Dropping-In on the Testbed

Last time out we started exploring one common application of so-called “drop-in” VI. The technique is based on the idea of creating VIs that are capable of performing something useful for the VI that is hosting it, but without interacting directly with that VI’s basic logic. The example we considered was manipulating the font and type size used to present textual data.

At the close of that post we has created a basic object-oriented structure that could manipulate the label or caption of any front panel control or indicator. I want to finish this discussion by looking at how to expand that basic implementation to allow it to set the text properties of text contained inside a control or indicator. For that we will return to our testbed application.

A Brief Recap

It has been a while since we have worked with this code, so a brief refresher on what it does is probably in order. The testbed application we will be modifying consists of several processes that run independently of one another. To begin with, there is a background process that oversees the reporting of errors that occur. Handling the user interface duties, a GUI process incorporates a subpanel that can display the front panels of several simulated acquisition and process-control VIs. The whole thing is kicked off by a launcher VI that loads the various processes into memory and starts them executing.

Our goal here will be to add the drop-in VI we created last time to all the user-facing VIs and add classes as necessary to allow it to handle the controls and indicators on those VIs. However, if you don’t already have a tool for editing database contents directly, you should first download a tool called Database .NET (the link is to a zip file, and is at the bottom of the page). The program is a simple utility that lets you examine and edit database data from a number of different DBMS. I don’t know the folks that wrote this, and have no vested interest in the program other than I have used it for years and found it very useful. Note that this program has no installer so it has a very small footprint – it will even run from a USB stick. To “install” the program, simply create a directory for it on your computer and then drag into it the program that is inside the zip archive you downloaded, and installation is complete. The easiest way to invoke it is to set it as the default application for *.mdb files.

  • Note that if you decide to install this utility in a subdirectory of the Program Files (x86) directory, you may have to play around with the folder permissions a bit before it will run. Because the program generates several temporary files when it’s starting up, the user has to have Full Access to the folder in which it is installed.

One other caveat to bear in mind before we dive into the modifications is that, these operations cannot override limits on these properties that might exist for other reasons. For example, these techniques will not work on controls that you have defined as strict typedefs. The reason: The strict typedef defines everything about the control’s appearance and the property node will throw an error if you try to change them. Likewise, a System-themed control will let you change the font characteristics, but will complain if you try to change colors.

Making With the Modifications

So where do we start? Well the first hing we need to do is to make a couple minor tweaks to the Display Font Manager.vi. First, we need to define what happens to the drop-ins errors. Because it’s important to preserve them, we will save the errors that arise in the drop-in to the same location that errors from the testbed application proper are stored – but without bothering the program’s operator. To accomplish that task, let’s reuse a the subVI that the error handling logic uses to store error data.

Drop-in Error Handling

Note that I had to add a case structure because the location where this subVI was originally used only executed if there was an error. So unless we want to have spurious records being posted, we have to add that logic here.

Next, as the code is currently written, the error chain in the drop-in’s logic starts with the Error In control and terminates in the Error Out indicator. Although this arrangement works fine during development and testing, when the time comes to deploy the code, this is not what we want. As I said last time, drop-in VIs should not interact with the host VI and should not inject their own errors into the host’s error stream. Still, it can be useful to be able to use the drop-in’s error IO to establish data dependencies that control when it runs. The solution is for the drop-in to have error clusters, but not have them be connected internally.

Errors - Straight Through

Changing the Testbed

Now that we are to install the drop-in, we need to look for where to install it. Completing that examination of the code, we see that there are 5 VIs that are user-facing:

  1. The Launcher (testbed.vi)
  2. The Main GUI (Display Data.vi)
  3. The Temperature Controller (Temperature Controller.vi)
  4. Two “Acquisition” VIs (Acquire Ramp Data.vi and Acquire Sine Data.vi)

So the first thing I do is modify each of these VIs by dropping a copy of the drop-in VI on to their block diagram outside the outer-most loop. For example, this is what the modified launcher block diagram looks like:

textbed.vi with drop-in installed

As promised earlier, this is all the modification that the application will need – which means we are ready to start testing.

The First Test

“But wait a minute…” you protest. “…we haven’t configured anything yet. There’s nothing to test!”

Well you’re half right. We have not gone into the database and configured any controls to be modified, but we still have something to test. We still have to verify the drop-in’s default behavior, which by the way, is to do nothing. Yes, you read that right, we have to test that nothing happens. You see, a major aspect of the drop-in concepts is that drop-ins don’t do anything unless they are explicitly told to through their configuration. Right now we have installed the drop-in code, but there are no controls configured in the database so we need to make sure that the main application continues to run as it did before: no side-effects and no errors. In short, the drop-in right now should do nothing, and we need to make sure that it fulfills that requirement.

So launch the top-level VI (testbed.vi) or run the standalone executable. As before, the launcher will show the names of the processes it’s launching and when it finishes the main GUI will open. Again as before, you will be able to switch between screens using the popup menu and the plugins will operate just as they did before. Finally, if you look at the contents of the event table in the database, you will see that no errors have been generated.

It’s All About the Children

Now that we have “nothing” working, we need to finish implementing all the “somethings”. You will recall that when we ended last time I had created a basic implementation of the font manager functionality that could change the label or caption of any type of control. The tricky part, I said was going to be implementing the subclass, or children, methods that would modify the font of a configured control’s contents. So let’s look at those children.

The String and Digital Subclasses

I choose to start with these two because they are the easiest to understand, and are very much alike. Here’s the child method for handing strings…

String Subclass Method

…and the one for digital numerics…

Digital Subclass Method

In either subclass, the logic starts by calling the parent methods (which handles labels and captions) and then extracting from the parent’s class data the reference to the control that will be manipulated. At the same time that is going on, the Font Parameters data is unbundled and the Component to Set value controls what, if anything, happens next. If the selected component is Label or Caption a case is selected which does nothing but pass through the error cluster. If, however, the selected component is Contents the associated case casts the basic control reference from the parent class data into the control’s specific control class, and then sets the appropriate properties.

The Boolean and RingSubclasses

The next two I want to consider are, again, similar each other, but differ from the preceding pair in that they represent control classes that don’t have any readily discernible textual value. Booleans represent logical true and false conditions, while rings are technically numerics, but the number that is their value doesn’t appear anywhere. In this sort of situation, the idea is to look for text that is not the control’s value but is associated with that value. For example, Boolean controls in LabVIEW can have textual displays that state the control’s condition. These strings are called Boolean Text and are often used to label push buttons or lights…

Boolean Subclass Method

Likewise, the Ring control appears to the user as a pop-up menu, so we can use this code to set the text properties of the text that appears in the menu…

Ring Subclass Method

The WaveformChart Subclass

Finally, we need to take the idea of strings that are only associated with data one more step. What about complex controls that can have multiple strings associated with their values? Objects like charts are good examples of what I am talking about. Just to start, there is text associated with the axis tick marks, there is text that forms the axis labels, and there is text in the plot legends.

The most flexible approach would be to figure out how to uniquely identify each of these components, however we must be careful to not create an API that is so flexible that it is unusable. One solution would be to simply make all the text the same font and size – which is what they are anyway. A look that I prefer however is to have the tick mark labels slightly smaller than the axis labels. Here is one way to do that:

WaveformChart Subclass Method

As you can see, the code treats the two axes the same by combining references to them into an array and then passing that array into a loop that manipulates the display parameters. This logic makes the axis labels the size specified in the configuration, but does a bit of math to make the tick mark labels about 10% smaller. This difference might not seem like much, but it works. If this isn’t exactly what you want, that’s OK. The point here is not to present a canonical solution, but to present concepts and ideas that help you find your own way.

Adding Configurations

Now we are ready to add the font definitions to the database. I have created a total of 12 definitions covering 9 different controls and indicators and you can see them all by examining the SQL file in the _repos subdirectory in the project (starting at line 27). However, to give you a taste of what the SQL code for this functionality looks like, here is the SQL for the table holding the font configurations, and the font definition for the string indicator on the front panel of the launcher.

CREATE TABLE ctrl_font_definition (
    owner_name  TEXT(50) WITH COMPRESSION,
    ctrl_name   TEXT(50) WITH COMPRESSION,
    font_name   TEXT(20) WITH COMPRESSION,
    font_size   INTEGER,
    ctrl_comp   TEXT(20) WITH COMPRESSION

INSERT INTO ctrl_font_definition
  (owner_name, ctrl_name, font_name, font_size, ctrl_comp)
  ('testbed.vi', 'progress', 'Segoe UI', 24, 'Contents')

The goal of these initial definitions is to “turn-on” the functionality without changing too much. For example, the ‘Segoe UI’ font is the default font that LabVIEW uses on recent versions of the Windows platform. If you are running this code on the Macintosh or Linux (or an older version of Windows), the default font will be different. So on other platforms you may need to modify these definitions before you install them.

Once we have the definitions in the database, let’s try the testbed application again. You might not notice a lot of difference, that is sort of the point. This initial test is to reproduce the default values. One place where you will notice a difference is if you are running Windows and you have the display font scaling on your display set to the non-default value. The text size will now always be the same relative to the size of the window regardless of how the display setting changes.

From here I would recommend that you play around a bit and manually change the font and size of the various controls to see the effect.

Testbed Application – Release 16
Toolbox – Release 12
Testbed Installer – Release 16

Please note that I have included in this release a built version of the application so you can practice working with the database. The LocalDB.mdb file included with this installer has the table defined for holding the font definitions, but the table is empty. This release has two purposes: One, by adding to and manipulating the data in its database, you can see that you really can modify the visual presentation without changing code. Two, I have started using LabVIEW 2015 and realize that some of you may not have upgraded yet. If this version change is a problem, post a comment and I will send you a version of the code back-saved to LabVIEW 2014.

The Big Tease

One of the things that I like about NI Week is the opportunity to meet friends both new and old. Before a keynote address one morning I was talking to another one of the LabVIEW Champions, Jack Dunaway by name, and the topic of this blog came up. To make a long story short, he suggested a topic that sounded so good, I’m going to get started on it next time.

One good of way showing a lot of data in a small space is what is known as a tree control. It’s valuable because its structure is inherently hierarchical and so can display a lot of data while not taking up a lot of screen real estate. In addition, it can reduce the overwhelm that you sometimes feel when looking at large datasets because, when done well, they allow you to start with a high-level view of the data and gradually drill down to the specific results you want.

If you are working in Windows, there are two such controls available: one that is part of Windows, and one that is native to LabVIEW. So next time: the Native LabVIEW Tree Control. Be there or be square.

Until Next Time…


Drop-Ins Are Always Welcome

One of the key distinctions of web development is that the standards draw a bright line between content and presentation. While LabVIEW doesn’t (so far) have anything as powerful as the facilities that CSS provides, there are things that you can do to take steps in that direction. The basic technique is called creating a “drop-in” VI. These functions derive their name from the fact that they are dropped into an existing VI to change the display characteristics, but without impacting the host VI’s basic functionality.

The Main Characteristics

The first thing we need to do is consider the constraints under which these VIs will need to operate. These constraints will both assist in setting the scope of what we try to accomplish, and inform the engineering decision we have to make.

No Fraternization

The first requirement that a VI to meet in order to be considered truly “drop-in” capable, is that there must be no interaction between its logic and that of the VI into which it is being dropped. But if there is to be no interaction with the existing code, how is it supposed to change anything? Given that we are only talking about changing the aspects of the data presentation, all we need is a VI Server reference to the calling VI, and that we can get using the low-level Call Chain function.

VI Server Accesses

As you can see, from the VI reference you can get a reference to the VI’s front panel, and from that you can get an array of references for all the objects on the front panel. It is those references that allow you to set such things as the display font and size – which just happen to be the two things we are going to be manipulating for this example.

One potential problem to be aware of is the temptation to use these references to do things that directly affect how the code operates. However, this is a temptation you must resist. Even though it may seem like you “got away with it this time”, sooner or later it will bite you.

To be specific, changing the appearance of data is OK, but changing the data itself is, in general, not. However, there is one exception that when you think about it makes a lot of sense: localization. Localization is the process of changing the text of captions or labels so they appear is the language of the user, and not the developer. This operation is acceptable because although you might be changing the value in, for example, a button’s Boolean text you aren’t changing what the button does. The button will perform the same whether it is marked “OK”, “Si” or “Ja”.

Autonomous Error Handling

The next thing a drop-in has to be able to do is correctly manage errors. But here we have a bit of a conundrum. On the one hand, errors are still important, so you want to know if they occur. However, you don’t on the other hand, want this added functionality to interrupt the main code because an error occurred while configuring something the user would consider as “cosmetic”.

The solution is for the drop-in to have its own separate error reporting mechanism that records errors, but doesn’t inject them into the main VI’s error chain. The error handling library we have in place already has the needed functions for implementing this functionality.

External Configuration Storage

Finally, the drop-in VI needs configuration data that is stored in a central location outside the VI itself – after all, we want this drop-in to be usable in a wide variety of applications and projects. For implementing this storage you have at your disposal all the options you had when creating the main application itself, and as with the main application, the selection of the correct storage location depends on how much of this added capability will be exposed to the user. If you intend to let the user set the values, you can put the settings in an INI file. You just need to make sure that you quality the data they enter before you try using it. Otherwise you could end up in a situation where they specify a non-existent font, or a text size that is impossibly large or small.

To keep things simple for this test case, we will store the data in the same database that we use to store all the other configuration values. The data that we store in the database will also be (for now) simple. Each record will store the data needed to modify one part of one control, so it will contain a field for the name of the VI, the name of the control, an enumeration for selecting what part of the control is to be set, and finally the font name and size. The enumeration Component to Set will have 3 values: Label, Caption and Contents. Note that to keep things organized and easy to modify or expand, this structure as a whole, and the enumeration in particular are embodied on the LabVIEW side in type definitions.

The Plan of Action

So how can we implement this functionality? The literary device of the “omniscient author” has always bothered me so rather than simply heading off in a direction that I chose before I started writing, let’s take a look at a couple of implementation options and see which one of the two will work the best for us. Remember that the only thing more important that coming up with the right answer, is knowing how you came up with the right answer.

The “Normal” Way

For our first try, let’s start with the basic logic for getting the control references we saw a moment ago, and add to it a VI that returns the font configuration data for the VI that is being configured. Note that the input to this fetch routine (which gets the data from the application database) is the name of the VI that is calling the drop-in. This name is fully qualified, meaning it contains not just the VI name, but also the names of any library or class of which it might be a member.

Font Manager - Deadend

The output from the database lookup consists of a pair of correlated arrays. Correlated arrays are arrays where the data from a given element in one array correlates to, or goes with, the data from the same-numbered element in the other array. In this case, one array is a list of the control names and the other array is a list of all the font settings that go with the various parts of that control.

The first thing the code does is look to see if there are any font settings defined for the VI by checking to see if one of the arrays are empty. It is only necessary to check one of the arrays since they will always have the same number of elements. If there are font settings defined for the VI, the code takes the array of control references from the VI’s front panel and looks at them one-by-one to determine whether the label for that particular control or indicator is contained in the array of control names. When this search finds a control that is in the list of control names, the code unbundles the font settings data and uses the Component to Set value to select the frame of a case structure that contains the property node for the specified component’s property.

This approach works pretty well for labels and captions because all controls and indicators can, regardless of type, have them. In addition, regardless of whether the control is a string, numeric, cluster or what have you, the properties are always named the same. (The property for manipulating a control’s Caption is shown.)

Unfortunately, things begin to get complicated once you move past the properties that all controls share in common and start changing the font settings for the data contained inside the control – what we are calling the Contents. For example, the property for setting the font of the contents of a string control is called Text.FontName, whereas the property for setting the corresponding information in a digital numeric is called NumText.FontName. Things get even stranger when you start talking about setting the font of the Boolean text in the middle of a button, or worse the lines in a listbox – there each row has to be set individually.

The fundamental problem that this simple approach has is that the settings for controls and indicators are built on object-oriented principles. Labels and Captions are easy because they are common to all controls, but as soon as you start talking about text that is contained inside a control, you have to deal with a specific type, or subclass, of control. Plus to even get access to the required properties you need to cast the generic Ctl reference to a more specific class like a Str (string) or DigNum (digital numeric). We could, of course, simply expand the number of items in the Component to Set enumeration to explicitly call out all the various components that we want to be able modify. Then in each case we could do something like this:

'fixing' a problem

Because we know that the String Text is only valid for strings, we could cast the reference to the proper subclass, set the appropriate property, and call it done. If you look at very much code you will see this sort of thing being done all the time. But looking closer in those situations you will also see all the code that gets put into trying to fix this implementation’s shortcomings. For example, because the subclass selection logic is in essence being driven by the enumeration, and the enumeration value is stored in the database; we have created a situation where the contents of the database needs to be kept “in sync” with the controls on the front panels. Hence if a string control should be changed to a digital numeric (or vice versa) the database will need to be manually updated to track the change. This fact, in turn, means that we will need to add code to the VI to handle the errors that occur when we forget to keep the code and the database in sync.

As bad as that might sound, it is not the worst problem. The real deal-breaker is that every time you want or need to add support for another type of control, or another Component to Set, you will be back here modifying this VI. This ongoing maintenance task pretty much means that reusing this code will be difficult to impossible. Hopefully you can see that thanks to these problems (and these are just the two biggest ones), this “simple” approach built around a single case structure ends up getting very, very messy.

But if the object-oriented structure of controls is getting us into trouble, perhaps a bit more object orientation can get us out of trouble…

Riding a Horse in the Direction it’s Going

When programming you will often find yourself in a situation where you are wanting to extend a structure that you can see in a way that you can’t yet fully see or understand. When confronting that challenge, I often find it helpful to take some time and consider the overall trajectory of the part of the structure I can see to see where it’s pointing. Invariably, if you are working with a well-defined structure (as you are here) the best solutions will be found by “riding the horse in the direction it’s already going”.

So what direction is this “horse” already going? Well, the first thing we see is that it is going in the direction of a layered, hierarchical structure. In the VI Server structure that we can see, we observe that the basic control class is not at the top of the hierarchy, but rather in the middle of a much larger structure with multiple layers both above and below it.


The other thing we can note about the direction of this architectural trendline is that the hierarchy we just saw is organized using object-oriented principles, so the hierarchy is a hierarchy of classes, of datatypes. Hence, each object is distinct and in some way unique, but the objects as a group are also related to one another in useful ways.

Taking these two points together it becomes clear that we should be looking for a solution that is similarly layered and object-oriented. However, LabVIEW doesn’t (yet) have a way to seamlessly extend its internal object hierarchy, so while developing this structure using classes of our own creation, we will need to be careful to keep “on track”.

Moving Forward

The basic for this structure is a class that we will call Display Properties.lvclass. Initially this class will have two public interface VIs: One, Create Display Properties Update Object.vi, does as its name says and creates an object associated with a specific control or indicator. This object will drive what is now the only other interface VI (Set Control Font.vi) which is created for dynamic dispatch and will serve as the entry point for setting the font and size of text associated with GUI controls and indicators. I am building the class in this way because it is easy to imagine other display properties that we might want to manipulate in the future (e.g. colors, styles, localization, etc.). This is the code I use to dynamically load and create display property update objects:

Create Font Object

In general, it is very similar to code I have presented before to dynamically create objects, but there are a few differences. To begin with, the code does not buffer the object after it is created because unlike the other examples we have looked at over the past weeks, these objects do not need to be persistent. In other words, these objects will be created, used and then discarded.

Next, to simplify in their identification, all VI Server classes have properties that return a Class ID number and a Class Name. The code uses the latter value to build the path and class name of the child class being requested.

Finally, after the code builds the path and name of the subclass it wants to use, it checks to see if the class exists and only attempts to load it if the defining lvclass file is found. If the file is missing, the code outputs a parent class object. The reason for this difference is twofold:

  1. Without it, if a control class was called that we had not implemented, the code would throw an error. Consequently, in order to prevent those errors I would have to create dozens of empty classes that served no functional purpose – and that is wasteful of both my time and computer resources.
  2. With it, the logic extends what normally happens when a method is not overridden in a subclass, to include the case where the subclass hasn’t even been implemented yet: the parent class and, – more to the point – the parent methods, are invoked.

Taken Care of Business

The dynamic dispatch VI Set Control Font.vi is obviously the parent method for what will eventually be a family of override methods that will address specific types of controls. But that begs the question: What should go in this VI?

Well think about it for a moment. In the first possible implementation we looked at, things initially looked promising because changing the font and size of labels and captions was so easy. You’ll remember that the reason they were easy, was because all controls and indicators can have them and the properties are always named the same. That sounds like a pretty good description of what we would want in a parent method – so here it is:

Set Font Parent

The structure is pretty simple, the code retrieves the control reference from where it was stored in the class data and passes it into a case structure that has cases for Label and Caption. In addition, it has an empty case that handles the Contents value of Component to Set. This case is empty because that value will be handled during override. So all we have left to do for right now is look at how these VIs look incorporated into the structure we looked at earlier – all we really needed to replace was the case structure…

Font Manager

…and here it is. Nothing much new to see here, so let me just recommend that you take a good look at this code because you probably won’t be seeing it again. Since we will be adding functionality in the context of the class structure we created, we won’t need to revisit this logic any time soon, and maybe ever.

The Big Tease

So with the basic structure in place, all we have to do is start populating the subclasses we need. But that will have to wait for next time when I will also post all the code.

Until Next Time…


Plotting Large Datasets Without Waiting Forever

Many years ago in his landmark 1977 book, Exploratory Data Analysis, John Tukey made a comment that in his pre-PC, pre-Excel, pre-most-everything-we-think-we-need-to-do-analysis time was pretty radical, what which is today pretty common-sense:

“…there is never a good reason to not look at a plot of your data.”

Any test engineer worth his or her salt will admit that we often do our best work when we heed that truly sage advice. Looking at a plot allows us to see the data’s basic “shape” and begin to develop something of an intuitive understanding of relationships that might exist in the data. The problem is that in a world where “Big Data” is the catchphrase of the day, that simple advice can be difficult to put into practice. It has become increasingly common to see people on the user forum wondering how to effectively view work with datasets that incorporate hundreds, thousands or even hundreds of thousands of datapoints.

Seeing the Problem

To assist us in visualizing some of the problems inherent in handling large datasets, I have put together a test dataset consisting of 3 traces, each with over 19,000 datapoints. Now when I just read the data and plot it, this is what I get:

Entire dataset with white bars

Clearly there is an issue here – I mean what is up with the wide vertical bars? But there is an even larger problem. Let’s say I change the size of the plot by making it just 9 pixels wider.

But now the bars have moved (9px)

Now what is going on? The white bars have changed and if you look at the peaks in the data carefully, some of them appear to have moved or even disappeared. In order to get your head wrapped around what is happening, consider what LabVIEW is having to do behind the scenes. I mentioned that the dataset had over 19,000 datapoints (19,555, to be exact) but the active plot area of the display is only 350 pixels wide. If you do the math, you discover that to generate this plot, each pixel has to represent about 57 datapoints. The problem of course is that you can’t subdivide a pixel into 57 pieces. So what is LabVIEW to do?

Well it does what any graphing package does when it is confronted with this challenge: it decimates the data. In other words it takes 57-datapoint chunks of the data, performs some sort of statistical operation on each chunk (min, max, mean, etc) and then uses the resulting summary value to represent that chunk of data on the plot. There are several potential problems with this way of handling the situation, but they typically don’t become an issue unless the dataset being plotted is very large relative to the size of the graph. For example, this is why the data on the graph appeared to change as a function of the size of the plot area. As the plot area changed (even slightly) the chunking changes so the data appears to change as well – you can think of it as sort of visual aliasing.

More subtle problems have to do with the way the graphing routines “summarize” the data chunks. Depending upon the shape of your dataset, the operations I mentioned earlier can give dramatically different output and to make matters worse you have no idea what techniques the graphing functions are using. But even if you can live with the visual effects there are good reasons to take action to address the issue.

Finally, in order to plot these huge datasets you have to be carrying them around inside your program. Consequently, rather than having just one copy of these monsters, you can have several – perhaps dozens – it all depends on how your code it written. From this discussion we can then see the two imperatives for our solution:

  1. The approach must minimize the number of copies that LabVIEW has to make of the dataset.
  2. It must reduce the number of datapoints that actually need to be plotted.

Let’s start by looking at the data management aspect of the problem, remembering of course that these two issues are inextricably linked together.

Low-Overhead Storage

Decades ago, people in the nascent computer-science discipline realized that if you had a value, like an array, that consisted of multiple items, the most efficient way of making it available throughout your code was to store it in one location in memory and give the code that needed to access it a “pointer” that served as a reference to that value. Originally this mechanism was pretty primitive with the pointer often consisting of simply the value’s starting address in RAM. In addition, there was no real way of preventing race conditions or security intrusions because there was no way of controlling access to the data. It would be nice to think that we have learned the errors of our ways and fixed all the holes, but such is not always the case. Remember the “Heartbleed” bug panic from last year?

The good news is that LabVIEW does not suffer from the same problems because while we have at our disposal a mechanism that fills the same role as the primitive pointer, it lacks the problems. I am talking about the Data Value Reference, or DVR. It meets the low-overhead storage mandate by accessing the data through a reference that is only 4 bytes long. The DVR is also secure because the buffer that is creates is strongly typed, meaning that you can’t just store anything in it or read whatever you want from it. The data going in and coming out must match the definition of the data structure that was used when the DVR was defined. Finally, the DVR removes problems resulting from simultaneous access to the same resource by defining a new structure that automatically serializes access on a first-come, first-served basis. So the first thing we need to do is get our data into the DVR, and here’s some code to do just that.

Load  Big Data

The VI starts by reading a binary file containing the data which, to simplify this example, is already formatted correctly for how we are going to use it. The resulting array drives a box called an inplace structure that guarantees there will be no other accesses to the DVR occurring in parallel with this one. However, the structure does something else too: Inplace structures operate something like compiler directives telling the LabVIEW compiler that its OK to attempt additional optimizations that would not otherwise be safe to make. For example, they allow to LabVIEW operates on the inplace data without making the copies that the compiler might otherwise need make.

The other thing to note is that funny-looking function in the middle of the inner inplace structure. It’s called Swap Values and its help description really doesn’t do it justice. If all you did was read the context help you might assume that it is simply some sort of switch for routing signals around, and stifling a yawn, go on to consider matters that you think look more exciting. To see why you should consider this function very exciting, we need to look under LabVIEW’s hood.

To store data internally, LabVIEW uses memory data buffers. In fact much of what we think of as “dataflow” consists of the manipulation of those buffers. Now when LabVIEW stores a complex datatype like a cluster (which is what the DVR in this case is holding) it uses a combination of techniques. For simple fix-sized data like numerics or booleans, LabVIEW simply includes the data values directly in the cluster’s memory space. However, it needs a different approach when storing data values like arrays or strings that can vary in length. When a cluster includes an item that can change in size, the item is stored outside the cluster in its own memory buffer and the cluster only holds a reference to that buffer. That way if the item changes in size it can do so without effecting the memory allocation of the cluster containing it.

However this explanation also reveals why the Swap Values node is so important. Let’s look at this code from the standpoint of buffers. Coming into the inner inplace structure there are two buffers allocated that are holding arrays: One contains the data I read from the file, and one the (empty) array that is contained in the cluster that is the contents of the DVR. Now there are two ways that we could initialize that array. The most obvious one is to leave the unbundle (left) side of the cluster inplace structure unwired and wire the array containing the data directly to the bundle (right) side of the cluster inplace structure. While this would work, coding it in that way would result in LabVIEW needing to copy the data contained in the incoming array’s buffer to the array buffer associated with the cluster – and the larger the dataset is, the longer this copy can take.

Now consider what happens when Swap Values is used. Although the node resides inside an inplace structure, it would seem logical that you can’t replace an empty array with an array containing thousands of datapoints in place. Well actually you can. The key point to remember is that at a very low level, the clusters don’t actually contain the arrays, rather they hold references that point to the arrays that are associated with them. So what Swap Values does is it leaves the two arrays in place and simply swaps the references that the clusters contain. Thanks to this optimization, populating this cluster with data will take the exact same amount of time whether the input data contains 2 datapoints or 200,000 datapoints because the only thing that is really being moved is a pair of 4-byte memory buffer references.

Getting Data Out

So we have gotten our data into the DVR as efficiently as we can, but if this storage is going to be of any use, there clearly needs to be a way to get data out of it as well. However, here we face the issue of plotting data that is too large. At the same time we are pulling it out, we also need to be reducing or decimating it to more closely match the size of the available graphing area. To meet those dual requirements I created this VI.

Read and Decimate Big Data

At first this code might seem intimidating, but if you take it step-by-step and analyse what it’s doing, it isn’t really so very different from the example we looked for initializing the data in the DVR. Starting at the left side, the code unbundles the data array from the DVR and passing it into a loop that will execute three times – once for each plot in the dataset. The first point of optimization is in how this loop operates. Note the node with the “P” in it. The presence of this node means that the for loop is set for parallel operation. There are many situations where, even though you specify a for loop, there is no logical reason that the iterations have to operate sequentially. When LabVIEW encounters a “parallelized” loop the optimizer essentially flattens the loop out, creates the necessary parallel code to execute each iteration simultaneously, and then reassemble the output data in the correct order. To find out if a loop is parallelizable, there is an option under Tools>>Profile called Find Parallelizable Loops…. This operation opens a dialog that allows you to identify the loops that can and cannot be run in parallel mode.

Inside the loop, the array drives an inplace structure that indexes out one element, and the resulting cluster feeds a second inplace structure that unbundles the two items in the cluster. The processing of this data occurs in two distinct steps. First the Start and Length inputs produce a subset of the total dataset representing the portion of the data that is to be displayed. Because this operation causes LabVIEW to copy the selected data into a new memory buffer, the code passes the resulting arrays into another inplace structure to ensure that the subset will also be manipulated inplace.

The code inside this inner-most inplace structure performs the second half of the processing – the decimation to reduce the size of the data being plotted. Note that if the selected portion of the dataset is already smaller than the width of plot area, the following code is bypassed. The first step in the decimation process is to reshape the 1D array into a 2D array where each row contains one chunk of data to be statistically summarized. To obtain the final X values, the code takes the first value of each chunk, while the final Y values are the maximum Y for each chunk. Note that this processing occurs in another parallelized loop that auto-indexes the output arrays, which are swapped into the output dataset as they work their way back out through the inplace structures.

Summarizing Options and Challenges

The real heart of this VI is the function that is being used to summarize the Y values for each chunk of data. Right now, I am using the function that returns the minimum and maximum values contained in the array. One of the advantages that it offers is that it is deals well with datasets containing missing datapoints represented by the value NaN. This consideration is important because it is a common (and valuable) practice to represent missing data points using that value. Without the NaN datapoints, any graph will simply connect the dots on either side of the missing datapoints resulting in a graph that visually misrepresents the data being presented. However, with the NaN values, the missing points are shown as breaks in the line (or gaps between bars), thus highlighting the missing data.

The statistical function I selected to summarize the data in the chunks simply returns the minimum and maximum values of the elements that are not NaN. However, most other analysis routines follow the basic rule that any calculation which has NaN as an operand will return an answer of NaN – which in this situation will not be real helpful. More often, what you will want is, for example, the average value of the datapoints that are present in the dataset chunk. If you are wanting to use the chunk mean or median value to summarize a dataset that you know contains NaN value, you should include something like this before the statistical operation:

Filtering out NaN

Basically it works by first sorting the array to move any NaN values to the end of the array. It then looks for the first NaN and simply trims off it (and anything after it). This works because a mean operation doesn’t care about data order, and the first thing a median function does is sort the data anyway.

Let’s See How it Works

When you run the top-level VI in the linked project, the graph that comes up will look a lot like the first image in this post, but minus the vertical white bars. As you make changes to the display that effect the X axis range, you will notice that the resulting image will zoom in on the data, showing ever greater levels of detail. Try manually typing in new X axis end points, or use the horizontal zoom tool on the graph palette to select a range of data points that you want to zoom in on.

Zoom in far enough and you will see why there were white bars on the original plot: There are a lot of missing datapoints. Using the default decimation resulted in wide white bars because the presence of the NaN values effectively hid dozens of real datapoints.

Plotting Large Datasets – Release 1
Toolbox – Release 11

Hopefully this discussion will give you something to think about, and experiment with.

The Big Tease

One of the things that developers often have to face is adding functionality to an existing VI without disrupting, or even modifying what is already there. One way to accomplish this (seemingly impossible) task is to use what are sometimes called “drop-in” VIs. These routines are simply dropped down on an existing block diagram and they do what they do without interaction with the existing code. To demonstrate how this could work, next time we’ll get back to our test bed application and give it the ability to customize the font and size of the test that are on its various displays.

Until Next Time…

More Than One Kind of Modularity

When learning something that you haven’t done before – like .NET – it’s not uncommon to go through a phase where you look at some of the code you wrote early on and cringe (or at least sigh deeply). The problem is that you are often not only learning a new interface or API, but you are learning how to best use that interface or API. The cause of all the cringing and sighing is that as you learn more, you begin to realize that some of the assumptions and design decisions that you made were misguided, if not flat-out wrong. If you look at the code we put together last time to help us learn about .NET in general, and the NotifyIcon assemble in particular, we see a gold-plated example of just such code. Although it is clearly accomplished it’s original goal of demonstrating how to access .NET functionality and illustrating how the various objects can relate to one another, it is certainly not reusable – or maintainable, or extensible, or any of the other “-ables” that good software needs to be.

In fact, I created the code in that way so this time we can take the lesson one step further to fix those shortcomings, and thus demonstrate how you can go about cleaning up code (of your own or inherited) that is making you cringe or sigh. Remember, it is always worth your time to fix bad design. I can’t tell you how many times I have seen people struggling with bad decisions made years before. Rather than taking a bit of time to fix the root cause of their trouble, they continue to waste hours on project after project in order to workaround the problem.

Ok, so where do we start?

Clearly this code would benefit from cleaning-up and refactoring, but where and how should we start? Well, if you are working on an older code base, the question of where to start will not be a problem. You start with where the most pain is. To put it another way, start with the things that cause you the biggest problems on a day-to-day basis.

This point, however, doesn’t mean that you should just sit around and wait for problems to arise. As you are working always be asking yourself if what you are doing has limitations, or embodies assumptions that might cause problems in the future.

The next thing to remember is that this work can, and should, be iterative. In other words you don’t have to fix everything at once. Start with the most egregious errors, and address the others as you have the opportunity. For example, if you see the code doing something stupid like using a string as a state variable, you can fix that quickly by replacing the strings with a typedef enumeration. I have even fixed some long-standing bugs in doing this replacement because it resolved places where states were subtly misspelled or contained extraneous spaces.

Finally, remember that the biggest payoffs, in terms of long-term benefit, come from improved modularity that corrects basic architectural problems. As we shall see in the following discussion, I include under this broad heading modularity in all its forms: modular functionality, modular logic and modular data.

Revisiting Modular Functionality

Modular functionality is the result of taking small reusable bits of code and encapsulating it in routines that simplify access, standardize the interface or ensure proper usage. There are good examples of all these usages in the application modified code, starting with Create NotifyIcon.vi:

Create NotifyIcon VI

Your first thought might be why I bothered turning this functionality into a subVI. After all, it’s just one constructor node. Well, yes that is true but it’s also true that in order to create that one node you have to remember multiple steps and object names. Even though this subVI appears rather simple, if you consider what it would take to recreate it multiple times in the future, you realize that it actually encapsulates quite a bit of knowledge. Moreover, I want to point out that this knowledge is largely stuff for which there is no overwhelming benefit to be gained from you committing it to memory.

Next, let’s consider the question of standardizing interfaces. Our example in this case is a new subVI I created to handle the task of assigning an icon to the interface we are creating. I have named it Set NotifyIcon Icon.vi:

Set NotifyIcon Icon VI

You will remember from out previous discussion that this task involves passing a .NET object encapsulating the icon we wish to use to a property node for the NotifyIcon object. Originally, this property was combined with several others on a single node. A more flexible approach is to breakup that functionality and standardize the interfaces for all the subVIs that will be setting NotifyIcon to simply consist of an object reference and the data to be used to set the property in a standard LabVIEW datatype – in this case a path input. This approach also clearly simplifies access to the desired functionality.

Finally, there is the matter of ensuring proper usage. A good place to highlight that feature is in the last subVI that the application calls before quitting: Drop NotifyIcon.vi.

Drop NotifyIcon VI

You have probably been warned many times about the necessity of closing references that you open. However, when working with .NET objects, that action by itself is sometimes not sufficient to completely release all the system resources that the assembly had been using. Most of the time if you don’t completely close out the assembly you many notice memory leaks or errors from attempting to access resources that still think they are busy. However with the NotifyIcon assembly you will see a problem that is far more noticeable, and embarrassing. If you don’t call the Dispose method your program will close and release all the memory it was using, but if you go to the System Tray you’ll still see your icon. In fact, you will be able to open its menu and even make selections – it just doesn’t do anything. Moreover, the only way to make it go away it to restart your computer.

Given the consequences of forgetting to include this method in your shutdown sequence, it is a good idea to make it an integral part of the code that you can’t forget to include.

Getting Down with Modular Logic

But as powerful as this technique is, there can still be situations where the basic concept of modularity needs to be expressed in a slightly different way. To see such a situation, let’s look at the structure that results from simply applying the previous form of modularity to the problem of building the menus that go with the icon.

Create ContextMenu VI

Comparing this diagram to the original one from last time, you can see that I have encapsulated the repetitive code that generated the MenuItem objects into dedicated subVIs. By any measure this change is a significant improvement: the code is cleaner, better organized, and far more readable. For example, it is pretty easy to visualize what menu items are on submenus. However, in cases such as this one, this improved readability can be a bit of a double-edged sword. To see what I mean, consider that for the structure of your code to allow you to visualize your menu organization, said organization must be hard-coded into the structure of the code. Consequently, changes to the menus will, as a matter of course, require modification to the fundamental structure of the code. If the justifications for modularity is to include concepts like flexibility and reusability, you just missed the boat.

The solution to this situation is to realize that there is more than one flavor of modularity. In addition to modularizing specific functionality, you can also modularize the logic required to perform complex and changeable tasks (like building menus) that you don’t want to hard code. If this seems like a strange idea to you, consider that computers spend most of their time using their generalized hardware to performed specialized tasks defined by lists of instructions called “programs”. The thing that makes this process work is a generalized bit of software called a “compiler” that turns the programs into data structures that the generalized hardware can use to perform specialized actions.

Carrying forward with this line of reasoning, what we need is a simple way of defining a menu structure that is external to our program, and a “menu compiler” that turns that definition into the MenuItem references that our program needs. So let’s build one…

Creating the Data for Our Menu Compiler

So what should this menu definition look like? Well, to answer that question we need to start with the data required to define a single MenuItem. We see that as a minimum, every item in a menu has to have a name for display to the user, a tag to identify it, and a parent tag that says if the item has a parent item (and if so which item is its parent). In addition, we haven’t really talked about it, but the order of references in an array of menu items defines the order in which the items appear in the menu or submenu – so we need a way to specify its menu position as well. Finally, because in the end the menu will consist of a list (array) of menu item references, it makes sense to express the overall menu definition that we will eventually compile into that array of references as a list (and eventually also an array).

But where should we store this list of menu item definitions? At least part of the to this question depends on who you want to be able to modify the menu, and the level of technical expertise that person has. For example, you could store this data in text files as INI keys, or as XML or JSON strings. These files have the advantage of being easy to generate and are readily accessible to anyone who has access to a text editor – of course that is their major disadvantage, as well. Databases on the other hand are more secure, but not as easy to access. For the purposes of this discussion, I’ll store the menu definitions in a JSON file because, when done properly, the whole issue of how to parse the data simply goes away.

To see what I mean, here is a nicely indented JSON file that describes the menu that we have been working using for our example NotifyIcon application:

		"Menu Order":0,
		"Item Name":"Larry",
		"Item Tag":"Larry",
		"Parent Tag":"",
		"Menu Order":1,
		"Item Name":"Moe",
		"Item Tag":"Moe",
		"Parent Tag":"",
		"Menu Order":2,
		"Item Name":"The Other Stooge",
		"Item Tag":"The Other Stooge",
		"Parent Tag":"",
		"Menu Order":3,
		"Item Name":"-",
		"Item Tag":"",
		"Parent Tag":"",
		"Menu Order":4,
		"Item Name":"Quit",
		"Item Tag":"Quit",
		"Parent Tag":"",
		"Menu Order":0,
		"Item Name":"Curley",
		"Item Tag":"Curley",
		"Parent Tag":"The Other Stooge",
		"Menu Order":1,
		"Item Name":"Shep",
		"Item Tag":"Shep",
		"Parent Tag":"The Other Stooge",
		"Menu Order":2,
		"Item Name":"Joe",
		"Item Tag":"Joe",
		"Parent Tag":"The Other Stooge",

And here is the LabVIEW code will convert this string into a LabVIEW array (even if it isn’t nicely indented):

Read JSON String

JSON has a lot of advantages over techniques like XML: For starters, it’s easier to read, and a lot more efficient, but this is why I really like using JSON: It is so very convenient.

Starting the Compilation

Now that we have our raw menu definition string read into LabVIEW and converted into a datatype that will simplify the next step in the processing, we need to ensure that the data is in the right order. To see why, we need to remember that the final data structure we are building is hierarchical, so the order in which we build it matters. For instance, “The Other Stooge” is a top-level menu item, but it is also a submenu so we can’t build it until we have references to all the menu items that are under it. Likewise, if one of the items under it is a submenu, we can’t build it until all its children are created.

So given the importance of order, we need to be careful how we handle the data because none of the available storage techniques can on their own guarantee proper ordering. The string formats can all be edited manually, and it’s not reasonable to expect people to always type in data in the right order. Even though databases can sort the result of queries, there isn’t enough information in the menu definition to allow it to do so.

The menu definition we created does have a numeric value that specifies the order of items in their respective menus and submenus. We don’t, however, yet have a way of telling the level the items reside at relative to the overall menu structure. Logically we can see that “Larry” is a top-level menu item, and “Shep” is one level down, but we can’t yet determine that information programmatically. Still the information we need is present in the data, it just needs to be massaged a bit. Here is the code for that task:

Ordering the Menu Items

As you can see, the process is basically pretty simple. I first rewrite the Item Tag value by adding the original Item Tag value to the colon-delimited list that starts with the Parent Tag. I then count the number of colons in the resulting string, and that is my Menu Level value. The exception to this processing are the top-level menu items which are easy to identify due to their null parent tags. I simply force their Menu Level values to zero and replace the null string with a known value that will make the subsequent processing easier. The real magic however, occurs after the loop stops. The code first sorts the array in ascending order and then reverses the array. Due to the way the 1D array sort works when operating on arrays of clusters, the array will be sorted first by Menu Level and then Menu Order – the first two items in the cluster. This sorting, in concert with the array reversal, guarantees that the children of a submenu will always be processed before the submenu item itself.

Some of you may be wondering why we go to all this trouble. After all, couldn’t we just add a value to the menu definition data to hold the Menu Level? Yes, we could, but it’s not a good idea, and here’s why. In some areas of software development (like database development, for instance) the experts put a lot of store in reducing “redundancy” – which they define basically as storing the same piece of information in more than one place. The problem is that if you have redundant information, you have to decide how to respond when the two pieces of information that are supposed to be the same, aren’t. So let’s say we add a field to the menu definition for the menu level. Now we have the same piece of information stored in two different places: It is stored explicitly in the Menu Level value while at the same time it is also stored implicitly in Parent Tag.

Generating the Menu Item “Code”

In order to turn this listing into the MenuItem references we need, we will pass this sorted and ordered array into a loop that will process one element at a time. And here it is:

Compiling the Menu-1

You can see that the loop carries two shift registers. The top SR holds a 1D array of strings that consists of the submenu tags that the loop has encountered so far. The other SR also carries a 1D array but each element in it is a cluster containing an array of MenuItem references associated with the submenu named in the corresponding element of the top SR.

As the screenshot shows, the first thing that happens in the loop is that the code checks to see if the indexed Item Tag is contained in the top SR. If the tag is missing from the array it means that the item is not a submenu, so the code uses its data to create a non-submenu MenuItem. In parallel with that operation, the code is also determining what to do with the reference that is being created by looking to see if the item’s Parent Tag exists in the top SR. If the item’s parent is also missing from the array, the code creates entries for it in both arrays. If the parent’s tag is found in the top SR, it means that one or more of the item’s sibling items has already been processed so code is executed to add the new MenuItem to the array of existing ones:

Compiling the Menu-2

Note that the new reference is added to the top of the array. The reason for this departure from the norm is that due to the way the sorting works, the menu order is also reversed and this logic puts the items on each submenu back in their correct order. Note also that during this processing the references associated the menu items are also accumulated in a separate array that will be used to initialize the callbacks. Because the array indexing operation is conditional, only a MenuItem that is not a submenu, will be included in this array.

Generating the Submenu “Code”

If the indexed Item Tag is found in the top SR, the item is a submenu and the MenuItem references needed to create its MenuItem should be in the array of references stored in the bottom SR.

Compiling the Menu-3

So the first thing the code does is delete the tag and its data from the two array (since they are no longer needed) and uses the data thus obtained to create the submenu’s MenuItem. At the same time, the code is also checking to see if the submenu’s parent exists in the top SR. As before, if the Parent Tag doesn’t exist in the array, the code creates an entry for it, and if it does…

Compiling the Menu-4

…adds the new MenuItem to the existing array – again at the top of the array. By the time this loop finishes, there should be only one element in each array. The only item left in the top SR should be “[top-menu]” and the bottom SR should be holding the references to the top-level menu items. The array of references is in turn used to create the ContextMenu object which written to the NotifyIcon object’s ContextMenu property.

What Could Possibly Go Wrong?

At this point, you can run the example code and see an iconic system tray interface that behaves pretty much as it did before, but with a few extra selections. However, we need to have a brief conversation about error checking, and frankly in this situation there are two schools of though on this topic. There is ample opportunity for errors to creep into the menu structure. Something as simple as misspelling a parent tag name could result in an “orphan” menu that would never get displayed – or could end up being the only one that is displayed. So the question is how much error checking do we really need to do? There are those that think you should spend a lot of time going through the logic looking for and trapping every possible error.

Given that most menus should be rather minimal, and errors are really obvious, I tend to concentrate on the low-hanging fruit. For example, one simple check that will catch a large number of possible errors, is looking to see if at the end of the processing, there is more than one menu name left in the top SR – and finding an extra one, asserting an error that gives the name of the extra menu. You should probably also use this error as an opportunity to abort the application launch since you could be left in a situation when you can’t shutdown the program because the “Quit” option is missing.

Something else that you might want to consider is what to do if the external file containing the menu definitions comes up missing. The most obvious solution is to, again, abort the application launch with some sort of appropriate error message. However, depending on the application it might be valuable to provide a hard-coded default menu that doesn’t depend on external files and provides a certain minimum level of functionality. In fact, I once worked on an application where this was an explicit requirement because one of the things that the program allowed the user to do was create custom menus, the structure of which was stored in external files.

Stooge Identifier – Release 2
Toolbox – Release 11

The Big Tease

So what are we going to talk about next time? Well something that I have seen coming up a lot lately on the user forum is the need to be able to work with very large datasets. Often, this issue arises when someone tries to display the results of a test that ran for several hours (or days!) only to discover that the complete dataset consists of hundreds of thousands of separate datapoints. While LabVIEW can easily deal with datasets of this magnitude, it should be obvious that you need to really bring you memory management “A” game. Next time will look into how to plot and manage VLDs (Very Large Datasets).

Until Next Time…


A Brief Introduction to .NET in LabVIEW

From the earliest days of LabVIEW, National Instrument has recognized that it needed the ability to incorporate code that was developed in other programming environments. Originally this capability was realized through specialized functions called Code Interface nodes, or CINs. However as the underlying operating systems continued to develop, LabVIEW acquired the ability to leverage such things as DLLs, ActiveX controls and .NET assemblies. Unfortunately, while .NET solves many of the problems that earlier efforts to standardize sharable code exhibited, far too many LabVIEW developers feel intimidated by what they see as unmanageable complexity. The truth, however, is that there are many well-written .NET assemblies that are no more difficult to use than VI Server.

As an example of how to use .NET, we’ll look at an assembly that comes with all current versions of Windows. Called NotifyIcon, it is the mechanism that Windows itself uses to give you access to programs through the part of the taskbar called the System Tray. However, beyond that usage, it is also an interesting example of how to utilize .NET to implement an innovative interface for background tasks.

The Basic Points

Given that the whole point of this lesson is to learn about creating a System Tray interface for your application, a good place to start the discussion is with a basic understanding of how the bits will fit together. To begin with, it is not uncommon, though technically untrue, to hear someone say that their program was, “…running in the system tray…”. Actually, your program will continue to run in the same execution space, with or without this modification. All this .NET assembly does is provide a different way for your users to interact with the program.

But that explanation raises another question: If the .NET code allows me to create the same sort of menu-driven interface that I see other applications using, how do the users’ selections get communicated back to the application that is associated with the menu?

The answer to that question is another reason I wanted to discuss this technique. As we have talked about before, as soon as you have more than one process running, you encounter the need to communicate between process – often to tell another process that something just happened. In the LabVIEW world we often do this sort of signalling using UDEs. In the broader Windows environment, there is a similar technique that is used in much the same way. This technique is termed a callback and can seem a bit mysterious at first, so we’ll dig into it, as well.

Creating the Constructor

In the introduction to this post, I likened .NET to VI Server. My point was that while they are in many ways very different, the programming interface for each is exactly the same. You have a reference, and associated with that reference you have properties that describe the referenced object, and methods that tell the object to do something.

To get started, go to the .NET menu under the Connectivity function menu, and select Constructor Node. When you put the resulting node on a block diagram, a second dialog box will open that allows you to browse to the constructor that you want to create. The pop-up at the top of the dialog box has one entry for each .NET assembly installed on your computer – and there will be a bunch. You locate constructors in this list by name, and the name of the constructor we are interested in is System.Windows.Forms. On your computer there may be more than one assembly with this basic name installed. Pick the one with the highest version (the number in parentheses after the name).

In the Objects portion of the dialog you will now see a list of the objects contained in the assembly. Double click on the plus sign next to System.Windows.Forms and scroll down the list until you find the bullet item NotifyIcon, and select it. In the Constructors section of the dialog you will now see a list of constructors that are available for the selected object. In this case, the default selection (NotifyIcon()) is the one we want so just click the OK button. The resulting constructor node will look like this:

notifyicon constructor

But you may be wondering how you are supposed to know what to select. That is actually pretty easy. You see, Microsoft offers an abundance of example code showing how to use the assemblies, and while they don’t show examples in LabVIEW, they do offer examples in 2 or 3 other languages and – this is the important point – the object, property and method names are the same regardless of language so it’s a simple matter to look at the example code and, even without knowing the language, figure out what needs to be called, and in what order. Moreover, LabVIEW property and invoke nodes will list all the properties and methods associated with each type of object. As an example of the properties associated with the NotifyIcon object, here is a standard LabVIEW property node showing four properties that we will need to set for even a minimal instance of this interface. I will explain the first three, hopefully you should be able to figure out what the fourth one does on your own.

notifyicon property node

Starting at the top is the Text property. It’s function is to provide the tray icon with a label that will appear like a tip-strip when the user’s mouse overs over the icon. To this we can simply wire a string. You’ll understand the meaning of the label in a moment.

Giving the Interface an Icon

Now that we have created our NotifyIcon interface object and given it a label, we need to give it an icon that it can display in the system tray. In our previous screenshot, we see that the NotifyIcon object also has a property called Icon. This property allows you to assign an icon to the interface we are creating. However, if you look at the node’s context help you see that its datatype is not a path name or even a name, but rather an object reference.

context help window

But don’t despair, we just created one object and we can create another. Drop down another empty .NET constructor but this time go looking for System.Drawing.Icon and once you find the listing of possible constructors, pick the one named Icon(String fileName). Here is the node we get…

icon constructor

…complete with a terminal to which I have wired a path that I have converted to a string. In case you missed what we just did, consider that one of the major failings of older techniques such as making direct function calls to DLLs was how to handle complex datatypes. The old way of handling it was through the use of a C or C++ struct, but to make this method work you ended up needing to know way too much about how the function worked internally. In addition, for the LabVIEW developer, it was difficult to impossible to build these structures in LabVIEW. By contrast, the .NET methodology utilizes object-oriented techniques to encapsulate complex datatypes into simple-to-manipulate objects that accept standard data inputs and hide all the messy details.

Creating a Context Menu

With a label that will provide the users a reminder of what the interface is for, and an icon to visually identify the interface, we now turn to the real heart of the interface: the menu itself. As with the icon, assigning a menu structure consists of writing a reference to a property that describes the object to be associated with that property. In this case, however, the name of the property is ContextMenu, and the object for which we need to create a constructor is System.Windows.Forms.ContextMenu and the name of the constructor is ContextMenu(MenuItem[] menuItems).

context menu constructor

From this syntax we see that in order to initialize our new constructor we will need to create an array of menuItems. You got to admit, this makes sense: our interface needs a menu, and the menu is constructed from an array of menu items. So now we look at how to create the individual menu items that we want on the menu. Here is a complete diagram of the menu I am creating – clearly inspired by a youth spent watching way too many old movies (nyuk, nyuk, nyuk).

menu constructors

Sorry for the small image, but if you click on the image, you can zoom in on it. As you examine this diagram notice that while there is a single type of menuItem object, there are two different constructors used. The most common one has a single Text initialization value. The NotifyIcon uses that value as the string that will be displayed in the menu. This constructor is used to initialize menu items that do not have any children, or submenus. The other menuItem constructor is used to create a menu item that has other items under it. Consequently in addition to a Text initialization value, it also has an input that is – wait for it – an array of other menu items. I don’t know if there is a limit to how deeply a menu can be nested, but if that is a concern you need to be rethinking your interface.

In addition to the initialization values that are defined when the item is created, a menuItem object has a number of other properties that you can set as needed. For instance, they can be enabled and disabled, checked, highlighted and split into multiple columns (to name but a few). A property that I apply, but the utility which might not be readily apparent, is Name. Because it doesn’t appear anywhere in the interface, programmers are pretty much free to use is as they see fit, so I going to use it as the label to identify each selection programmatically. Which, by the way, is the next thing we need to look at.

Closing the Event Loop

If we stopped with the code at this point, we would have an interface with a perfectly functional menu system, but which would serve absolutely no useful purpose. To correct that situation we have to “close the loop” by providing a way for the LabVIEW-based code to react in a useful way to the selections that the user makes via the .NET assembly. The first part of that work we have already completed by establishing a naming convention for the menu items. This convention largely guarantees menu items will have a unique name by defining each menu item name as a colon-delimited list of the menu item names in the menu structure above it. For example, “Larry” and “Moe” are top-level menu items so their names are the same as their text values. “Shep” however is in a submenu to the menu item “The Other Stooge” so its name is “The Other Stooge:Shep”.

The other thing we need in order to handle menu selections is to define the callback operations. To simplify this part of the process, I like to create a single callback process that services all the menu selections by converting them into a single LabVIEW event that I can handle as part of the VI’s normal processing. Here is the code that creates the callback for our test application:

callback generator

The way a callback works is that the callback node incorporates three terminals. The top terminal accepts an object reference. After you wire it up, the terminal changes into a pop-up menu listing all the callback events that the attached item supports. The one we are interested in is the Click event. The second terminal is a reference for the VI that LabVIEW will have executed when the event you selected is fired. However, you can’t wire just any VI reference here. For it to be callable from within the .NET environment it has to have a particular set of inputs and a particular connector pane. To help you create a VI with the proper connections, you can right-click on the terminal and select Create Callback VI from the menu. The third terminal on the callback registration node is labelled User Parameters and it provides the way to pass static application-specific data into the callback event.

There are two important points here: First, as I stated before, the User Parameters data is static. This means that whatever value is passed to the terminal when the callback is registered is from then on essentially treated as a constant. Second, whatever you wire to this terminal modifies the data inputs to the callback VI so if you are going to use this terminal to pass in data, you need to wire it up before you create the callback VI.

In terms of our specific example, I have an array of the menu items that the main VI will need to handle so I auto-index through this array creating a callback event for each one. In all cases, though, the User Parameter input is populated with a reference to a UDE that I created, so the callbacks can all use the same callback VI. This is what the callback VI looks like on the inside:

callback vi

The Control Ref input (like User Parameter) is a static input so it contains the reference to the menu item that was passed to the registration node when the callback was created. This reference allows me to read the Name property of the menu item that triggered the callback, and then use that value to fire the SysTray Callback UDE. It’s important to remember when creating a callback VI to not include too much functionality. If fact, this is about as much code as I would ever put in one. The problem is that this code is nearly impossible to debug because it does not actually execute in the LabVIEW environment. The best solution is to get the selection into the LabVIEW environment as quickly as possible and deal with any complexity there. Finally, here is how I handle the UDE in the main VI:

systray callback handler

Here you can see another reason why I created the menu item names as I did. Separating the different levels in the menu structure by colons allows to code to easily parse the selection, and simultaneously organizes the logic.

Future Enhancements

With the explanations done, we can now try running the VI – which disappears as soon as you start it. However, if you look in the system tray, you’ll see its icon. As you make selections from its menu you will see factoids appear about the various Stooges. But this program is just the barest of implementations and there is still a lot you can do. For example, you can open a notification balloon to notify the user of something important, or manipulate the menu properties to show checkmarks on selected items or disable selections to which you want block access.

The most important changes you should make, however, are architectural. For demonstration purposes the implementation I have presented here is rather bare-bones. While the resulting code is good at helping you visualize the relationships between the various objects, it’s not the kind of code you would want to ship to a customer. Rather, you want code that simplifies operation, improves reusability and promotes maintainability.

Stooge Identifier — Release 1

The Big Tease

So you have the basics of a neat interface, and a basic technique for exploring .NET functionality in general. But what is in store for next time? Well I’m not going to leave you hanging. Specifically, we are going to take a hard look at menu building to see how to best modularize that functionality. Although this might seem a simple task, it’s not as straight-forward as it first seems. As with many things in life, there are solutions that sound good – and there are those that are good.

Until Next Time…


Helping a Window to Remember

One of the most common, most basic, and most mindless, things we do with computers every day is open windows. Launching a program or opening a document is often synonymous (on a practical level at least) with opening a window. As common as this action is, we rarely give any thought to what is going on behind the scenes when we open a window – hence the wisecrack about it being a mindless operation.

However, if we want to make the most of our design efforts, be need to replace this “mindlessness” with “mindfulness” by really thinking about the things that make windows easy and comfortable to use.

Defining a Well Behaved Window

As we begin looking at the behavior of windows, I want to emphasise that I am not talking about user interface design. User interface design deals with the details of what a window does functionally. Rather what I’m talking about is an examination to the behaviors a window should exhibit, regardless of what happens to be on the screen.

If we think about the window as a kind of frame that supports the interface’s core functionality, we see that one of the big things a window can do is remember things. Over the years, people have developed, and posted on the LabVIEW forums, a variety of toolboxes for storing generic window information like screen customizations, positions and settings. One of these toolsets combined with an event-driven structure can make it easy to significantly pump up the convenience factor of just about any application.

To see how these sorts of tools work, we’re going to enhance our undockable windows application with a simple addition that automatically saves a windows last position and restores that position the next time the window undocks. Although the basic logic is simple, it provides us with the opportunity to discuss many of the major issues that impact this sort of functionality.

The Data, and Where to Keep it

When considering the data that this sort of functionality requires and uses, the operative word is: “convenience”. By that I mean that this data may make using the screen more convenient, but nobody is going to be crying if it gets lost. In fact, a valuable behavior is the ability basically “reset” all the stored data back to its default value by simply deleting the data from the file that is storing it and letting the application rebuild it as needed.

Likewise the data should be of low “intelligence” value. In other words, we don’t want to include things in this data that could constitute a security risk. However, having said that, we also want to make sure that a well-meaning user doesn’t mess up the program’s operation by manually editing the data. My approach to blocking such edits has three major points:

  • Be careful about what you name things: You want to give identifiers that are, of course, meaningful. However, you don’t want to use names that will call attention to themselves in a way that says, “Hi, I’m a setting you might want to play with…”
  • Use a non-obvious data structure: For example, in our example we don’t save a window’s position as a simple list of four values. The problem is that a user looking at these values might decide to try to edit them manually – a simple act that could have some significant side effects. To see why consider that the way you move a window around the screen is by changing the VI’s WinBounds property. However, this property defines a windows position by essentially specifying the location of the window’s upper-left and lower-right corners. Consequently, while it does set the window’s location, it is also specifying the window’s size.
  • Provide a simple way to validate the data: Given that there is no way to know ahead of time what sort of data you might be wanting to store, validation might seem like a huge task, but it’s really not. Remember, when validating the data you don’t have to prove the data values are valid, just that they haven’t changed since the program wrote them.

As we get into how the position saving is implemented, you’ll see how I put these ideas into action, but first we need to look at how we are going to implement the capability from a high-level view.

Our Basic Approach

When adding in new or enhanced functionality, you want to do so in a way that requires as few modifications to (and has as little impact on) the existing structure, as possible. This ability to easily incorporate new functionality is a large part of the meaning of the term, “maintainable”. It is also why it is always good to think about your overall application in terms of functional blocks – or specific VIs that do specific things, and handle specific situations.

With that point in mind we know we have two basic operations we need to add: one sets the position when we open a VI and one writes a new position when we close it. Of these two operations, the simplest is the one that reads the last saved location and moves the window to that location. It’s simple because there is only one place in the code where that opening takes place, and that is right here:

Read Position Installed

This is the VI Float the VI.vi and if you compare it to the version that I presented last week you will note that it has one extra subVI that uses a reference to the VI being opened to look up and set the window position. we’ll look at exactly how it does that in just a moment. The operation that saves a VI’s last open position can also be boiled down to a single subVI, but due to the nature of our application, it will have to be installed in two locations.

Save Position Installed 1

Here’s the first of those locations. It occurs in the subVI Unfloat the VI.vi and it handles the case where the user closes any of the floating windows. Again you’ll notice one added subVI. Using the VI reference supplied to it, the subVI determines the target VI’s new window position and saves it. The other place where this VI occurs is in the event that stops the application.

Save Position Installed 2

Here the event logic checks to see if a window is docked, and if not calls the same subVI to save the window position of the VI associated with the reference.

Digging for Details

Now that we see where the modification fits into the application, let’s look at how the subVIs work – starting with the routine that saves the window position.

Save Window Location

As you can see, I am using the configuration file VIs to store the data in a text file using the INI file structure. However, it’s not the application’s INI file but rather one that I am creating in the user’s “My Documents” directory. This selection has at least a couple of implications. First it means that every user that logs into the computer will have their own set of customizations. Second, if the user wants to reset all their customizations back to default, all they have to do is delete or rename that one file.

Next, notice that this VI was designed to be usable in two different ways. If the VI reference input carries a valid reference the code uses that reference to get the data it wants to save. Alternatively, if the VI reference provided is not valid, the true case of the structure (not shown) open a reference to the VI calling this subVI and save the data for it.

Finally, let’s look at the subVI that the code uses to convert the window bounds data into the string that will be saved to the custom INI file.

Pack WinBounds

In keeping with the concepts I cited earlier, I obfuscate the datatype by flattening the structure to a string, convert the string into an array of U8s, and finally format the array as a string of 2-character hexadecimal values. However, before making that last conversion I provide a mechanism for ensuring data validity: I append a 16 bit CRC. The result is a string that will allow you to detect if it has been manipulated outside the program.

Turning now to the VI that retrieves the data, we see logic that reverses what was done in the position save routine. However, there is one added twist: if this VI is being called for the first time, the position of the target VI might not be in the INI file. Consequently the code needs to be able to recognize that situation and just let the windows open in its default position.

Read Location and Position Window

The subVI (below) that converts the string from the INI file back into the LabVIEW data structure for defining a windows bounds, generates a structure containing all 0s when it is passed an empty string – which is what you get when you try to read a string value from an INI file that doesn’t exist.

Unpack WinBounds

Also notice how this VI verifies the CRC. It’s commonly believed that in order to verify a CRC you have to split the CRC from the message, calculate a new CRC on the message and compare the result to the CRC received as part of the message. However, such is not the case. Due to the way the CRC calculation works, if you simply perform a CRC on the entire message including the original CRC the results will always be 0 for a valid message and CRC. Hence, the logic only goes to the trouble of splitting the message from the CRC after it has determined that the message is valid. In the case where the second CRC calculation is not zero (indicating a corrupted message) the logic outputs the same invalid data structure that you get from a null input string.

Given these facts, the VI for reading the target VI’s last position only has to look for that known-invalid data structure and if it finds it, bypasses the logic that set the window’s bounds.

Undockable Windows – Release 2
Toolbox – Release 10

The Big Tease

So there we have it, a basic framework that you can use to implement a variety of “window memory” functions. But what about next time? I have talked a lot about processes that run in the background. What happens though, if you want to be able to provide a minimal interface that isn’t always there but can be easily called up when needed. Next time I’m going how you can utilize the Windows system tray to house just such an interface. At the same time we’ll look at one of the more interesting backwaters of LabVIEW development – .net callbacks.

Until Next Time…


Using VI Server to Interact with Executables

We all want to be able to reuse code and a good way of accomplishing that goal is by repurposing executables that you wrote for other projects. The problem is how you control them. Last week we started addressing this challenge by looking at some of the general tools that are at our disposal for manipulating executables — regardless of where you got them. This time out we will complete the discussion by looking at some of the things you can do that are specific to LabVIEW-created executables.

First, we need an executable

As the title says, if we are going to talk about making VI Server calls to an executable, the first thing we need is an executable — and an executable, we have. Although the functionality it implements is, to be honest, rather sparse, it is sufficient to demonstrate what we need. Here’s what its front panel looks like:

A Small Test Executable - FP

Starting in the top left, it sports an indicator where you can see the command line that was used to launch it. Immediately below that string is an indicator showing the current time, a button for stopping the program manually, and a pair of LEDs that indicate when two different events are triggered: Application Instance Close? and Panel Close?.

To the right is a path indicator that displays one of two different paths depending on the state of the checkbox that is next to it. Below the path indicator are two numerics. One is the PID of the instance that is running. The other is the TCP/IP port number that was assigned to the executable when it was launched. Note that if you don’t provide a port number in the command line parameters, the executable will terminate almost immediately — though you may see it briefly appear in the Windows Task Manager.

Handling the front panel

Most of the code that makes this interface work is pretty straight forward, so I won’t take the time to describe it. The one exception is this bit in the initialization logic:

Intialization Logic

The reason that I’m pulling it aside for special attention is that it illustrates (at least part of) the solution for a problem that you will encounter the first time you create a VI that is designed to run entirely in the background. The heart of this problem is mismatched expectation: When LabVIEW runs an executable it expects to open the window associates with the top-level VI. You, on the other hand, wanting the executable is run unseen in the background, expect the window to stay closed. Consequently, what happens is that LabVIEW opens the window and starts the VI running in that order and your code immediately hides the front panel. What the user sees is a windows that open and then immediately closes without explanation, and if there is one things that worries users more than things happening too slowly, its things happening too fast — like windows flashing open and then closing.

The solution lies in a VI property called Transparency. The setting to control it can be found in the custom appearance dialog.

Transparency Dialog

When the box is checked and the percentage is set to 100, the window will be open but totally transparent. Hence, when the runtime engine launches the application, the window will still open but it will be invisible. A moment later, the code above will hide the window and set the transparency to 0 so that when we do decide to open it, we will be able to see it.

VI Server Operations

Last time, I presented this block of settings from the application’s INI file. Before we continue, we need to take a moment all consider what these settings mean — at least to the extent that anyone knows what they mean…

; server.tcp.port=3363
; server.tcp.serviceName=""

The first four parameters in this list control overall access to the application via TCP/IP. Consequently, they are the four that you are most likely need to muck with:

  • server.tcp.enabled=True: This setting enables the TCP/IP interface that VI Server uses. If this setting is False, nothing is happening.
  • ; server.tcp.port=3363: This setting specifies the port that the associated TCP/IP listener will be monitoring for a connection. Note that I have this line commented out because we will be assigning this value via command line parameter.
  • ; server.tcp.serviceName="": Also commented out, this optional parameter allows you to define a name that you can then use to reference the application, instead of a Port number.
  • server.tcp.acl=???: This setting defines the TCP/IP access control list (ACL) — or who is allowed to connect to the application. Already I can hear you wondering, what is the deal with that long string? Well, if you ever find out be sure and let me know. The bottom line is that the original interface included an ACL that simply listed the IP addresses that were and were not allowed to access the application. For reasons unknown, NI decided to change this common sense approach to something more enigmatic. So how do you generate this string? Glad you asked. According to LabVIEW’s documentation, you need to set up your development environment to have the same access list as you want your application to have, and then copy and paste the resulting string from LabVIEW’s INI file to your application’s INI file, seriously…

The remaining parameters specify in one way or another the specific resources that the remote program can access in the target application.

  • server.vi.access="+*": This parameter contains a list of VIs that are accessible through the VI Server interface. The default value shown allows access to all VIs.
  • server.vi.callsEnabled=True: This parameter specifies whether the remote program is allowed to run VIs contained in the executable. We leave this True so I can demonstrate that ability.
  • server.app.propertiesEnabled=True: This parameter gives the remote program access to the executable’s application-class properties — like the names of all the VIs currently in memory.
  • server.vi.propertiesEnabled=True: This parameter gives the remote program access to the VI-class properties of individual VIs. This category include things like a reference to the VI’s front panel.
  • server.control.propertiesEnabled=True: This parameter gives the remote program access to the properties associated with individual controls on VI front panels. For example, you need to have this parameter enabled to do things like programatically set the value of a control. This value is True as I will be demonstrating this ability as well.

Finally, I want to state one thing that I hope is obvious. All these “security” settings are contained in a plain text file that can be edited by anyone who knows how to use a simple text editor. The point here is that while recent versions of Windows are making it harder and harder to modify files in the “Programs” directories, it is not by any stretch of the imagination bullet-proof. Hence, if there are truly sensitive things that need restricted access, don’t depend on these settings.

What we can do with these controls

So let’s put some of what we have been learning about into action. If you download the code from the SVN repository you will find, in addition to the source code, a compiled executable. For the following tests, you can either run the executable I have included, or compile it on your own — it’s up to you. You will also want to be sure to update your copy of the toolbox as I have added a couple useful VIs. One of the executable management VIs is where we will start:

Launching the Executable

Start by opening small test executable.lvproj and then open the routine Launch 3x.vi. It’s job is to launch three copies of the test application (small test executable.exe) so on the front panel click the path browser button next to the path control and navigate to and then select the test application. Now, run the VI.

When it finishes, launch the Windows Task Manager. Nothing new under Apps, or Programs (depending on your version of Windows), so look in the Background Processes. Ah, there’s the executable, but why is it listed here? And why is there only one instance? The VI clearly looped 3 times, and there were no errors. Go to the directory where the test application is located and open its INI file. There are your answers. There is only one instance running because the INI file has multiple instances turned off, and the one executable that did launch shows up as a background process because the INI file also says to hide the root window. Leave the root window setting the way it is, but change the AllowMutipleInstances to True, and save and close the file.

Now back in the Task Manager, abort the one instance of the test application that is running now, and rerun Launch 3x.vi. You should see when it finishes that there are now 3 instances of the test application running. The instances were given sequential TCP/IP port numbers from 3365 to 3367.

Firing Remote Events

The next thing I want to do is open the front panels of the 3 instances so we can observe their operation. Now if you look at the test application’s source code, there is a UDE that will make the front panel visible, so all I need to do is fire that event. But wait, those are three instances of a compiled executable — you can’t fire events in other executables! Well actually, you can. To see how, open the test VI, Open the Executable’s Window.vi.

Open Executable Front Panel

No magic here. All the code is doing is dynamically running a VI. But check out the function before Open VI Reference, it’s called Open Application Reference. Its job is to open a reference to a copy of LabVIEW or the LabVIEW runtime engine that is running somewhere else. That “somewhere else” is defined in terms of a machine name and a port number or service name. The machine name can be a DNS name, an IP address or (as in our case here) localhost to point to the local computer.

By the way, if you think it sounds like I just said that you could make this same code access an executable residing on a remote computer by simply changing localhost to an IP address, your right. I did just say that.

But as cool as that feature might be, how does it allow me to fire an event in a compiled executable? Look at the name of the file being run: Open Window.lvlib:Generate Event.vi. It’s the VI that fires the event, and since VIs called in this way actually execute in the remote LabVIEW environment, the event gets fired in the targeted executable.

To see this code in action, run it three times with the port number 3365, 3366 and 3367. Three windows will open.

Setting Control Values

Another way of interacting with an executable is to directly manipulate controls on its front panel. However, if the target VI is event-driven like our test application, we need to remember that there is a difference between setting a value and firing any value change events associated with that control. If all you need to do is set a value, there is a VI method called Control Value:Set that will do the job nicely. However, if you want to fire the value change event you have to set the control’s Value(Signaling) property — which frankly is a bit more work.

set the selector control value with signalling

This picture is the block diagram of the test VI Toggle the selector checkbox.vi, but the good news is that for this little bit of extra effort, you can set (or read) any control property that can be changed while a VI is running.

Shutting Down the Executable

Finally, we need to be able to stop an application that is running. But the problem here is figuring out how to test it such that we can see that it really did what it was supposed to do. The solution is to turn to the trace technique we discussed a while back when we were learning about command line arguments. I have written the code such that if the executable is run with the argument “d1” in the command line, the code will write a line to the trace file saying how the instance was stopped. And to help demonstrate how this works, I have created a test VI (Stop the Executable.vi) that can execute some of these termination paths.

To start off, leave both controls in their default state, and run the VI. This example stops the targeted executable by clicking the Stop button on its front panel. The instance with the port number 3365 will immediately close.

Now increment the Port Number to 3366 and set the Method control to Windows shutdown - Forced. This example stops the targeted executable by telling to Windows to abort it. The instance with the port number 3366 will immediately close.

Finally, we want to test the remaining instance’s response to the Application Instance Close event. To do that, restart your computer now. (That’s right, restart your computer. Don’t stop anything, don’t shut anything down — just restart.) When your computer is restarted and you are logged back in, go to the directory where the test application is installed and open the trace file. You will see two lines that look something like this:

07:29:39 05/24/2015 -- Shutdown 3365 -- Just Stop
07:42:52 05/24/2015 -- Shutdown 3367 -- Appl Inst Close

The second line shows that when you restarted your computer Windows did in fact generate the Application Instance Close? event and the application caught the event. You’ll note that there is no entry for the instance with the port number 3366. Remember, we stopped it by forcing an abort and a Windows abort is very much like clicking the red abort button when LabVIEW is running a VI: It just stops. No orderly shutdown. No deinitialization.

A Small Test Executable — Release 1
Toolbox — Release 9

The Big Tease

So that was, I hope, interesting. Starting next time I’m going to start delving into how to use LabVIEW code as the data collection and control backend for an application that has as its only customer-facing interface a web site. While there are many companies offering options that claim to be developer-friendly I have found that many of the marketing claims are largely based on FUD (Fear, Uncertainty and Doubt). Simply put, they build up a “strong man” of supposed complexity and complication, and then tell you that the best (and perhaps, only) way to get past this obstacle is to buy their product. The truth, however, is that their “strong man” is really made of straw, and if you understand how it all fits together, doing it yourself isn’t really very hard.

Until Next Time…


Managing Standalone Executables

A while back when we were discussing opportunities for code modularization, I made the comment that it is even possible to incorporate processes into a new application that have already been compiled into standalone executables. At the time we didn’t have the time to go into detail on the point, but now we will take the time to consider the various tasks required to manage an existing standalone executable that you wish to leverage for a new application.

So What’s to Manage?

When I talk about managing an executable, I’m including all those tasks needed to control the executable from an operating system level. Therefore, to complete these tasks successfully we need a certain basic understanding of Windows and how it interacts with the programs that it is running. As you might imagine, most of these tasks will involve simply starting and stopping programs, but there are nuances that are important too. For example, when I double-click on a program the first time, Windows will launch it, but what if I double-click on it again? In this situation Windows has two possible options: launch another instance of the program or activate (bring to the front) the instance that is already running. One of the things that we will look at is how to specify that action for the executables we create,

Likewise, sometimes it isn’t enough to simply launch a program, sometimes we need to take high level look at its operation to determine such things as: How many instances of it are currently running or how much memory it is using. This information can help us decide how to best husband the computer resources that our application as a whole uses. Consequently, we will spend a little time considering the alternatives.

Finally, there can even be subtleties when it comes to stopping an executable. Can we simply signal the executable that it needs to stop, or do we need to force it to stop Now.

Getting Things Started

The first thing we want to cover is how to programmatically launch standalone applications. So to state the obvious, this topic is first and foremost about how to tell the operating system to launch the application for us. Luckily, this task is easily handled by the built-in System Exec.vi. This function allows you to do from within LabVIEW anything you would do from a command prompt — though at times the operations aren’t done in the exact the same way. In fact, for some older versions of Windows the differences can be significant. But even in Windows 8 there can be small inconsistencies. For example, say you have a program called Small Test Executable.exe that can accept a couple of command-line arguments like p3365 fpd2. You could launch this program from the command line like so:

C:\"Program Files\Test Executable\Small Test Executable.exe" -- p3365 fpd2

Unfortunately, this doesn’t work with System Exec.vi, which simply returns an (undocumented) error. To get the command to work you have to either write it to a temporary batch file and then execute the batch file, or modify the command like this:

"C:\Program Files\Test Executable\Small Test Executable.exe" -- p3365 fpd2

Don’t see a difference? Check out the location of the first double quote… To simplify dealing with these variations, I have created a subVI to encapsulate the functionality called Launch Executable w-Command Line Parameters.vi and included it in the new release of the toolbox.

Launch EXE with Parameters

The other thing about getting an executable running is that there are a few important launch parameters that we can only set in the application INI file, so we’ll deal with those next.

A Windows Convention

One of the conventions that Windows imposes is that the INI for an executable will have the name as the executable (but with the file extension ini) and the standard configuration parameters will reside is a section with the same name as the file (but minus the file extension). It is in this section that the runtime engine expects to find the parameters that it needs to successfully launch the executable.

The first such parameter we need to discuss controls the behavior when the executable is called more than once. By default, when you create an executable with LabVIEW only one copy of it will launch. If you call it again, Windows will simple bring to the front the one instance of the program that is already running. However, you can change this behavior by including this line in the INI file:

AllowMultipleInstances = TRUE

With this parameter set as shown, Windows will launch another instance of the program each time it is called. An important point to remember, however, is that no matter how many instances you launch, they all share the same INI file. Consequently, if you want to pass unique parameters to each instance, you will have to do so through the command line when the instances are launched.

The second INI file parameter I want to mention controls the program’s visibility to the computer user. Regardless of whether its GUI is visible, when a program normally launches it has a tab that appears on the taskbar. But what if you are creating a program that is intended to run unseen in the background? This setting handles that case.

HideRootWindow = True

When this setting is true, the program can still have a user interface but there is no tab associated with its window on the taskbar. As a side effect, the program also doesn’t appear in the task manager as an “App”, but rather lists it as a “Background Process”. Please note that this terminology is what Windows 8.x uses, older versions make the same distinction but use the word “Programs” and “Processes” to describe it.

The third INI file setting to discuss, is really not so much of a setting, as it is a family of settings that work together to control the program’s ability to respond to external connections via TCP/IP and VI Server.


Some of these settings are easy to understand, while others are rather obtuse. For right now, don’t worry about what these settings all mean. Next week we will.look at these parameters in detail.

Passing Parameters

To reiterate a point I made earlier, creating a standalone executable that will support multiple instances pretty much necessitates the use of command line arguments. In some ways, this situation is analogous to the situation concerning reentrant clones. The instance needs to be able to identify itself in the midst of a cloud of identical instances.

Given that point, one of the things you should consider when creating such an application is how the program should respond if the required command line parameters are missing. The answer to that question depends, to a large extent, on the nature of the parameters. If the input is essentially a customization for which there is a valid default value, it might be acceptable to simply accept that default and go on. However, in some situations there is no default value possible, so you should consider shutting down the program if the command line parameter is invalid or missing.

Getting Status

After we have gotten the instance (or instances) of a program up and running, one of the things that we might want to do is check on how, and perhaps what, it is doing. For now we will consider this matter in terms of the performance aspects that are visible to Windows. It is certainly possible to access the executable programmatically to obtain internal status information, and even control its operation, but that discussion will have to wait until next week.

General Information

One of the most basic indicators of the health of a program is its memory usage. You should expect the memory that a program has allocated will rise and fall over time. It should not treat trend steadily upwards without regard for what the program is actually doing. If a program’s memory consumption does consistently increase over time, that condition is referred to as a “memory leak” and it is a condition that needs to be addressed. Typically this remediation takes the form of figuring out why the program wants ever-increasing amounts of memory (like programming errors resulting in file or I/O being left open) and fixing the problem. Occasionally however, you will find that the leak is occurring in a place where you have no visibility — like inside LabVIEW itself. In those situations all you can do is try forcing Windows to deallocate the memory or change the way you are doing something so the problem doesn’t arise.

A simple way to read memory usage is to call the simple command line function tasklist and parse the result. Here is the code I wrote to do the job. Its name is Read Task List.vi.

Read Task List

If you leave the Image Name control empty, the call returns information on all the tasks currently running. However, if you populate that control with the name of a program, it returns a list of all the programs with that name. For example, on my computer right now LabVIEW.exe returns 1 item, while chrome.exe returns 13. (Why does Chrome need 13 instances of itself running in the background?)

The two most valuable pieces of information that you can get from this command is the application’s memory usage (expressed in kilobytes) and its PID. The PID is a numeric tag that uniquely identifies each program that is currently running. It is important because other commands or system calls will often require the PID of the program that you are wanting to check.

Digging for the Details

Beyond this basic information there is a variety of other details that you can find, For example, here you can find a community-developed VI that makes direct calls to a .NET assembly in order to return a plethora of information about your computer in general, and the programs that are running in particular. From the standpoint of accessing this information programmatically, this VI has two big problems:

  1. The code is at least 7 years old: This is a problem because there are places where the same operations could be done much more efficiently using techniques that LabVIEW has introduced in the intervening time.
  2. It’s essentially undocumented: To figure out how to really use it you will have to spend time researching the calls and the specific parameters — which can vary from one version of Windows to the next.
  3. It was designed for manual operation: In other words, it was designed to be used interactively, and not programmatically.

Because we need something that is usable from within a program, I created two subVIs that essentially repackage the functionality in a more program-friendly form. The first subVI is called Get Available Instances.vi. To see why this VI is needed, think back to the output of the tasklist function. In that output multiple instances of the same executable (like the 13 copies of chrome) all had the same name. They were distinguished from one another by a numeric id number called a PID. The designers of the .NET interface must have thought this approach confusing because they took a different approach that did away with the PID in favor of a name that was modified to make it unique — as in chrome through chrome#12. Likewise, any file extension was also dropped from the name.

Get Available Instances

The other subVI (Get Task Performance.vi) accepts as inputs an instance name and an enumeration that lists the available performance parameters that the VI can fetch under Windows 8.

Get Task Performance

But what is the deal with the loop and all the “extra” logic? This loop is needed because, while some of the parameters are instantaneous values that can be read at any time, others are not. As a case in point, consider the processor usage measurements. Everyone knows (or at least everyone in this business should know) that a CPU can really only do one thing at a time. In fact, the illusion that is central to much of modern technology is the programmatic slight of hand that makes it look like computers are doing multiple things at once.

Now since a CPU can only do one thing at a time, logically there are only two possible immediate answers to the question of how much processor are you’re using. Either you are not executing (in which case you have 0%) or you are executing (resulting in 100% usage). In short, for this type of measurement to have any sort of meaning it has to be approached as a statistic, not an absolute measurement. In the VI, this task is implemented by making two calls to the same measurement counter. The first call — which always returns a 0 — starts the measurement process, with the second and subsequent reads returning the statistical results for the previous measurement period.

Oh, and the logic for stopping the loop after just one iteration? That’s to optimize operation for parameters that are absolute.

Stopping: Fast and Half-Fast

It has been said that, “All good things must come to an end” and this is true for programs as well. However Windows actually provides two different types of shutdown events. The first causes Windows to simply abort the targeted program. This approach, while very fast, is logically equivalent to clicking the abort button on a LabVIEW program — and carries with it all the same dangers. The other type of shutdown event takes longer, but is much safer. It basically sends a message to the targeted program that requests it to stop. However, to reiterate something we learned before, for this messaging to work properly the executable must be written such that the Application Instance Close event will always initiate an orderly shut down.

In addition to using these messages for its own purposes, Windows also provides a command line function that allows users and individual programs to generate these messages as well. I implement both types of shut down requests in the subVI Task Killer.vi.

As a polymorphic VI, the routine incorporates instances that can shut down processes by name and by PID. You simply pick the version that gives you the level of control that you need. For instance, if I wanted to shut down all 13 instances of chrome, using the name version of the code would get the job done all at once. However, if I wanted to stop just a few select instances, I would use the version that uses the PID input. Here is the name version of the code.

task killer

But why start and stop at all?

Before ending this time, there’s one more idea that we should consider. To this point we have been thinking about the management of executables in a rather conventional sort of way: Start something just before we use it and close it when we are done. But might not there be situations where the “conventional” doesn’t make sense? I would assert that there are more than you might at first imagine.

The whole point of interacting with a process that is deployed as a separate standalone executable is that it obviously implements some bit of functionality that more than one application would need. Given that fact, starting and stopping it — especially stopping it — can get complicated. Just because one application is stopping and no longer needs it, that doesn’t mean there might not still be another application that does. One obvious solution to this problem is to simply let Windows handle it all. Install the background task such that it starts when either the computer boots, or the user logs in; and then stops when Windows shuts down. This solution might seem wasteful, but what is the real impact of such a solution? Oh sure, the program will be in memory, but if it isn’t being used it will probably be shuffled off to the page file pretty quickly. Likewise, if the executable is written correctly, it will be event driven which means that unless it is accessed by a program that is using it, the process won’t be burning up any CPU cycles either — so what is there to lose?

Just something to be thinking about.

Toolbox Release 8

The Big Tease

Ok, this is all for now. Next time I’ll introduce a little executable that will let us practice some of the theory we have examined today — and expand the conversation to include how to use VI Server to interact with executables created in LabVIEW.

Until Next Time…


Pay Attention to Units

The title for this post was a common quote from a favorite teacher of mine in high school. Mr Holt taught chemistry and physics and it seemed like I spent most of my life in his classroom — and I guess I did because in four years of high school I took 3 years each of chemistry and physics. Being a small school in a small Missouri town with a small budget we spent a lot of time (especially in physics) working on what would today be called, “mathematical modelling”. Back then, we just called it, “figuring out what would likely happen if we did something that we can’t really try because there isn’t enough money available to actually run the experiment”.

It was in those classes that I began to develop an understanding of, and appreciation for, all the pieces of contextual information that surrounds numbers. For example, if you have the number 4 floating around in a computer the most basic thing you can define about that number is what that 4 is counting or quantifying. Is it the mass of something (4 pounds)? Is it the cost of something in England (also, interestingly, 4 pounds)? Or is it a measure of how much your college roommate could drink before passing out (4 beers)?

In my work today, where I sometimes still have budgetary constraints, the need to remember that I have to, “…pay attention to the units…” remains. The difference is that now I have a powerful ally in the form of the facility that LabVIEW provides for assigning units to floating point numbers.

LabVIEW Units Aren’t Smarter Than You Are

Before we get into the meat of the discussion, however, we need to deal.with a few concerns and consideration that developers — even some very senior developers — have about using units. While there are a few behaviors that are clearly bugs, there are many more that people think are bugs but really aren’t. We should, of course, not be surprised by this situation. Anything related to floating-point will exhibit similar issues. For example, there used to be claims that LabVIEW didn’t do math correctly because the results of some calculations were different from what you got from Excel. Such claims ignored the fact that at the time Excel did all math in single-precision, while LabVIEW has always used double-precision math.

Similarly, a lot of the upset over “bugs” in units handling comes from an incomplete understanding of the entire problem. The simple truth is that there are a lot of things we do unconsciously when handling values with units so when we try to automate those operations, things can get messy. For example, we realize that sometimes when we square a number with units we want to square the unit, so meters become meters2. But sometimes we just want to square the number. Does it after all really make sense to convert volts into volts2? In implementing the units functionality, NI had to deal with similar ambiguity in something approaching a systematic manner so they made some decisions and then documented those decisions. So we may disagree with those decisions, but disagreement is not the same thing as a bug.

Another complaint that you sometimes hear is that using units complicates simple math. People will say things like, “…all I want to do is increment a number and LabVIEW won’t let me.” Ok, let’s look at that problem. You say you want to add 1 to a value, but 1 what? Even assuming a given type of unit, like length, how is LabVIEW supposed to know what you want to do? Do you want to add 1 meter, 1 furlong, 1 light-year?

Finally, there can be issues caused by shortcuts we take in our everyday thinking and communications, that are (to put it kindly) imprecise. For example, there are two units that can be, and often are, referred to as “PSI”. However, one is a force and one is a pressure, so mathematically they are very different beasts. In the end, a good way of summarizing a lot of this discussion is that using explicit units requires us to be more precise and careful in our thinking — which is always a good thing any way.

How Units Work

The basic idea behind LabVIEW’s implementation of units is that it draws a sharp distinction between how a number is stored and how it is expressed. The first step in making this distinction is deciding what kind of unit the developer selected and then expressing the value they entered in the “base unit” for the type of unit being used. For example, lengths are converted meters, temperatures are converted to degrees Kelvin, and time is converted to seconds and it is this converted value that exists on the wire. Likewise, when this wire encounters an indicator, LabVIEW converts the value on the wire to the units specified in the indicator before display. It is this basic functionality that makes simple math on timestamps possible.

pi time

In this snippet, each data source (the constants) converts the number it contains into seconds using its associated time unit. For example, 3 hours is converted into 10,800 seconds (60 x 60 x 3), 14 minutes is converted into 840 seconds (14 x 60) and the last input is left as is because it is already in seconds. Because all the numbers are now expressed the same unit, we can simply add them together and then add the result to the timestamp — which is also expressed in seconds. By the way, this is one of my favorite places to use units because it does away with so many magic numbers like 86,400 (the number of seconds in a day).

In terms of selecting the units to use, we have already seen one technique. On a control or constant you can show the units label and then enter or select the unit you want. To select the units, right-click on the units label and select the Build Unit String… option from the popup menu. The resulting dialog allows you to browse the standard units that LabVIEW supports and select or build the desired units.

However, if you have an existing number to which you want to apply a unit, you need to use a Convert Unit node. It looks a lot like the Expression Node but instead of typing simple math expressions into it, you enter unit strings.

unit conversion nodes

This snippet is one that I sometimes use when specifying timeouts. Note that a Convert Unit node is used to both apply are remove units.

When applying units, the unit string tells LabVIEW how to interpret the unitless number. It does not, as some suppose, specify the specific unit that value will have. For example, you may tell LabVIEW to interpret a particular number as feet, but internally the value will still be stored (like all lengths) in meters. When removing units, the node’s unit string tells LabVIEW how to express the internal value. In the above snippet, time is always carried as a floating point number of seconds, so the node driving the output causes LabVIEW to express the number of seconds as milliseconds.

Dealing with Temperatures

Another place I like to use units is with temperatures, but let’s be honest things can seem to get a bit strange here — or it can seem strange until you understand what is going one. The root of this apparent strangeness lies in the fact that there are absolute temperatures (like the room is 72° Fahrenheit) and there are relative temperatures (like the temperature went up 5° Fahrenheit in the last hour). Moreover, as we think about and manipulate temperatures we unconsciously, and automatically, keep track of the difference between the two. Unfortunately, when using a computer nothing happens unconsciously or automatically. Things happen because we tell them to.

The first thing NI had to decide when implementing this functionality was what base unit to use for temperature, and they picked Kelvin because it is metric and 0° K is the coldest something can get. Being metric is important because it means that the magnitude of change in 1° K is the same as for 1° C. By the way, did you notice what your brain just did? It went from thinking about absolute temperatures to thinking about relative temperatures — without thinking about it. This is what computers can’t do. Computers need some sort of indication or hint to tell them how to think about things, and in the context of temperature, this hint needs to provide a means for drawing a distinction between the units used for absolute temperature and units for relative temperatures. To see this issue in action, let’s revisit the previous time example but recast it to use temperature unit:

bad temperature calculation

Here we are taking a temperature value 20° C and incrementing it by 5° C. Unfortunately, when you run this example, the result is 298.15° C. “But,” you protest, “I thought that converting everything to the same units allowed you to perform any math you wanted with impunity. Where did this wild result come from. Surely this is a bug!”

Actually, it’s not. The only “problem” is that LabVIEW did exactly what the programmer told it to do, and not what they wanted it to do. LabVIEW added together two absolute temperature values and came up with an answer they didn’t expect. While its true that adding absolute temperatures probably doesn’t make any logical sense — how is LabVIEW supposed to know that? The real error is with the way the developer framed the problem and not with LabVIEW or its handling of units. What the developer obviously intended was to add a relative temperature value to the absolute temperature. To do that, you have to change the units of the increment input.

The way that LabVIEW identifies a relative temperature is to change the unit label from degC or degF, to Cdeg or Fdeg. By the way, now that you have the unit definitions correct, you can freely mix and match temperature scales. For example, you can add a 5° F increment to a 23° C room temperature and LabVIEW will happily give you the right answer (25.7778° C).

good temperature calculation

More Calculations

Other math operations can sometimes be confusing as well. For example, we have already discussed why you can’t simply increment or decrement numbers with units, but multiply and divide operation do not have the same restriction. You can multiply or divide a number with units by one without units with no problems. This apparent inconsistency makes sense when you remember that these two math operations used in this way simply scale the value that is there, regardless of how it is stored internally. Also NI has decides that many operations only operate on the numeric value. For example, if you square 5 meters using the square function, you get 25 meters. If you want to operate on the unit, you have to use the multiply function which will generate any unit squared.

But moving beyond these issues, there are many operations that using units will simplify, and even prevent you from making some kinds of logical errors. For instance, dividing a distance by a time will automatically generate a velocity unit (meters/sec by default). In the same way dividing a velocity by time will calculate an acceleration. And these conversions work the other way too: multiply a velocity by time and you get a distance (by default, meters).

The only negative here is that some of the unit labels can look a bit odd at first glance — though they are mathematically correct. For example, meters per second is shown as s^-1 m. However, despite the strangeness of this representation, you can alter the unit displayed by changing either of the base units shown. If you wanted feet per second, the unit string would be s^-1 ft, or miles per hour would be h^-1 mi. By the way, if you want to hide these potentially confusing representations from your users, the units string is a writable property so you could create a popup menu on a control that shows prettier labels.

unit selector

Note that he enumeration making the selection from the array has three values: Miles per Hour, Kilometers per Hour and Meters per Second.

Carrying on further, the units understands Ohm’s Law. Divide a voltage by a resistance and you get amps. Multiplying ohms and amps returns volts, and the calculations even know that the reciprocal of a frequency is time. All and all, this is some very cool stuff here that can significantly improve your code — and perhaps prevent you from making some silly mistakes. But as with all things new, work into it slowly, and thoroughly double-check the results when you try something different.

The Big Tease

There seems to be rumor running around that I don’t like queues. Well that is not true. They have a couple features that make them uniquely qualified for certain kinds of operations. My next couple posts will demonstrate a really good place use a queue — a data processing engine for manipulating data that will dynamically resize itself on the fly to provide more or less processing bandwidth.

Until Next Time…


Command Line Arguments aren’t a relic of the past

Back when phones were something with wires attached to them, computer programs had nearly non-existent user interfaces. However, the need still existed to be able to pass data to programs. The standard way that developers used for accomplishing this vital task was through cryptic codes called command line arguments. These codes which were cryptic by necessity (a data entry line couldn’t be more than 80 characters), were simply appended to the name of the program the user was wanting to run.

Despite the advances that have come along over the years, operating systems still support command line arguments — and LabVIEW applications still sometimes have a need for them.

How they work

Even today, commands still get sent to operating systems in the form of strings, and curiously, the primary delimiter still used to separate the name of the program from other parameters is the space. This is why if you look at the shortcut for a program, the path to said program is almost always in double quotes: path and program names contain spaces.

The way the process works is that anything before the first space not inside double quotes is considered to be the name of a program and is sent to the part of the OS that is responsible for launching stuff. Everything after that first space is sent to the program that is being launched, which is then responsible for determining whether the remaining string contains valid information or trash.

When LabVIEW (or the LabVIEW runtime engine) parses the remaining text, the first thing it looks for is an initial delimiter in the form of two hyphens in a row, like this:

command line arguments

Anything after the hyphens are returned to the program through an application property as an array of strings. Note that the double hyphens have a space on either side.

But what’s it good for?

Unfortunately, while it may be easy to describe what this feature does, it can be tough to identify a good use case. To help us identify where this feature might be useful, let’s consider some of the technique’s major attributes.

  • It’s not very convenient
    Because the technique depends on creating what is essentially a custom command for launching your application, it can only be used within the context of a Windows shortcut — and did I mention that you have to create the shortcut manually?

  • It can still be pretty cryptic
    Although you have complete freedom in defining how the arguments are formatted and what they do, you still have a maximum line length constraining how verbose your complete command line can be.

  • It’s not secure
    The command line arguments are unprotected and can be changed by anyone with even a smidgen of technical savvy. Moreover, they can be bypassed completely if the user simply chooses to launch the program by double-clicking the application itself.

  • It’s error-prone
    There is no way to error check arguments before they get passed to the application. Consequently, the application needs to be very careful and validate all inputs.

Doesn’t sound too promising does it? Clearly command line arguments not suitable for anything critical, so the application needs to run just as well if the arguments aren’t there. Likewise, you usually don’t want to have users needing to muck around with them. Well, believe it or not there is at least one bit of functionality that falls well within this technique’s capability. Check it out.

Tracking execution

A common problem can arise when you deploy your application as an executable and it doesn’t work on the customer’s computer. Although, LabVIEW provides facilities for remote debugging that are really nice, it sometimes isn’t usable. First of all, to use it requires you to make a special build of your application that includes the remote debug capability. Second, this debugging technique assumes a level of network access that might not be available. Third, because you don’t want to sit for hours connected to a customer’s computer waiting for something bad to happen, it is only useful for problems that you can quickly and easily duplicate. However many problems are difficult to recreate on command.

One of the earliest software troubleshooting techniques can be particularly useful in isolating this type of intermittent error. The technique involved inserting code in your application that simply prints program values to a log file. These log values can range from critical internal values like the result of an intermediate calculation, to an electronic version of bread crumbs that simply says, “I made it to the error checking state”. But if you are going to include this sort of logic, you are also going to need some way to control it. After all, we are trying to avoid creating special “debug” versions of our code, but every installation doesn’t need to be generating debug files all the time — this where command line arguments comes into play.

The basic idea is to go through you code and identify places where information is available that would be of potential value in debugging, and then create VIs that will write that information to a file. To save memory, you can even make these trace VIs dynamic so they are only loaded if they are enabled. But how do you enable them? A structure I often use is to assign each potential trace operation one bit of a U32 number. I pass the bit-mapped number into the application using a command line parameter, and store it in a FGV. Then each trace operation looks at its bit in the number and only generates its trace if the bit is set.

Putting theory into practice

To see how this approach would actually work, let’s add some trace capability to our test bed application. The following figure shows the mappings of trace operations to bits:

bit mappings

Note that we will actually be using 10 bits because three of the trace operations are in our reentrant temperature controller state machine, and we want to be able to control the trace for each clone individually. Next, we want to define the syntax for the command line parameter. To keep it simple, we will use the structure I showed above, where the value following the “d” is the bit-mapped number that will provide our enables. To generate this number, simply total the numeric values for each on the trace options you wish enabled. For example, the argument “d1” would turn on just the Sample Period Change trace, while the argument “d146” (2+16+128) would turn on the Fan State Change trace for all three temperature controllers.

In case you’re wondering how users are supposed to generate these magic numbers, that’s easy — they don’t. The intended use is that a support person will tell the customer what to enter with the instruction, “When you see the problem we are troubleshooting, go to the directory where the application is installed. You will find there a file named ‘trace.log’. Email it to us.”

Reading the arguments

In the LabVIEW environment, you gain access to the command-line arguments by reading the Command Line Arguments application property. The data is returned as a 1D array of string that LabVIEW creates using the space as a delimiter between elements. This property is always available in the development environment, but in a runtime system you have to enable it in the Advanced section of the application builder parameters. To retrieve our debugging parameter I have built a VI that searches this array for the first element that starts with the letter “d” and then converts the remainder of the string into a number.

initialize debugging

Once it has isolated the number, the code converts it into a Boolean array and stores the result in a FGV for later use.

Creating the traces

With the array of Boolean values safely tucked away in a FGV, we can begin building the code that implements the trace functionality. However, not being stupid, the first thing I do is create a subVI that will format the trace entries in a standard way. Assuming there were no errors generated before the trace operation is called, each entry will consist of a timestamp, the name of the trace point generating the data, and the trace data itself.

save trace data

If an error did occur earlier in the error chain, the trace subVI instead saves the error. Ideally, all errors should be saved to the error log as part of your program’s normal operation — but mistakes in propagating errors can easily happen.

save trace data - error

With that foundation laid, we move up a layer in the code hierarchy, and the first application-specific trace option we create is one that will record changes in the sample period that the two acquisition processes use to pace their operation.

sample period trace option

Thanks to the subVI created a moment ago, you can see that all the trace VI really has to do is check to make sure that its bit is set in the array of Boolean enables from the FGV and (if it is) write pass the trace data to the subVI. To simplify things, I also built a subVI to encapsulate the indexing process and return an enumeration giving the trace state.

The remaining options we want to implement are for the temperature controller. First, we want to save a trace message whenever the fan or cooler state changes. This is the trace VI for the Fans State Change trace. The Cooler State Change trace is almost identical:

fan state change trace option

A new challenge that this code needs to address is that for it to work properly, the VI needs to know which of three clones is calling it. To enable this new requirement, I added a new parameter to the data that is passed to the clones when they are launched. Called Launch Order it tells each clone its ordinal position in the launch sequence. With that number and a little basic math, the trace VI can calculate the correct index for its enable.

The last trace operation (also in the temperature controller) records the state machine’s state transitions. Due to its similarity to the others, it’s not worth showing its code. However, this trace option is a good example of a type of operation that can be very helpful, but which you want to be sure isn’t left enabled for an extended period of time. Every state change will be recorded — an operation that can generate a lot of entries really fast. Consequently, if you put in this sort of trace you should think about augmenting it, for example, with the ability to only report certain state transitions, or perhaps only save the last 100 trace messages.

Testing our work

To see this code in action you can build an executable, or run it from within the development environment. In either case you need to start by creating a shortcut on your desktop that points to either LabVIEW.exe or Testbed.exe. Next, edit the shortcut to add in a debug argument with a numeric value of you choosing.

Now use the shortcut to launch LabVIEW or the testbed application and monitor the project directory. A trace.log file will appear and be updated as the application executes. A handy tip is if you are going to be testing a number of set configurations, it can save time to create a number of shortcuts with different debug configurations preconfigured.

Testbed Application Release 14
Toolbox Release 7

So this is one use I came up with for this technique. Where does you intuition and imagination say to try it? In the mean time, the requisite teaser for the next post…

Objectifying LabVIEW

The National Instruments training course on object-oriented programming recommends trying it in small doses as you are learning — an attitude I heartily endorse. Next time we are going to take a small step in that direction by developing a more general solution for at least part of our data management. Along the way we’ll discover that while it may not be the ultimate programming paradigm that will revolutionize civilization as we know it (as some Object-Orient Fanboys proclaim) it can still be real useful — even in small doses.

Until Next Time…