Update 2020 Feb
- All dependencies cut.
- Logging streamlined
- It should work with any C++ version you might have.
Update 2019 December
No dependency on the logger. See the
test.cpp on how to install your own logger.
Please see the readme in the Github project.
Update 2018 December
In case you need it, here is one very relevant use case, for proving the dynamic dll loading usability.
Sometimes that one Windows API function you need, has been added between the major releases. It is non trivial to compile the code that will be able to reach that function or proceed smoothly if that function is not available on some particular machine.
If you need
GetGeoInfoEx (for example) you are faced with a very tricky issue, impossible to be solved just by using WIN32 compile-time macros. The issue is, the existence of this function is not tied to the particular version of Windows, but to a particular, not major release of Windows 10. Namely,
GetGeoInfoEx exist in kernel32.dll but only if run-time is Windows 10 Redstone 3 or better. Macros are for compile-time, and this is a run-time issue.
To explain the complexity better, you will be able to compile and link your code that calls this function. But that will fail at run-time. Because the system is not checking before run-time if a particular function exists in a particular dll. On a particular machine!
Using WIn32 versionhelpers.h, in this case, will not help. You need a more fine-grained check than that. Yes, you can use WIN32 API to develop a more fine-grained runtime check function, but you might soon realize it is far from straightforward.
I might suggest, to be in control of this situation you can use my dll dynamic loader-user as described below. Here is the snippet showing you how:
First, declare the function pointer required
function signature as documented
This exist in kernel32.dll but only above W10 REDSTONE 3
First declare the function pointer with signature as above
using GGIEX =
int (*)( PWSTR ,GEOTYPE, PWSTR ,int);
Now one function call, to get to the pointer to the instance of this function inside kernel32.dll
Required function pointer type, the dll name, function name and that is it. And here is the secret sauce:
If function is not in the dll, returned value will be
nullptr. No other harm will be done.
Next we have the function pointer, which we use as any other function. With this crucial run time check included.
// ONLY is this version of WIN has it.
(int)geoDataCount ) ;
You can not know if your exe is running on W7 or something older or W10 bellow REDSTONE 3. In that situation the function pointer will be null, and you will code easily around that, no harm done. To replace that
if(fp) line with something else, an large amount of WIN32 sorcery is required, to find out do we use the right Windows and the right build of it. At runtime.
Without dynamic dll loading and using mechanism, this code will be much more involved and complex. As mentioned below the github project is here.
The Original Article
And more importantly, why would anybody do it?
This article contains a Windows programming code and a C++ code of moderate to high complexity.
Why loading dynamically a dll and calling a function from it?
Among other things, because you do not need a header file. You do not need to find it, use it and potentially “fight” with issues some other completely unrelated stuff in the same header, might do to your project.
That is a simple use case. And here are more reasons you will want DLL dynamic loading.
You have written some snazzy C++ and you are very proud of it. Being C++ it might be some deep system “mumbo-jumbo” that you have been amazed about. And every good deep system “mumbo-jumbo” does not require Graphical User Interface (aka GUI). After all, you are a system programmer and your code runs on servers! But it does require some way of signaling to its human administrator’s servants, and you have decided you want it to release sounds! Like squeaking when not well, that sort of a thing.
OK. Of to yet another masterful dollop of (server-side) C++. It is kernel32.dll and yes, there is a WIN32 function called Beep() in there. Nothing easier than using it. Bam! Done!
This is where I stop you, my dear C++ sorcerer.
Think. Your C++ code produces a very small and fast and perhaps useful and invisible Windows program. The last thing you want to do it is to start including DLL’s in its build. Why?
Including DLL’s in a build means your program will need to make sure (at startup time) each of these DLL’s is found and then loaded into memory. And each of them, like kernel32.dll, will do the same for itself with a myriad of other DLL’s it depends on. And so on with those other dll’s. This is called “DLL Dependency Tree”
All of a sudden the file size and the memory footprint of your little, beep emanating C++ wonder, has grown. A lot. And you wanted “just” to call the Beep(), and very rarely at that.
This is why a concept of dynamically loading (Windows) dll’s has been invented. This is the sequence of activities it is made of:
- Load a dll
- Ask it (nicely) for a function you need
- Call a function
- Unload the library. (This is a crucial step)
By now I am sure, you might have lost all the patience and are about to leave, and copy-paste next C++ code from somewhere. This is why I have made this little DBJDBJ tool, to do the dynamic loading of Windows dll’s.
Basically, there is one function that does it all. For it to be used you need three things
- the dll name
- the function name
- the function signature
// the Beep function signature
typedef BOOL(*BeepFP) (DWORD, DWORD);
// load the dll + get the function
auto fp = dbj::win::dll_call<BeepFP>(
// just use it if found to exist
dll will be properly unloaded when an instance of
dll_load goes out of scope. For the fancy test unit please look into test.cpp.
To start the dll dynamic loading, one needs to know where is it, and give the full path to the dll location.
For understanding the “finding rules” please read the article here. Note: we are using
LoadLibrary() and not the
LoadLibraryEx(). Why? Primarily because we want to enforce the usage policy titled: “Keep it simple”.
We obviously could provide more functionality and make everything more complex. I suggest that a small percentage of readers needing the extended functionality, take this code and extend it.
The full working code is on the Github here. I am sure you know what is testing code and what is a library code in there. Hint: The lib is all in one header. It is made with 2017 Community Edition of Visual Studio. It uses standard C++17.
Any questions, or bugs, fire away and ask. Please use the issues tab, on this GitHub repo.