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:
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.
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.
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:
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.
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.
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.
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.
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:
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
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.
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…
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…