Rolf Kalbermatter

We recently came across this problem. Not really Report Generation Toolkit related but in our own library to interface to Excel. Microsoft seems to have changed the interface to the Save and SaveAs methods once again in Office 2016. LabVIEW as a statically compiled system implements the interface to ActiveX as a static dispatch interface at runtime. Only at compile time does it reevaluate the method interface to the actually installed type library on the current computer. This is a choice that works fine in most cases and is pretty fast performance wise but fails if someone changes the ActiveX interface of a component and you try to call that component from LabVIEW without wanting to bother about the actual version that is installed on the final target system. Microsoft however does not provide compatibility methods that support the old interface, since they feel that the ActiveX dynamic dispatch capability, where a caller can find out about and construct the necessary dispatch interface at runtime, makes that unnecessary. Unfortunately ActiveX is considered both by Microsoft and NI as a legacy technology so neither party has much interest to invest any time at all into this beyond keeping it working the same as until now. Basically there is no easy solution for this. You have to compile an app on a computer that uses the same office version as what the target computer will use. The only way around that is to actually create separate wrappers for the Save methods on two different computers with each their own version of MS office installed and then in your app determine the actual Office version that is installed and then invoking the correct VI dynamically. A possible but painful workaround. For older Office versions I believe NI already incorporated such a fix into the RGT Toolkit for the save method (and used a somewhat sneaky trick to avoid accidental recompilation of the relevant dynamic VIs during an application build, which would adapt them to whatever Office version is currently installed on the machine) but obviously this hasn't been updated for Office 2016 yet. But it's a maintenance nightmare for sure for them, but the alternative of implementing runtime dynamic dispatch to Active X methods would be a major investment with several possible problems for existing application in terms of performance, and that is very unlikely to happen, since ActiveX is already considered a legacy technology for about a decade.

Except that from the languages I do know, only Java, C# and C++14 support a deprecated keyword. Yes you can do it with gcc with the __attribute((deprecated)) and with MSVC with the __declspec(deprecated) keyword too, but that is a compiler toolchain specific extension which is not portable. So for many languages it either ends up as a custom decorator of a specific library (Python or Lua can do that) or it's just a comment that nobody will read anyhow!

It would actually help if you saved the VIs for previous. I haven't installed LabVIEW 2016 here. However as far as calling the function in the header file is concerned, something like this should definitely work, if you didn't mess up the configuration of the DLL build script (which I can't control for lack of LabVIEW 2016). #include "Password.h" #define BUF_LENGTH 100 char input[] = "some text"; char output[BUF_LENGTH]; MakePassword(inpupt, output, BUF_LENGTH); printf("This is the converted text: %s", output); I'm not sure about your C programming experience, but if you have little to none, the trick is most likely in providing a valid buffer for the output string and not expect the function to handle that automatically like you are used in LabVIEW!

Reading a bit further on the technical mailing list it seems there was an initial clash of some sorts between a few people who were on the two opposite ends of wanting to get code into the kernel and wanting to maintain a clean kernel source code base. Both points are pretty understandable and both sides have sort of resolved to some name calling in the initial phase. Then they sat down together and actually started working through it in a pretty constructive manner. None of the latter seems to have been picked up by the mainstream slashdotted media, and the quick and often snarky comments of more or very often less knowledgeable people, concentrated mostly on that initial fallout. It's very understandable that the kernel maintainer didn't want to commit 100k lines of code into the kernel code just like that. Apparently the AMD guys didn't expect that to happen anyways and actually were more of proposing the code as it was as a first RFC style submission, after having worked a bit to long in the shadows on the huge code base. They didn't however make this clear enough when submitting the code. And the maintainer was a bit quick and short in his answer. In hindsight the way this was handled from both sides isn't necessarily optimal, but there is no way this code would have been committed if Linus himself would still control the kernel sources. Yes developing for the Linux kernel is a very painful process if you are used to other device driver development such as on Windows. There you develop against a rigidly defined (albeit also frequently changing) kernel device interface. In Windows 3.1 and Windows 95 days you absolutely had to write assembly code to be able to write a device driver (VxD), in Windows 98 and 2000 they introduced the WDM model which replaced both the VxD and NT driver model completely. With Windows Vista the WDF framework was introduced, which is supposed to take away many of the shortcomings that the WDM model had for the ever increasing complexity of interactions between hardware drivers, such as power saving operations, suspend, or also IO cancelation or pure user mode drivers. In the Linux kernel, a driver is normally an inherent part of the kernel sources. As such they are very tightly coupled with the specific kernel interfaces, which is not static at all but changes as needed and all the drivers in the kernel source then have to be modified to adhere to these changes. It's obviously a very different development model than what you see in closed source OSes but there is something to be said about not involving several layers of complex intermediate abstraction that are generally difficult to debug and even more difficult to keep in sync with modifications on both the upper and lower boundary of each layer. It's not a very good model in terms of scaling when adding many different device drivers, as a single change in the kernel interface will easily require to change every single device driver in the source tree too, but that's what the kernel developers decided in. The alternative is to not only strictly specify the interface between the kernel and user space, which the Linux kernel does, although often with a twist in that they seem to prefer to do it in a different way to BSD and other Unix variants, seemingly for the sake of being different, but also define a stringent and static device driver API inside the kernel that will never change except maybe between major kernel versions. And even-though that might seem like a good idea for device driver developers as it would allow closed source drivers that won't need to be recompiled with every kernel upgrade, it's also a model that requires enormous architectural work upfront, before any driver can be written, only to find out that at the time the interface has been defined and the necessary infrastructure has been developed, that it is already obsolete. Another factor that might play in here is that the only way a device driver is actually easy to maintain in such a development model, is by actually open sourcing it, which is of course one of the main motivations of GNU in general and the Linux kernel especially. Unfortunately this leaves users of hardware that the manufacturer doesn't want to document openly, such as most NI hardware too, pretty much in the cold when they want to use Linux. One can get angry at NI or the Linux kernel guys that they each maintain their position, but that doesn't help and in the end both sides have the right to deal in this as they wish and currently do. Linux is not going to have a static device driver interface and while that is a pita for anyone not wanting to donate their device driver source with lots of their own support and sweat into the kernel mainline, it's how the Linux world is going to work for as long as there are people who want to work on Linux. It seems that many open source developers favor this model also outside the rather confined albeit extensive kernel development, but they forget that if you write a library you are not operating in a closed environment such as the kernel sources, but in a world where others will actually want to interface to that library and then arbitrarily changing the contract of a library API, simply because it is convenient to do so, should not be something that is considered without the uttermost care

First check that your host provider does allow external connections to the database server. Almost every webspace provider nowadays lets you choose to install mySQL, (usually as MariaDB now) in your webspace environment so you can implement webstores, blogs and what else on your hosted website. However most do not allow connections to that database from outside the virtual website environment for security reasons. Once you determined that such external connections are allowed you have to determine which type of database server is used. Besides mySQL (MariaDB), you can also get hosted database servers based on MS-SQL or possibly even Oracle, for some high throughput commercial services, and that will largely influence the possible selection of your interfacing strategy. The SQL Toolkit you so profoundly excluded would support almost all possible servers. Alternatives are LabSQL, which is based on the same ADO interface that the SQL Toolkit uses, or the ADO-Tool. Depending on the used server you might also get lucky with the mySQL native driver from Saphir.

There are certainly problems with storing and retrieving fractional seconds from database timestamps and that depends on database, according database driver and such. We had many trouble with that on MS-SQL and Oracle in the past and the only thing that works reliably across various versions of databases is to use stored procedures that take either a custom number format or the fractional and second part as separate numbers and then using DB specific functions to combine the two values into a native timestamp. Both ODBC and ADO/DAO lack an unified standard for this that all database drivers would support and traditionally the timestamp was only supporting full second resolution in ODBC and accordingly ADO, as well as in most database servers. You can't really blame the database toolkit for this, since it is really a pretty thin wrapper around ADO, and can't make up for historical shortcomings of the underlying infrastructure. As to implementing a native T-SQL protocol through TCP/IP VIs, there is at least one library out there that is definitely not as extensive and well tested as the Saphir toolkit for MySQL but also workable. The problem about the T-SQL protocol or more precisely TDS is that it is not fully documented. The Open Source implementation in C, called FreeTDS is based in part on an older public specification of an older Sybase SQL server version which MS-SQL server is derived from. That documentation is for version 4.2 of the TDS protocol but the current MS-SQL Server versions use version 7.4. MS has added various extensions to it since the 4.2 version and current MS SQL Servers refuse to connect with a TDS client that doesn't support at least 7.0. Quite a bit of the 5.0 and higher support in FreeTDS was basically reverse engineered through network logs and as such can be considered working for many cases but likely isn't fully protocol compliant. While one can implement a native LabVIEW library for the TDS protocol using the TCP/IP VIs, this would have to be based in large parts on the openly available protocol documentation of the TDS 4.2 specification with extra info from the preliminary protocol description in the FreeTDS documentation, possibly helped by peeks into the FreeTDS source code. But that source code is under GPL license, so looking to much at that code is not a good idea to implement a non GPL implementation of said protocol. Also an additional problem with trying to implement this in pure LabVIEW is the fact that newer protocol versions add various encryption and compression features, that are not easily implemented in pure LabVIEW.

I'm not sure you can blame Linux for this. Packed Library is an entirely LabVIEW specific feature. It's basically a ZIP archive with an executable header for reasons I don't know. It definitely is not instantiated through OS loader code, so the executable header looks mostly be tacked on for version resource purposes. It seems unneccessary. The entire code in packed libraries is LabVIEW specific code, the precompiled executable code for the VI and optionally the VI diagram for debugging purposes. The loading and linking of these code resources is done entirely through LabVIEW itself. So what the problem with packed libraries on NI Linux RT systems is, I have no idea.

What do you expect? The DSC system is a collection of many shared libraries that work together. Someone cracked the Windows version apparently that you are using. The realtime variants of those shared libraries have to be different because they can't rely on the same Windows API (Pharlap ETS) or are for completely different CPUs (VxWorks) or operating system (NI Linux RT). They can't use the license manager as used for the Windows version, but I'm sure NI is smart enough to employ some kind of protection there too. But you will be hard pressured to find a script kiddie who has such hardware available and is willing and able to crack this for you. Asking in a public forum about this is definitely not the smartest move you can do!

While I think that the remark in itself wasn't helpful I do understand where it comes from. In many open source projects trying to interface to them from another software is like trying to continuously keep a moving target in focus. Granted, maintaining backwards compatiblity can be a painful process and there is something to say about starting with a clean slate at some point. And of course often the open source programmer is dedicating his own free time to the cause, so it is really his decision if he rather spends it to keep the software compatible or develop new exciting features and change whatever is needed to change during that without considering the possible consequences. Still I think a bit more discipline wouldn't normally hurt. It's sometimes the difference between a cool but for many applications pretty unusable solution and a really helpful and useful piece of software. Another thing are changes made on purpose for the sake of disallowing their use from certain types of clients. That I have a pretty ambivalent feeling about. It seldom prevents what they try to block, but it causes lots of mischief for the users. The IMAQdx link you provided refers to a forward compatibility issue. That is something that is very difficult to provide. There are techniques to help with that somewhat but they more often than not tend to take up more code and complexity than the entire rest of the library, so in short basically never worth the effort. Working in regulated industries might be an exception here.

Well Python 2.3 should be indeed ok, although I never tested with numpy and similar in that version. But that is so old, it's like requiring people to work with Linux 2.2 kernels or Windows 2000. Right! You can spend many man hours to get LabPython working correctly with current version, quite a few more man hours to get the PostLVUserEvent() working as well (it's asynchronous operation and while no rocket science really, involved enough that I have to wrap my mind around it every time again, when trying to implement it somewhere). Or you implement a client server RPC scheme in LabVIEW and Python and just pass around the information that way. The second is a lot easier, easily expandable by other people with absolutely no c knowledge, and much easier to debug too.

Of course. I never said otherwise. But we were not really discussing LabPython at this point since it has quite a few issues that would require some serious investment into the code. The solutions we were discussing where more along the lines of running Python in its own process and communicate between Python and LabVIEW through some means of interapplication communication like nanomsg, Zeromq or a custom made TCP/IP or UDP server client communication scheme. Refer to this post for a list of problems that I'm aware of for the current version of LabPython.

I have not moved anything to github and am very unlikely to do. Besides that I find git not very easy to use I have come across way to many projects that were taken from somewhere, put on github and then abandoned. The 4.0.0.4 version of LabPython is on the old CVS repository for the LabPython project on sourceforge, but I did add the LabPython project with some initial improvements to the shared library to the newer SVN repository of the OpenG Toolkit project on sourceforge. That is as far as I'm concerned the current canonical version of LabPython, although no new release package has been created for a few reasons: -The changes I did to the C code are only a few minimal imrovements to make LabPython compile with the Python 2.7 headers. Only very brief testing has been done with that. More changes to the C code and a lot more testing would be needed to make LabPython compatible with Python 3.x. - More changes need to be made to the code to allow it to properly work in a 64 bit environment. Currently the pointer to the LabPython private management structure which also maintains the interpreter state is directly passed to LabVIEW and then treated as a typed log file refnum. LabVIEW refnums however are 32 bit integers, so a 64 bit pointer will not fit into that. The quick and dirty fix is to change the refnum to a 64 bit integer and configure all CLNs to pass it as a pointer sized variable to the shared library. But that will only work fro LabVIEW 2009 on onwards which probably isn't a big issue anymore. The bigger issue is that a simple integer will not prevent a newby user to wire just about anything to the control and cause the shared library to crash hard when it tries to access the invalid pointer. -There is currently serious problem when trying to use non-thread safe Python modules like numpy and similar from within LabPython. These modules assume that its functions are always executed from within the same OS thread and context. LabPython doesn't enforce that and LabVIEW happily will call it from multiple threads if possible, which makes those modules simply fail to work. LabPython tries to use the interpreter lock that the Python API does provide, but either that is not enough or they changed something between Python 2.3/2.4 and later versions in this respect that makes LabPython not correctly use this lock. Getting this part debugged will be a major investment. Documentation about the interpreter lock and thread safety of the Python interpreter are scarce and inconsistent.

I can only echo neds remarks. Calling any of the LabVIEW manager functions from a different process than LabVIEW itself is doomed to fail. If you wanted to call this function through the Python ctypes interface, the according Python interpreter has to run inside the LabVIEW process, just as what LabPython attempts to do. Trying to do that from a seperate Python execution interpreter is doomed without proper interprocess communication like through nanomsg, ZeroMQ or your own TCP/IP, UDP deamon. This is no fault of LabVIEW or Python but simply proper process separation through protected mode memory and similar involved techniques, fully in effect since Windows NT.

I'm not sure I understand you well here. If the library offers to install semaphore callbacks that is of course preferable from a performance viewpoint but you can still choose to protect it on the calling side by a semaphore instead (and you could even use an implicit serialization by packing all CLNs into the same VI with an extra function selector and setting the VI to not be reentrant) instead of wrapping each CLN into an optain semaphore and release semaphore. A library offering semaphore callback installation is pretty likely to only use them around critical code sections so yes there might be many function calls that don't invoke a semaphore lock at all as it is not needed there. Even when it is needed it may choose to do so only around critical accesses, freeing the semaphore during (relatively) lengthy calculations so that other parallel calls are not locked, which can result in quite a bit of performance when called from a true multitasking system like LabVIEW.

As has been already pointed out, there are a number of possible reasons why a library could be not thread safe. The most common being the use of global variables in the library. One solution here is to always call the library from the same thread. Since a thread can't split magically into two threads, that is a safe method to call such a library. Theoretically a library developer could categorize each function if it makes use of any global and sort the library API's into safe functions who don't access any global state and into non-safe functions who need to be called in a protected way. Another way is to use a semaphore. That can be done explicitedly by the caller (what drjdpowell describes) or in the library itself but the later has the potential to lockup if the library uses multiple global resources that are each protected by their own semaphore. OpenSSL which Shaun probably refers to, requires the caller to install callback functions that provide the semaphore functionality and which OpenSSL then uses to protect access to its internal global variables. Without having installed those callbacks OpenSSL is not threadsafe and dies catastrophally rather sooner than later when called from LabVIEW in multithreaded mode. An entirely different issue is thread local storage. That is memory that the OS reserves and associates with every thread. When you call a library that uses TLS from a multithreaded environment you have to make sure that the current thread has the library specific TLS slots initialized to the correct values. The OpenGL library is such a library and if you checkout the LabVIEW examples you will see that each C function wrapper on entry copies the TLS values from the current refnum to the TLS and on exit restores those values from TLS back into the refnum. In a way it's another way of global storage but requires a completely different approach. But for all of these issues guaranteeing that all library functions are always called from the same thread solves the problems too.

Well, Lua for LabVIEW would give you a lot of the things you hope for but it is not free. So that is the main reason I didn't really push it as a viable option.

While finding the root cause is of course always a good thing, networking is definitely not something that you can simply rely to work always uninterrupted. Any stable networking library will have to implement some kind of retry scheme at some point. HTTP did this traditionally by usually reopening a connection for every new request. Wasteful but very stable! Newer HTTP communication supports a keep alive feature, but with the additional provision to close the connection on any error anyways and on the client side reconnecting again on every possible error including when the server closed the connection forcefully despite being asked to please keep it alive. Most networks and especially TCP/IP were never designed to guarantee uninterrupted connections. What TCP guarantees is a clear success or failure on any packet transmission and also proper order of successful packets in the same order as they were send, but nothing more. UDP on the other hand doesn't even guarantee any of these.

It's no magic really, although I haven't used it myself yet. I make use of other features related to so called UserDataRefnums that are although not really documented a bit more powerful and flexible than the (IMHO misnamed) "DLLs Callbacks". Basically each Call Library Node instance has its own copy of an InstanceDataPointer. This is simply a pointer-sized variable that is associated with a specific Call Library Node. You have the three "Callback functions" Reserve(), Unreserve() and Abort(), each with the same prototoype MgErr (*proc)(InstanceDataPtr *instanceState); So each of them gets a reference to the the Call Library Node instance specific pointer-sized variable location.You could store in there directly any 32 bit information (it's of course 64-bit on 64-bit LabVIEW but you do not want to store more than 32-bits in there for compatibility reasons for the case where you might need to support 32-bit LabVIEW and OSes, such as Pharlap, VxWorks and NI Linux ARM targets) but more likely you will allocate a memory block in Reserve() and return the pointer to that memory block in this parameter. In addition you should make sure the memory is initialized in a meaningful way for your other functions to work properly. The Unreserve() callback is called before LabVIEW wants to unload the VI containing the CLN in order to deallocate anything that might have been allocated or opened by the other functions in the InstanceDataPointer including the InstanceDataPointer itself. Abort() obviously will be called by LabVIEW when the user aborts the VI hierarchy. Now these three functions in itself are not very helpful on their own but where it gets really useful is when you add the special function parameter "InstanceDataPointer" to the parameter list in the Call Library Node configuration. This parameter will not be visible on the diagram for that Call Library Node. Instead LabVIEW will pass the same InstanceDataPointer to the library function as what is passed to the three callback functions. Your function can then store extra information during execution of the function in that InstanceDataPointer that Abort() can use to properly abort any operation that the function itself might have started in the background, including closing files, aborting any asynchronous operation it started, etc, etc. Depending on the complexity you can probably even get away with not implementing the Reserve() function specifically but instead have each function invocation check if the InstanceDataPointer is NULL and then allocate the necessary resources at that point. It may be a performance optimization in not allocating an InstanceDataPointer on load of the VI but only on first execution, so if someone only loads the code without ever starting it, you won't unnecessarily allocate it. If you ever had the "joy" of using Windows API functions with asynchronous operation you will recognize this scheme from the LPOVERLAPPED data pointer those functions use. Remains to stress the fact that every Call Library Node instance has its own private InstanceDataPointer. So if you have 10 Call Library Nodes on your diagram all calling the same library function you still end up with at least 10 InstanceDataPointers. I say here "at least" since this would be multiplied with the number of clones that exist for this particular VI when you have a reentrant VI. As to providing ready made samples with code, that is a crux with this kind of advanced functionality. As it involves asynchronous programming it really is a rather advanced topic. Anyone who understands the explanation as above will pretty readily be able to apply it for their specific application and others who don't won't be helped much with an example that doesn't match their specific use case almost perfectly. Even I get myself regularly lost in the pointer nirvanas where an asynchronous task is accessing the wrong pointer somewhere that the debugger is having a hard time to reach into.

I would guess that it has to do with dynamic dispatch. Most likely dynamic dispatch would get significantly slower (and I'm talking here more than a few 100 nanoseconds which some people already found an insurmountable problem when NI changed something in the dynamic dispatch code between LabVIEW 2014 and 2015) if there was the possibility that a VI is not already loaded!

While the middle-layer is indeed an extra hassle, since you have to compile a shared library for every platform you want to support, it is for many cases still a lot easier than trying to play C compiler yourself on the LabVIEW diagram. Especially since not all LabVIEW platforms are equal in that respect (with 32 bit and 64 bit being one but by far not the only possible obstacle). Yes you can use conditional compile structures in LabVIEW to overcome this problem too, but at this point I really feel like using duct tape to hold the Eiffel tower together. Maintenance of such a VI library is a nightmare in the long run. Not to forget about performance. If you use a middle layer shared library you can often directly use the LabVIEW datatype buffers to pass to the lower layer library functions, with MoveBlock you often end up copying any and every data back and forth multiple times. And smithd points out another advantage of a middle layer. You can make sure that all the created objects are properly deallocated on a LabVIEW abort. Without that the whole shenanigan is staying lingering in memory until you close LabVIEW completely, possibly also keeping things like file locks, named OS pipes, OS events and semaphores alive that prevent you from rerunning the software again.

There is no way to directly access LaVIEW controls from a Python script. You would have to somehow write a Python module and an accompanying LabVIEW module that can communicate with each other. But I'm not sure that is the approach I would choose. It requires the Python script to know your LabVIEW user interface exactly in order to be able to reference controls on it, which I find to be a rather brittle setup. Technically LabPython is best suited when you can write a library of Python routines that you then call from your LabVIEW code. In that way your LabVIEW program does provide all the information the script would need by passing it as parameters to the routines. Calling back from a Python script into LabVIEW was never really the main intention when I developed LabPython back in the old days :-). We eventually did Lua for LabVIEW which does support some limited calling back into LabVIEW (limited in that it only works from LabVIEW to Lua or Lua to LabVIEW but not in a recursive loop back and forth) but that is in fact one of the most complicated (and brittle) parts of Lua for LabVIEW. From the initial introduction of Lua for LabVIEW in LabVIEW 6 or so until the latest LabVIEW version, almost all problems that arose with a new LabVIEW release were related to this part of the package. As to support for LabPython the most likely place to get any feedback at all is probably here, but there are not many people using it nowadays and I haven't written any Python script in at least 10 years. I did a few minor updates in the past to the LabPython shared library to fix some minor quirks but in order to make it work properly with Python 3.0 and newer it would require some real work, also on the C side of the code. It was developed for Python 2.3 or so and works pretty ok up to Python 2.7.x but 3.0 added several changes that also have effect on the C code behind LabPython.

My experience with this is that under Windows it is pretty easy and non-problematic but if you end up having numerous class hierarchy levels that depend on each other and build the various classes all into its own PPL you have to be careful if you do this for Linux realtime targets. For some reasons only known to I don't know who, if you for whatever reason rebuild one of the base class packed libraries you absolutely have to rebuild every depending class packed library or LabVIEW will start to complain that the depending classes can't be loaded. I have no idea what the reason is, I did assume that a packed library is an isolated container that only exports its public interface to the outside world, so as long as nothing on the signature of the public methods changes this should be a no-brainer, but that doesn't seem to be the case for NI Linux RT targets. I didn't seem to have these problems on Windows nor VxWorks realtime targets.

If your packed library is really just a wrapper around your child class implementation, a better way would most likely be to employ a default naming scheme for the PPL name that follows the class name. Then using "Get Default Class from Path" you simply load the class into memory at runtime and cast it to the Base class and then you can call all the Base class methods and properties from that and the dynamic dispatch will make that the child methods are invoked.

You did redistribute the No Debug version of your DLL? The Debug version will link to a different C runtime library that is not redistributable to other computers and only works on PCs where you have the Visual C compiler installed.

Actually you should not really need to change anything code wise. The Linux kernel sources support to be compiled for just about any architecture that is out there, even CPUs that you would be nowadays hard pressured to find hardware to run it on. Of course depending on where you got your kernel sources they might not contain support for all possible architectures, but the kernel project supports a myriad of target architectures, provided you can feed the compiler toolchain with the correct defines. Now figuring out all the necessary defines for a specific hardware is a real challenge. For many of them the documentation is really mostly in the source code only. Here come various build systems into play that promis to make this configuration easier by allowing you to select different settings from a selection and then generating the necessary build scripts to drive the C toolchain with. What is the real challenge, is the configuration that needs to be done to tell the make toolchain for which target arch you want to compile, what hardware modules to link statically and what modules to compile as dynamic kernel modules if any. Without a thorough understanding of your various hardware components that are specific to your target that can be a very taunting task. Obviously there are certain popular targets that you will find more readily some sample configuration scripts than others. To make matters even more interesting, there isn't just one configuration/build system. Yocto which is what NI uses, used to be a pretty popular one for embedded systems a few years ago but lost a bit of traction some time ago. It seems to be back in activity a bit but the latest version is not backwards compatible with the version NI used for their NI Linux RT system. And NI probably does not see any reason to upgrade to the newest version as long as the old one works for what they need. It uses various other tools from other projects such as Open Embedded or BitBake internally. Buildroot is another such build system to create recipe based builds for embedded Linux. The real challenge is not to change the C code of the kernel to suit your specific hardware (that should basically be not necessary except adding driver modules for hardware components that the standard kernel does not have support for out of the box). It is to get the entire build toolchain installed correctly so that you can actually start a build successfully and once you got that, select the correct configuration settings so that the compiled kernel will run on your hardware target and not just panic right away. This last part should be fairly simple for a Virtual Box VM since the hardware that is emulated is very standard and shouldn't be hard to configure correctly.