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Is the while loop really a while loop? No. Strictly speaking the while loop is a "do while" (C) or repeat until (PASCAL) programming structure. In other words the loop always executes at least once, and the Boolean test for continuation is done at the end of the loop execution. With a true while loop in a text based program the test is done prior to executing any of the commands contained within the loop. To create a "real" while loop wire the output of your terminating condition to a case structure surrounding the rest of the code within the loop. How do I stop a for loop? A for loop by definition executes the requisite number of times (N). If the number of iterations is not known then you have to use a while loop. Is there some religious prohibition against the while loop that precludes its use? There are ugly, inelegant things you could do by putting a case statement in the for loop to avoid doing anything after some number of iterations but there is no reason to do this except to introduce wasteful, inefficient, useless code. Use a while loop, you will like it. An underappreciated feature of while loops is their ability to index arrays or produce array outputs similar to for loops. To do this "enable indexing" on the wire coming into or out of the while loop. Likewise you can disable indexing on for loops to avoid the indexing feature. What does a 0 msec wait function do in a while loop? If you have multiple loops in your application that don't need to run as fast as possible, then placing a 1ms delay will limit your loops to at most 1000 iterations per second. Without a delay, the loop rate can be in the millions of iterations per second range depending on what is inside it. So that means that your CPU has about 1/1000th as much work to do and can go off and tend to other tasks. If your loop was already taking several ms, then the 1ms delay is likely in parallel and it won't affect the loop speed noticeably. So placing the delay in the loop can drop your CPU usage noticably and allow time for the OS to do other work like send messages that your application may be waiting for, etc. But what does a wait of 0 ms do? Let's consider the LV execution system to be a xerox copying machine. Everyone that has something to copy heads for the copying machine and lines up. If you have never had the pleasure of waiting at a copy machine, then consider any other time consuming task where people wait in line. Everytime a LV diagram executes a wait function, it is like releasing the copier and staying out of the line for some time delay. After the delay has passed, you will get back in line to make your next copy. A wait of 0 ms is like immediately going to the end of the line to let a smaller copy-task take place. If nobody is in line behind you, you immediately start your next copy task. If someone is in line, it lets them take a turn. This 0 ms wait is a pretty cool little trick to make LabVIEW loop parallelism less chunky. It naturally adds some overhead because the loops have some setup time, but when the loops are doing significant work it is tiny. Use it whenever you think you need to, but beware that if some loops don't have any delay, they are still going to hog the CPU, and the wait of 0 ms may turn into much larger waits because the loops with no waits play by different rules. How do I execute a shell command via system exec? The exec VI function can be used to send shell commands. Include the shell call with the commands that are desired. For example under Windows operating systems the call is to command.com. For example, the string to copy filea to a drive would be command "copy filea a:" with the arguments enclosed in quotes. The exact shell call will depend on the operating system. What is a state machine? The state machine is a convenient LabVIEW construct where a case structure is contained with a while loop. Execution of particular cases in the structure is determined by the output from the previous case (or in the instance of the first execution) by the control selector input. The control of the order of case execution is controlled through the use of a shift register. By wiring the output from one case as the determinate to the subsequent case (wiring to the shift register on the right hand side) and wiring the input to the case selector from the left hand side shift register, operations can be ordered according to which state is executed. The state machine has the advantage of allowing several steps to be linked in series so that each individual step can be executed (or debugged) easily. The trap for the inexperienced programmer is the use of the state machine to step in a haphazard fashion through the cases in the loop. This can lead to complicated pathways through the state machine reminiscent of convoluted goto statements in conventional text based languages. However state machines are a valuable tool and can be used very effectively with this in mind. How do I control the order of execution in a state machine? There are three possible ways to order a LabVIEW state machine. The first and easiest method (although NOT recommended) is to use an integer. The integer output from one case is wired as the selector for the subsequent case. The disadavantage of this method is that the numbers themselves are not very helpful in tracking through the code and the case selector becomes somewhat meaningless. Either of the following methods are preferred and there are pros and cons for each of them. Debate (strong at times) arises on the list quite frequently as to the best of these methods, but in practice either will work very nicely. The second method is to use a string as the case selector. With the correct choice of string states, this has the advantage of labelling each case so that your state machine becomes self documenting. A further advantage of this method is that all cases need not be written in the first instance and it is somewhat easier to include new states should the need arise (especially in earlier versions of LabVIEW (< 6.0)). The disadvantage is that it is easy to misspell a state and have the state machine buggy because of his. An easy way to overcome this disadvantage is to include a default case that pops up a warning including the spelling of the failed case. The third method is to use an enumerated type as the case selector. In preference this enum should be a typedef'ed so that changes to the states are refleced easily and transparently when new states are added to the enum (via the LabVIEW auto update from typedef setting). Note that there are reports where this method causes problems in version of LabVIEW prior to 6.0, because the enum auto updating did not correctly handle newly inserted cases. This method has the advantage of the string method in code documentation, but requires the use of a default case if not all the cases are required to be written when the code is first created. How do occurrences occur? There seem to be misconceptions about how exactly occurrences work and what exactly the "ignore previous" parameter on "Wait on Occurrence" (WoO) does. The following tries to explain, in a long and winded way, how they (functionally) behave: How Occurrences are "generated": When a VI is loaded (!) each "Generate Occurrence" function allocates exactly one unique occurrence. When the VI is running and this function is called it simply returns this one occurrence -- no matter how many times it is called (try putting one in a loop and examining its value using the probe, it will always have the same number). If you stop the VI and run it again, you will get the same value; only removing the VI from memory and loading it again will give you a "fresh" value. How Wait On Occurrence (WoO) works: Each WoO functions "remembers" what occurence it last waited on and what time it continued (because the occurence fired or because of a timout). When a VI is loaded (!) each WoO is initialized with a non existing occurrence. When a WoW is called and "ignore previous" is FALSE there are 1) The occurrence has *never* been set -> in this case WoO waits 2) The occurrence has been set since this WoO last executed -> WoO does not wait 3) The occurrence has last been set before this WoO last executed and last time this WoO was called it waited on the *same* occurrence -> WoO will wait 4) The occurrence has last been set before this WoO last executed but last time this WoO was called it waited on a *different* occurrence -> WoO will *not* wait!!!! The first three cases are pretty clear, the last one may seem a bit strange. It will only arise if you have a WoO inside a loop (or inside a *re-entrant* VI in this loop) and it waits on *different* occurrences (out of an array, for example) or if it is inside a *non re-entrant* VI and the VI is called with different occurrences. These cases do generally not happen. The reason the WoO behaves this way is due to its implementation. Each A possible (implementation) problem: It appears that occurances are "remembering" that they have been set during previous invocations of the program. One would think that generating an occurance should create a clean "non-set" occurance. This problem can be illustrated in a program that has three parallel loops with an abortable wait in each using occurances. If the program is stopped with the stop button things are fine. But if one waits until one of the random stop conditions triggers the end of the loops (generated from the other loops), the next time the program is run, the loops will execute only once and not loop at all. (The random terminate condition in the actual program is an error occuring in some piece of equipment.) Either this is a bug, or we are completely wrong on the use of occurances. Our example here is using the occurances in the "do not clear previous" mode. We would think that we will not remember ccurances from previous runs of the program since a new clear occurance should be created with the generate occurance icon. In this instance we can not use the clear previous occurances mode since we need a single occurance to stop multiple parallel loops. The reason for the problem: The first time the occurrence is set because of an error, the loops terminate, but the "stop button" loop is still running. When you click on the stop button, the occurrence gets triggered again (unneccessarily) and the program stops. The next time the VI runs, the WoO will not wait because of this extra trigger; and since you'll trigger again in the "stop button" loop, the VI won't work until it's reloaded from disk. A solution to the problem: Due to this behavior of occurrences, it is clear that one cannot use the "timed out" flag to determine when the occurrence fired. You will have to maintain some global information about your state, let's say in a global boolean called "FINISHED". At the beginning of the program you would initialize it to false. If you have an error, set FINISHED to true, and then trigger the occurrence. After the WoO see if FINISHED is true (make sure you don't read the global until *after* WoO finished executing), if FINISHED is false continue with the loop. In the "Stop button" loop, also set FINISHED before you trigger the occurrence. BTW, if you don't like globals, you could use a VI with an uninitialized shift register (LabVIEW 2 global), but the effect would be the same. Comments: One may think that occurrences are quirky, a pain to use and that they Why does an typecast enum output not change? If a typecast function is used to convert a number related, for example, to the button pressed (in an array of buttons) to an enum type then a problem observed in that the type cast doesn't seem to work. The button press is changing the number on the input, but the enum output does not change. The answer is that most likely the two inputs to the typecast function have different representations, probably I32 and U16. The type cast is reading the high order bits of the I32 (which aren't changing) and casting them into the U16 enum output. The solution is to convert both to representations having the same length (signed and unsigned mix fine), either by changing the representation of the controls themselves or by using a convert function at the input to the type cast. How do I make cool looking icons smaller than the entire square? First, draw the border for the icon in the B&W icon view. Be sure to align the icon to the wiring terminals by using the Show Terminal check box. This defines the icon outline. If you have no border drawn in the B&W view, it defaults to the whole square (in my limited tests, you get a "no outline in B&W view default will be used" dialog). The icon border will actually be the largest of the drawn areas in the three icon views. So actually, you need only draw a single dot on the B&W view to define your 256 colour drawing as the border limit. Something must be drawn in the B&W view to get the selection border for the icon on your diagrams to conform to the drawn outline (is this a useful feature? seems like you could have nothing drawn in this view by defaults and have it still work). Second, Click on the 256 colour icon view (if that's what you use), and click Copy from Black & White to get your border. Then finish drawing the icon to your liking, link it up, and presto, small icon. Wires to border, transparent outside, etc. You can even make irregularly shaped icons. The white area outside will be transparent in your diagrams. Interestingly, if the border you define is not contiguous, the inside of your icon will also be transparent (you can see a wire go all the way to its connection point (visible as a little cross in the "show terminals" icon view). You can actually have multiple non-transparent areas in your icons if you wish. Very cool! You could have a wire pass between two blobs to represent some subtle transformation on the data if you really wanted to go all out, but this actually requires drawing wires between the connection points on the icon to make them look contiguous. Are there any other icon tricks I should know? Double clicking on some (any?) of the tool icons in the editing palette will apply that tool to the entire icon square. For instance double click on the dotted square selection tool will select the entire icon area (handy for deleting the icon as a starting point in making small icons). Double clicking on the bordered square will draw a border around the current icon in the currently selected colour, without affecting the inner pixels, etc. Why doesn't my FOR loop return a value? As a good programming rule in LabVIEW, NEVER output a value from a for loop with indexing disabled. When the loop does not execute (0 iteration) a non indexing output stays undefined and will hold any garbage left there by previous memory usage. This is because the wire output from the for loop has no code or data source to get the value from when the loop does not execute. Instead, use a shift register (SR) to output the value and at the left SR enter a default value for the case when the loop does not execute. When the for loop does not execute this default value of the left SR is passed to the right SR. Similarly passing a refnum through a for loop that never executes (0 iterations) destroys the reference. Either pass the reference using a SR as described above or wire around the loop. Note that while loops always execute at least once so the outputs are always defined. What is a reentrant VI and what are the implications of reentrancy? First, consider three VIs, foo.vi, bar.vi, and sub.vi. Both foo.vi and bar.vi call sub.vi. If sub.vi is normal (not reentrant), then if foo.vi tries to call sub.vi but sub.vi is busy servicing a call from bar.vi, then foo.vi has to wait. This can be both a very GOOD thing and a very BAD thing depending on circumstances. It's very GOOD when sub.vi controls access to something like a serial port, where you only want one part of your program accessing it at a time. It's very BAD when sub.vi controls something like ALL the serial ports, because you may want foo.vi to be able to use one serial port while bar.vi is busy using a different serial port. Another very bad circumstance is where foo.vi is in a critical loop and bar.vi is not, yet because of the contention for sub.vi, bar.vi can end up blocking foo.vi. If sub.vi is reentrant, then both foo.vi and bar.vi can call sub.vi at the same time. In order for this to work, each call to sub.vi needs to have its own "data space" which is all the internal storage sub.vi uses in order to execute its code. Now at this point I need to point out a distinction between LabVIEW and most other languages. LabVIEW doesn't want to allocate a data space on the fly, because for LabVIEW that would slow down performance. LabVIEW allocates all the VI data spaces it needs when VIs are being loaded. Except when you use VI Server to call VIs dynamically, all the loading happens before any VIs execute. Therefore, for each place that sub.vi appears on foo.vi's block diagram, a copy of sub.vi's data space gets embedded in foo.vi's data space, assuming sub.vi is reentrant. If sub.vi isn't reentrant, it just has its one data space allocated that each call will use in turn. In most (all?) other languages, reentrant functions allocate their data spaces on the fly, so there's no storage that goes with each place that a particular function is called. How does LabVIEW's unusual implementation affect us in practical terms? There are really two ways: 1. If you use uninitialized shift registers to store information, then you can get two different behaviors depending on the reentrancy of your VI. For a non-reentrant VI, you get a data sharing function that lets you move large quantities of information between parallel loops without making copies of it. For a reentrant VI, you get a reusable storage function that can keep independent copies each place you use it. There is a PowerPoint presentation and some example code related to uninitialized shift registers at: 2. The second implication is that you can't do recursion (functions that call themselves) easily in LabVIEW. In most languages, if a function is reentrant then it's OK for it to call itself. In LabVIEW, that would require that the data storage for sub.vi would include a copy of the data storage for sub.vi which would ... to infinity. You can do recursion in LabVIEW if you use VI Server to have a VI call itself dynamically, but as I said, allocating data spaces on the fly is inherently slow. In LabVIEW, it's best to convert recursive algorithms to their iterative equivalents, which I hear is mathematically proven to always be possible. In the iterative version, you'll end up changing the sizes of arrays at each iteration, which is also one of the slower operations in LabVIEW, but is not nearly as slow as dynamic VI calls. And Greg McKaskle added: To expand on this, reentrant means that more than one execution is allowed to take place at the same time. In other languages, it is more a situation than a setting. You never mark a C function as allowing or disallowing reentrancy, it is either safe to do so or a source of bugs. In LV it is a setting, and many times its setting doesn't affect the correctness of a VI, but in some cases, it can be a source of bugs. It depends on what the VI does. The setting in LV determines two major attributes about how a VI executes. First is access. With reentrancy turned off, only one call to the subVI can be active at a time. When the current call finishes, the next one can begin. The subVI calls queue up while the VI is busy. For functions that execute quickly, this is normally fine and reentrancy doesn't affect much. If you have a function that uses TCP to talk to another computer and waits for responses, these waits also affect the other subVI calls that are queued up. So if you have an operation that can occur in parallel and doesn't consume the CPU, you can make the VI reentrant and the multiple subVI calls don't enter a queue, and multiple VIs can talk TCP and wait for responses at once. This allows the wait time of one subVI to be used as work time in another and increases overall performance. On the otherhand, given a VI that reads a global modifies it and writes it back, a reentrant subVI means that more than one subVI call at a time can be modifying the global -- a race condition which will cause incorrect answers. Lots of real-world devices also get confused when more than one subVI tries to control them at a time. So when trying to protect a global resource, the one of the tools, and frequently the easiest to use is to simply make sure that the access goes through a non-reentrant VI. The second attribute is data side-effects. If a VI has unconnected controls or uninitialized shift registers on its diagram, then it remembers some amount of information from call to call. A good example of this is a PID or a filter. Data from previous calls affect the result of the next call. For these sorts of VIs, if they are reentrant, then each call gets its own place to store the previous call's state information. If made non-reentrant, there will be only one storage location for all calls to share, so the data will get all jumbled, likely causing an incorrect answer. What are some of the pitfalls of the event structure? In LabVIEW v6.1 a new structure was introduced: the Event Structure. This structure allows the user more power over the UI, but at the same time with the new power comes the potential to lock a LabVIEW application hard. Below are a compilation of tips for using the UI Event Structure: 0. Read the LabVIEW Help on "Loops and Case Structures, Case Sequence and Event Structures, Event Structures, Caveats and Recommendations when Using Events in LabvIEW" 1. Only place a single event structure within a while loop. In reality the while loop and the event structure are intimately linked so there should only ever be one event structure per loop. 2. Never place an event structure within an event structure. It is better to solve issues where you might want to do this with some thought and perhaps a second while loop in parallel. 3. It is possible to have more than one event structure while loop combination on the same block diagram. There are valid reasons why you might want to do this: eg to have some events handled without pausing the while loop (Lock Panel Until Handler Completes = False) and others to wait until the handler completes (= True). The advice here is to separate out those events that lock the handler and those that do not lock the handler into separate while loops. 4. Whether the panel is locked until the handler completes is set individually for EACH case in the event structure. 5. It is advisable to notify the user (ie with a message popup or mouse busy) when tasks that lock the panel take a long time to execute (> 0.5 s). Another way is to set some busy status message visible as the first step and then hide this message when the case exits. 6. Boolean switches and their state require some thought when used within the event structure. With the default button style ("latch when released") LabVIEW reads the state of the boolean and only after reading does the button change back to it's other state. If the boolean is the "switched" type then TWO events are generated when the boolean is switched (ie F->T and T->F). If a single case of the event structure is used for "value chaged" on a boolean that is of the "switched" type, make sure that the code is only executed when the true event is processed (ie with a true/false case selector). This is most relevant where to use a boolean as a local variable then the boolean type must be of the "switched" type. Why does the event structure behave as it does? LV BE (Before Events) had the panel and diagram behaving as asynchronously as possible WRT one another. A control periodically handled UI events from the user and dropped the value into its terminal. Independently, the diagram takes the current value when it is needed and does its work. There is no synchronization between when the UI event is handled, when the value in the terminal changes, and when the diagram reads it. It is often a very simple way of writing your code and mimics how most hardware works. So why do events? The primary reason for the events feature is to allow synchronization between the UI and the diagram. First off, the diagram gets notification of a value change. It is guaranteed not to miss user changes or to burn up the CPU looking for them. In addition to the notification, the diagram gets a chance to respond, to affect the rest of the UI, before the rest of the user input is evaluated. Maybe this part needs an example. Lets look at polled radio buttons. In theory, your diagram code polls fast enough to see each change in the radio buttons so that it can make sure that the previous button is set to FALSE. But when the user is faster than the diagram and presses multiple buttons, what order did they press them in? There has to be a fixup step to break the tie and pop out all but one button to FALSE regardless of the order the user clicked them. With events, when the user clicks on a button, the panel is locked and will not process the next user click until the diagram finishes. This allows the diagram to see the button changes one at a time in the same order as the user presses. Another example, perhaps a better one, is a panel with three action buttons: Save, Acquire, and Quit. The order that the presses are responded to is important, so the polling diagram has to "guess" whether to Save and then Acquire or Acquire, then Save. The Event Structure knows the order and the code in it is synchronized with the UI allowing for a better, more friendly UI. Getting back on topic, the event structure introduces the synchronization, but the downside is that as with all synchronization, it allows for deadlocks when not used in the right way. As already discovered, nesting event structures is almost never the right thing to do, at least not yet. As noted earlier, it is possible to leave the Event Structures nested, but turn off the UI locking. This appears to solve the problem, but it isn't how I would do it. I think a much better solution is to combine the diagrams of two structures into one. The value change for hidden controls will not happen, but it doesn't hurt to have it in the list of things to watch for. Another option is to place them in parallel loops. This will let the first structure finish and go back to sleep. Why doesn't the event structure register local variable changes? >The main reason for not sending events for programmatic value changes is to avoid feedback. A happens. In responding to A, you update B and C. If responding to B or C results in changing A, then you have feedback and your code will behave like a dog chasing its tail. Sometimes the feedback will die out because the value set will match what is already there, but often it will continue indefinitely in a new type of infinite loop. One solution to this is what is called User Events. You define what data they carry and when they fire. Then you either have the value change and the set local both fire the user event, or you can combine the event diagram to handle both and just fire the event when writing to the locals. Today, you can accomplish this with the queued state machine using the state machine to do all the common work and just having the event structure pass things to the queue. Why is it necessary to lock the panel after an event fires? Given the synchronized mechanism of events, it is pretty easy to repost events to another queue or turn off locking and synchronization. If the event structure isn't synchronized, it would be impossible for the diagram to add it and become synchronized with the UI, so it is at least necessary for event diagrams to be able to lock the panel. Should it be the default? In our opinion, yes. When responding to an event, it is pretty common to enable/disable or show/hide some other part of the display. Until you finish doing this, it is wrong for LV to process user clicks on those controls, and LV doesn't know which controls you are changing until you are finished. Additionally, it isn't the best idea, but what happens when the event handler does something expensive like write lots of stuff to a database inside the event structure? If the UI is locked, then the user's events don't do much, ideally the mouse is made to spin and this works the same as a C program. The user is waiting for the computer, and the UI more or less tells the user to be patient. If the UI isn't locked, the user can change other things, but you can't execute another frame of your event structure until this one is finished. This is a node. It must finish and propagate data before it can run again, and the loop it is probably in can't go to the next iteration until it completes. You would have low level clicks being interpretted with the current state of the controls before the diagram has a chance to respond. This is sometimes the case, so it is possible to turn off event handling on the cases where you know that you may take awhile and you do not affect the state of the UI. If you need to respond to the events in parallel, you can make a parallel loop, add an event handler for the other controls, and handle them there while this one churns away. That will work fine and it is IMO clear from the diagram what is synchronized and what is parallel. Taken to an extreme, each control has its own loop and this approach stinks, but it is a valid architecture. Note that for this to work well, you need to turn off the UI lock or have a node to release it. Another way of doing expensive tasks is to have the Event Structure do the minimum amount necessary before unlocking -- treat them like interrupts. Have the event structure repost expensive operations to a parallel loop or fire up an asynchronous dynamic VI. Now your event structure is free to handle events, your UI is live, LV is still a nice parallel-friendly language, and your diagram just needs to keep track of what parallel tasks it has going on. In the end, I'm not sure I can convince you, but if you continue to experiment with the different architectures that can be built using the Event Structure, I think you will come to agree that it normally doesn't matter whether it is locked or not. There are times where it is really nice that it is locked, and occasionally you may turn off locking so that the user can do additional UI things up to the point where synchronization is necessary again. For correctness, we decided that locking should be the default. I'd suggest reading the article on devzone sooner or later. It is in the Developer Insights/LabVIEW Guru section, and it will help start you down the right path. What does an "Insane Object" mean? Should I report this as a bug to NI or do they already know about it? Firstly, there is no need to necessarily report this to NI, as they do know about insane objects. Secondly they are NOT just bugs, insane objects can be generated as part of the LabVIEW verification process. The insane object message is what LabVIEW puts in a dialog when one of the objects on the diagram does not meet its "checksum". In other words, this object isn't in a state we expect it to be in. Most of the time these errors are not fatal -- we simply put the object in the state we do expect. But it raises questions about how the object got into that bad state and what might have been done with that object between the last time we checked and it was good and the time it became insane. The insane object messages are something we work on with each version of LV. But as it is a generic error message that can apply to anything from a simple cosmetic to the front panel itself, you'll still see them in any version of LV that has a bug -- a fact of life unfortunately for the foreseeable future. If you get such a message, it is good to check the known bugs of LV at ni.com and if the particular insanity in your dialog is not listed, report the insanity to NI technical support. The cryptic nature of the message can be deciphered as follows: Most of the time, deleting the offending object and recreating it from scratch is sufficient to fix your VI and allow you to continue working. What are LabVIEW 2 style globals, functional globals, and uninitialised shift registers (USR's)? LabVIEW 2 style globals and functional globals refer to the same code construct in LV. Most people in the mailing list refer to these structures as LV2 globals because this was the version of LV when they were introduced. Functional globals is how NI refer to these structures because they can be more than just global variables in that they can contain code and data. Uninitialised shift registers (USR's) are the LV code primitives used in making these functional global variables work. A functional global is created by placing a shift register upon a while loop and wiring a boolean constant to the condition terminal such that the loop executes a single time. When the shift register is not wired with an input from the left hand side (ie it is uninitialised) LV retains the memory in use for that vi between calls. This means the the vi stores the value of the shift register variable between calls (executions) in the code. Extensions can be done by adding cases within the loop, eg one for read and one for write with a 'mode' enumerated type selecting which case is executed when the vi is called. These globals can contain as many cases as needed and because these are very similar to state machines, they can be made to execute several cases at each call to add increased functionality. Of course all recent versions of LV have global variables, accessed from the structures palette, so why do people still refer to them or even use them? Well, there are a couple of good reasons: So how does it work? In the LV execution system, a single vi, even if called from multiple places in the LV code, can only be executed exclusively in each place. In other words, the vi will not execute until a previous execution is complete. With a functional global, because it is a vi, LV determines that the functional global can only be executed once. Therefore updating of the shift registers is protected by the LV execution system. This mechanism of a vi 'protects' the data preventing any race condition. It should also be noted that USR's only retain this ability to share data across the code when they are not reentrant. If the vi is reentrant then the USR still stores the data, but this is only stored between calls of the vi in that place in the code - another call to the vi in another section of the code would have its own dataspace and its own saved USR values. It should be stressed that the reentrant vi would not transfer data from one section of the code to another and by definition not be a functional global. |