National Instruments 320685D-01 Error Checking

Chapter 9 Checking for Errors in LabWindows/CVI

LabWindows/CVI Programmer Reference Manual 9-2

National Instruments Corporation

Note Status codes are integer values. These values are either common to an entire

library of functions, or specific to one function. Libraries that have a common set

of codes have a listing at the end of the chapter or manual they appear in. You can

find the error message for each integer value there. In addition, each of these

libraries contains a function you can call to translate the integer value to an error

string. When an error code is specific to a function, you can find a description for

it in the function description in the LabWindows/CVI manual set. The error

description also appears in the online help of the library function panels in

LabWindows/CVI.

Error Checking

LabWindows/CVI functions return status codes in one of two ways— either by a function

return value, or by updating a global variable. In some cases, LabWindows/CVI uses both of

these methods. In either case, it is a good idea to monitor these values so that you can detect

an error and take appropriate action. A common technique for error checking is to monitor the

status of functions, and when a function reports an error, pause the program and report the

error to the user through a pop-up message. For example, LoadPanel returns a positive

integer when it successfully loads a user interface panel into memory. However, if a problem

occurs, the return value is negative. The following example shows an error message handler

for LoadPanel.

panelHandle = LoadPanel (0, "main.uir", PANEL);

if (panelHandle < 0) {

ErrorCheck ("Error Loading Main Panel", panelHandle,

GetUILErrorString (panelHandle));

}

When a function reports status through a separate function, as in the RS-232Library, check

for errors in a similar way. In this case, the status function returns a negative value when the

original function fails.

bytesRead = ComRd (1, buffer, 10);

if (ReturnRS232Error() < 0) {

ErrorCheck ("Error Reading From ComPort #1", ReturnRS232Error(),

GetRS232ErrorString(ReturnRS232Error()));

}

Notice that the above function also returns the number of bytes read from the serial port. You

can compare the number of bytes read to the number you request, and if a discrepancy exists,

take the appropriate action. Notice that the error codes differ between the RS-232 Library and

the User Interface Library. A section describing how each LabWindows/CVI library reports

errors follows this section.