You are here: NI Home > Support > Product Reference > Manuals > LabVIEW 8.2 Help

No matter how confident you are in the VI you create, you cannot predict every problem a user can encounter. Without a mechanism to check for errors, you know only that the VI does not work properly. Error checking tells you why and where errors occur.

When you perform any kind of input and output (I/O), consider the possibility that errors might occur. Almost all I/O functions return error information. Include error checking in VIs, especially for I/O operations (file, serial, instrumentation, data acquisition, and communication), and provide a mechanism to handle errors appropriately.

By default, LabVIEW automatically handles any error when a VI runs by suspending execution, highlighting the subVI or function where the error occurred, and displaying an error dialog box.

To disable automatic error handling for the current VI, select File»VI Properties and select Execution from the Category pull-down menu. To disable automatic error handling for any new, blank VIs you create, select Tools»Options and select Block Diagram from the Category list. To disable automatic error handling for a subVI or function within a VI, wire its error out parameter to the error in parameter of another subVI or function or to an error out indicator.

You can choose other error handling methods. For example, if an I/O VI on the block diagram times out, you might not want the entire application to stop and display an error dialog box. You also might want the VI to retry for a certain period of time. In LabVIEW, you can make these error handling decisions on the block diagram of the VI.

Use the LabVIEW error handling VIs and functions on the Dialog & User Interface palette and the error in and error out parameters of most VIs and functions to manage errors. For example, if LabVIEW encounters an error, you can display the error message in different kinds of dialog boxes. Use error handling in conjunction with the debugging tools to find and manage errors.

VIs and functions return errors in one of two ways—with numeric error codes or with an error cluster. Typically, functions use numeric error codes, and VIs use an error cluster, usually with error inputs and outputs.

Error handling in LabVIEW follows the dataflow model. Just as data values flow through a VI, so can error information. Wire the error information from the beginning of the VI to the end. Include an error handler VI at the end of the VI to determine if the VI ran without errors. Use the error in and error out clusters in each VI you use or build to pass the error information through the VI. The error clusters are flow-through parameters.

As the VI runs, LabVIEW tests for errors at each execution node. If LabVIEW does not find any errors, the node executes normally. If LabVIEW detects an error, the node passes the error to the next node without executing that part of the code. The next node does the same thing, and so on. At the end of the execution flow, LabVIEW reports the error.

Error Clusters

The error in and error out clusters include the following components of information:

• status is a Boolean value that reports TRUE if an error occurred.
• code is a 32-bit signed integer that identifies the error numerically. A nonzero error code coupled with a status of FALSE signals a warning rather than a error.
• source is a string that identifies where the error occurred.

Some VIs, functions, and structures that accept Boolean data also recognize an error cluster. For example, you can wire an error cluster to the Boolean inputs of the Select, Quit LabVIEW, or Stop functions. If an error occurs, the error cluster passes a TRUE value to the function.

Using While Loops for Error Handling

You can wire an error cluster to the conditional terminal of a While Loop to stop the iteration of the While Loop. When you wire the error cluster to the conditional terminal, only the TRUE or FALSE value of the status parameter of the error cluster is passed to the terminal. When an error occurs, the While Loop stops.

When an error cluster is wired to the conditional terminal, the shortcut menu items Stop if True and Continue if True change to Stop on Error and Continue while Error.

Using Case Structures for Error Handling

When you wire an error cluster to the selector terminal of a Case structure, the case selector label displays two cases—Error and No Error—and the border of the Case structure changes color—red for Error and green for No Error. If an error occurs, the Case structure executes the Error subdiagram.

Use the SubVI with Error Handling template VI to create a VI with a Case structure for error handling.