I've dynamically linked libhunspell.dll (HunSpell) to my application. It works, but there is a dumb problem which I don't know why it happens.
Even before I use LoadLibrary("path\\to\\libhunspell.dll"); to load it and use it, on the start of the application it attempts to load the library by itself. If I place the libhunspell.dll into the path where my main executable resides, it can load it, otherwise it reports an error, immediately after starting the application - This application has failed to start because LIBHUNSPELL.DLL was not found. Re-installing the application may fix this problem. and the application doesn't start.
I would understand if the LoadLibrary would use invalid path but this happens as soon as the executable runs, even before the first statement in WINAPI _tWinMain(HINSTANCE, HINSTANCE, LPTSTR, int) executes (I've tried to place a breakpoint and it doesn't even reach it, so this happens before).
So, as a result, I must place libhunspell.dll in the same folder as my application executable, and not in the path I want.
This is probably easy to fix although I don't what to look for.
So the question is - how do I avoid it loading it immediately and have it wait until I use LoadLibrary call?
Here is how I linked if it can help:
1) compiled libhunspell.dll in Visual Studio 2015 (I used /MT option to link it statically so it doesn't have VC++ Redistributable as a dependency).
2) created import library (libhunspell.lib) using implib.exe -a -c -f libhunspell.lib libhunspell.dll
3) linked that to the source .cpp unit which is using it using #pragma comment(lib, "libhunspell.lib") (it is RAD Studio 2010 so the .lib is required unlike newer versions).
4) later in the same .cpp used LoadLibrary to load this library and used it.
By linking in the import stubs (libhunspell.lib) the OS will load the DLL for you as it is now a static dependency.
One approach would be specify the library as a delayload dependency: /DELAYLOAD:libhunspell.lib via the linker options. You can then call LoadLibrary on the DLL.
The only other option is to stop including the .lib in the linker step, making it truly a dynamic dependency.
I assume you did Add to project a *.lib file for your DLL. That is a kind of "static" linkage done in the App initialization (prior to your forms are created). So it has two disadvantages.
You DLL must be in the same path as the Apps EXE file
Sometimes DLL file name is locked (can not be changed)
The advantage is that you do not need to do any coding for the DLL loading as the VCL do it for you ... so your app should not contain the LoadLibrary,GetProcAddress calls you just include the *.h file with propper import declarations ...
For dynamic linkage you need to remove the *.lib from your project and use WinAPI LoadLibrary + GetProcAddress for loading your DLL as josh poley suggested. Here an example:
Builder C++ calling VC++ class
Beware there was/(is?) a bug in the GetProcAddress preventing from loading all the functions from your DLL in some cases. Especially if the DLL has old legacy mangling of names the count of functions is high and the DLL was created on compiler incompatible with the mangling in question.
Related
I have built Game Music Emu from source to use with Love2d. (Note: I am not very familiar with C/C++.)
In lua I load the dll with FFI and on my computer it works great, but when I sent my friend the app for testing, his machine doesn't recognize the DLL.
I sent him the love2d binaries with the libgme DLL included to make sure he didn't just misplace the DLL file. So what he is running is the exact same thing I am running.
My code looks like this:
ffi.cdef[[ ... ]]
local gme = ffi.load("libgme")
This is the exact error my friend gets:
lovegme.lua:4: cannot load module 'libgme.dll': The specified module could not be found.
Depending on how libgme is compiled you may have some dependencies that are satisfied on your computer (for example, mingw libraries), but not satisfied on the other computer.
I'd try several things: (1) use the full filename in the load command, (2) use "profile" mode in dependency walker to check what is failing during DLL load, or (3) use the same dependency walker on your machine to see what other DLLs libgme may depend upon and include those in your package/installation as well.
I've built a inproc com server dll which I can package as 1 file or many via the build utility py2exe. When I allow all the dependencies to remain external, I have no issues, but bundling as 1 file produces problems.
When the dll is utilized (either registering it or instantiating a com object from it), it immediately loads MSVCR90.DLL from the path c:\windows\winsxs\x86_microsoft.vc90.crt_1fc8b3b9a1e18e3b_9.0.30729.6871_none_50944e7cbcb706e5\MSVCR90.DLL no matter what I do, I can't change that. There is no information that I can find (using Dependency Walker) to indicate what is causing that to load. It just happens magically...
Then, later on it loads that dll again via an explicit call to LoadLibraryA("MSVCR90.dll") (part of some py2exe black box?), but this time it does not look into the winsxs manifests / directory. Instead it looks to the system path and/or will respect a dll redirection. That's when the problem occurs. If I set the system path to start with c:\windows\winsxs\x86_microsoft.vc90.crt...\ it will load the exact same dll and be happy - but if ANY other file is utilized - inclusive of a copy of the EXACT same dll - but at a different path - then the whole thing blows up. It can't handle using two different files.
How can I fix this? Ideally, I've love to make the initial magic loading of the dll draw upon a private assembly, but no matter what I do with manifests or .dll.local etc it will not respect that until this second dll loading takes place.
Note that with the non-bundled dll (external dependencies) it always uses the winsxs MSVCR90.DLL.
I can "fix" my failure to use the dll by forcing the system path to load the winsxs copy, but that is pretty useless for a deployable com server!
The reason is that you DLL has a manifest that tells the module loader to search also in the SxS storage.
You have several choices
Build your DLL using static linkage. Not using any of the MFC-DLLs (see project settings)
Don't use a side by side manifest for the DLL and still use the MFC DLLs. But beware you have to ship those DLL with your DLL in the local path (see DLL search sequence docs)
Use a later build of VS. Later versions of VS don't use the SxS storage any more and there are no manifests for those DLLs any more.
For the 2. see this article in code project. There is an update for VS-2008 [here].
2
Build your DLL
When I was going to set up my developing environment for (SDLSimple DirectMediaLayer), I downloaded the package provided by the website, from the readme.txt file I found that I need both .lib and .dll...
My question is :
I am not sure if my understanding is correct : in my thought, .lib for windows is like .a for linux, is static library, and .dll for windows is like .so for linux, is shared library, is this correct ?
If the idea above is correct, I think we can only use .lib or .dll, since the work they do is the same ? So why bother to get both two in one program ?
And I do not quite understand .dll, if my memory servers me right, the one of the advantage for shared library is they can be updated while the program is running, so how can people do this, if we update the .dll file, how can an running program get to know the changes of the .dll and reload it to memory ?
In VC, there are two kinds of libs.
First type is a "real library", generated by the "static library project", which contains everything you can link and run, without dll.
The second type is "import library", generated by the "dll project", which just contains the symbols the link.exe needs. You can link to the library, but you need the dll to run.
There are two ways to use a dll. Link to the "import library" or "use LoadLibrary API".
You can not change the dll file when the dll is loaded.
But if you use LoadLibrary to load the dll, you can use FreeLibrary to unload the dll, change the dll and then use LoadLibrary to load the new dll. Of course, you need to monitor something to invoke this procedure.
Still the easier way is that, use a loader to do the update, then load the real exe.
I have a solution in VS 2008 that creates a DLL. I then use that DLL in another application. If I go in to the DLL projects property pages and change the following configuration for a DEBUG build then the built dll no long provides the desired functionality. If I change it back and rebuild the DLL, then the DLL does provide the correct functionality:
Property Pages => Configuration Properties => C/C++ => Code Generation => Runtime Library
If set to "Multi-threaded Debug DLL (/MDd)"
then everything works as it should. I get the correct functionality from the DLL
If set to "Multi-threaded DLL (/MD)" then the DLL does not function properly...no runtime errors or anything, it just doesn't work (The DLL is supposed to plot some lines on a map but does not in this mode).
So the question is, why does using the /MDd flag result in correction functionality of the underlying code, while /MD results in incorrect functionality?
A little background...somebody else developed the DLL in C++ and I am using this DLL in a VB.net application.
All DLL's/debug code generation must match across everything that uses them. There may be another referenced library or object or dll or some code in there that is built using the wrong options; or specific options for an individual element that override the global project options.
The only way of figuring it out is to meticulously check all of the options for each file, checking the included and referenced libraries (.lib and .dll) and object files. Check the linker options too.
The reason why it doesn't work is probably because the debug version adds extra guard blocks around memory to allow detection of errors.
I had similar problems. My application which "used" a 3rd party DLL crashed when its runtime library was set to "Multi-threaded DLL (/MD)", but worked when its runtime library was set to "Multi-threaded Debug DLL (/MDd)".
It has something to do with passing std::strings and std::lists across the DLL interface.
Our guess was the low level definition of these types was somehow different in the two runtime libraries.
We solved our related problems using this rule...
The DLL and the DLL user must be build using the exact same runtime library.
The main difference between the two options is in the libraries that your code will be linked at later. for the debug version for example this will include LIBCMTD.LIB and a few others. if your library is going to be built as debug the you should always link with MDd. failing to do so will result in lots of unresolved external linker errors at best. and sometimes the code compiles normally but crashes at runtime. if this happens in vb.net then a catch can easily hide the error. I guess you should make sure you build setting is correct. for more detailed information check this.
I am facing the the problem to link to a third party dll. It is windows mobile application, where I am try to link to this third party dll.
Here first I had the dll and lib file. I was not able to link to it explicitly, but implicit linking is working. In the explicit linking the getprocaddress was failing. The dumpbin showed only the dllmain functions being exposed and no other function being exposed, hence the getprocaddress was failing.
However my application doesnot start if the dll is not found/installed in the device. It is expected as it is imlicit linking of a dll hence my application does not start.
I reported this to the third party dll provider and said that I want the explicit linking as the dll takes more space in my application if it is linked implicitly.
They replied by providing the .def file and said that I can use this .def file in my app to explicitly link to a dll.
I don't know how to use this .def file to explicitly link to a dll. Can any one please explain briefly about how to use this .def file in my app to explicitly link to a dll.
Use the *.def file when you build the DLL, to specify what function names the DLL is supposed to export.
After the DLL is built, use dumpbin /exports to verify that the functions are indeed exported from the DLL.
After you have verified that the DLL is exporting functions, you can either link to them at run-time using LoadLibray/GetProcAddress, and/or you can link to them at build-time by passing the DLL's *.lib file (which was created when you built the DLL using its *.def file) as an argument to your application's linker.