_sofec]

how did I miss that?

[hn_sofec]

by the way, I do not see the arguments passed into this callback function.  how does it get its arguments then?

[Wolfgang]

I already provided the link to the help of this function... : The last argument of DAQmxRegisterDoneEvent () is the callback data, in this case the NULL pointer

[hn_sofec]

Sorry for the confusion, DAQmxRegisterDoneEvent() calls DoneCallback(). However, as you can see:

DAQmxErrChk (DAQmxRegisterDoneEvent     (taskHandle,0,DoneCallback,NULL)     );

DoneCallback() does not come with any argument or parentheses.  So I wonder where they went.

[Wolfgang]

ah, I see. This is because the type of callback function (int32 CVICALLBACK Callback (TaskHandle taskHandle, int32 status, void *callbackData)) is well defined, so only a pointer to the function is needed.

[hn_sofec]

I do not quite get how that works.  do you not have to have all arguments when calling a function?  otherwise, where does it get all the arguments from, when called?

[Wolfgang]

Yes, but all the arguments are already available...

So: DAQmxRegisterDoneEvent will internally call the callback function DoneCallback with all its arguments, i.e., it will call

DoneCallback (taskHandle, status, callbackData); those arguments are known to DAQmxRegisterDoneEvent because, for example, the taskhandle is the first parameter to DAQmxRegisterDoneEvent...


[hn_sofec]

so the declaration of DoneCallback() also specifies its arguments?