Debugging Techniques
Execution Highlighting
View an animation of the execution of the block diagram by clicking the Highlight Execution button, shown as follows.
Execution highlighting shows the movement of data on the block diagram from one node to another using bubbles that move along the wires. Use execution highlighting in conjunction with single-stepping to see how data values move from node to node through a VI.
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Note Execution highlighting greatly reduces the speed at which the VI runs. |
If the error out cluster reports an error, the error value appears next to error out with a red border. If no error occurs, OK appears next to error out with a green border.
Single-Stepping
Single-step through a VI to view each action of the VI on the block diagram as the VI runs. The single-stepping buttons, shown as follows, affect execution only in a VI or subVI in single-step mode.
Enter single-step mode by clicking the Step Over or Step Into button on the block diagram toolbar. Move the cursor over the Step Over, Step Into, or Step Out button to view a tip strip that describes the next step if you click that button. You can single-step through subVIs or run them normally.
If you single-step through a VI with execution highlighting on, an execution glyph, shown as follows, appears on the icons of the subVIs that are currently running.
Probe Tool
Use the Probe tool, shown as follows, to check intermediate values on a wire as a VI runs.
Use the Probe tool if you have a complicated block diagram with a series of operations, any one of which might return incorrect data. Use the Probe tool with execution highlighting, single-stepping, and breakpoints to determine if and where data is incorrect. If data is available, the probe immediately updates during single-stepping or when you pause at a breakpoint. When execution pauses at a node because of single-stepping or a breakpoint, you also can probe the wire that just executed to see the value that flowed through that wire.
Types of Probes
You can check intermediate values on a wire when a VI runs by using a generic probe, by using an indicator on the Controls palette to view the data, by using a supplied probe, by using a customized supplied probe, or by creating a new probe.
Generic
Use the generic probe to view the data that passes through a wire. Right-click a wire and select Custom Probe»Generic Probe from the shortcut menu to use the generic probe.
The generic probe displays the data. You cannot configure the generic probe to respond to the data.
LabVIEW displays the generic probe when you right-click a wire and select Probe, unless you already specified a custom or supplied probe for the data type.
You can debug a custom probe similar to a VI. However, a probe cannot probe its own block diagram, nor the block diagram of any of its subVIs. When debugging probes, use the generic probe.
Using Indicators to View Data
You also can use an indicator to view the data that passes through a wire. For example, if you view numeric data, you can use a chart within the probe to view the data. Right-click a wire, select Custom Probe»Controls from the shortcut menu, and select the indicator you want to use. You also can click the Select a Control icon on the Controls palette and select any custom control or type definition saved on the computer or in a shared directory on a server. LabVIEW treats type definitions as custom controls when you use them to view probed data.
If the data type of the indicator you select does not match the data type of the wire you right-clicked, LabVIEW does not place the indicator on the wire.
Supplied
Supplied probes are VIs that display comprehensive information about the data that passes through a wire. For example, the VI Refnum Probe returns information about the VI name, the VI path, and the hex value of the reference. You also can use a supplied probe to respond based on the data that flows through the wire. For example, use an Error probe on an error cluster to receive the status, code, source, and description of the error and specify if you want to set a conditional breakpoint if an error or warning occurs.
The supplied probes appear at the top of the Custom Probe shortcut menu. Right-click a wire and select Custom Probe from the shortcut menu to select a supplied probe. Only probes that match the data type of the wire you right-click appear on the shortcut menu.
Refer to the Using Supplied Probes VI in the labview\examples\general\probes.llb for an example of using supplied probes.
Custom
Use the Custom Probe Wizard to create a probe based on an existing probe or to create a new probe. Right-click a wire and select Custom Probe»New from the shortcut menu to display the Custom Probe Wizard. Create a probe when you want to have more control over how LabVIEW probes the data that flows through a wire. When you create a new probe, the data type of the probe matches the data type of the wire you right-clicked. If you want to edit the probe you created, you must open it from the directory where you saved it.
After you select a probe from the Custom Probe shortcut menu, navigate to it using the Select a Control palette option, or create a new probe using the Custom Probe Wizard, that probe becomes the default probe for that data type, and LabVIEW loads that probe when you right-click a wire and select Probe from the shortcut menu. LabVIEW only loads probes that exactly match the data type of the wire you right-click. That is, a double precision floating-point numeric probe cannot probe a 32-bit unsigned integer wire even though LabVIEW can convert the data.
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Note If you want a custom probe to be the default probe for a particular data type, save the probe in the user.lib\_probes\default directory. Do not save probes in the vi.lib\_probes directory because LabVIEW overwrites those files when you upgrade or reinstall. |
Breakpoints
Use the Breakpoint tool, shown as follows, to place a breakpoint on a VI, node, or wire on the block diagram and pause execution at that location.
When you set a breakpoint on a wire, execution pauses after data passes through the wire. Place a breakpoint on the block diagram to pause execution after all nodes on the block diagram execute.
When a VI pauses at a breakpoint, LabVIEW brings the block diagram to the front and uses a marquee to highlight the node or wire that contains the breakpoint. When you move the cursor over an existing breakpoint, the black area of the Breakpoint tool cursor appears white.
When you reach a breakpoint during execution, the VI pauses and the Pause button appears red. You can take the following actions:
- Single-step through execution using the single-stepping buttons.
- Probe wires to check intermediate values.
- Change values of front panel controls.
- Click the Pause button to continue running to the next breakpoint or until the VI finishes running.
LabVIEW saves breakpoints with a VI, but they are active only when you run the VI. You can view all breakpoints by selecting Operate»Breakpoints and clicking the Find button.
You can remove breakpoints individually or throughout the entire VI hierarchy.
Suspending Execution
Suspend execution of a subVI to edit values of controls and indicators, to control the number of times the subVI runs before returning to the caller, or to go back to the beginning of the execution of the subVI. You can cause all calls to a subVI to start with execution suspended, or you can suspend a specific call to a subVI.
To suspend all calls to a subVI, open the subVI and select Operate» Suspend when Called. The subVI automatically suspends when another VI calls it. If you select this menu item when single-stepping, the subVI does not suspend immediately. The subVI suspends when it is called.
To suspend a specific subVI call, right-click the subVI node on the block diagram and select SubVI Node Setup from the shortcut menu. Place a checkmark in the Suspend when called checkbox to suspend execution only at that instance of the subVI.
The VI Hierarchy window, which you display by selecting View»VI Hierarchy, indicates whether a VI is paused or suspended. An arrow glyph, shown as follows, indicates a VI that is running regularly or single-stepping.
A pause glyph, shown as follows, indicates a paused or suspended VI.
A green pause glyph, or a hollow glyph in black and white, indicates a VI that pauses when called. A red pause glyph, or a solid glyph in black and white, indicates a VI that is currently paused. An exclamation point glyph, shown as follows, indicates that the subVI is suspended.
A VI can be suspended and paused at the same time.
Determining the Current Instance of a SubVI
When you pause a subVI, the Call list pull-down menu on the toolbar lists the chain of callers from the top-level VI down to the subVI. This list is not the same list you see when you select Browse»This VI's Callers, which lists all calling VIs regardless of whether they are currently running. Use the Call list menu to determine the current instance of the subVI if the block diagram contains more than one instance. When you select a VI from the Call list menu, its block diagram opens, and LabVIEW highlights the current instance of the subVI.
Commenting Out Sections of Block Diagrams
You can run a VI with a section of the block diagram disabled, similar to commenting out a section of code in a text-based programming language. Disable a section of the block diagram to determine if the VI performs better without it. Place the section you want to disable inside a Diagram Disable structure.
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