C++ - Import of explicitly specialised templates on Windows - c++

I am having some trouble getting a program to link on Windows with VC2008 SP1.
I am explicitly specialising a template member function in a DLL, which appears correctly as an exported symbol in dependency walker, for the correct type, and with the correct arguments.
When I try to call the symbol from an .exe, the linker complains that it can't resolve the symbol (although it compiles the object ok), however the decorated name of the symbol it says that it can't resolve exactly matches the decorated name of the exported symbol in my DLL (as viewed with dependency walker) The problem only occurs for explicitly specialised exported template member functions. If I comment the calls to these, all other exported functions classes will link ok, so the .lib looks like it is being generated ok.
I have tried changing my declaration of the template function in the headers to include the 'extern' directive, and it has the customary __declspec(dllimport) correctly specified, but this makes no difference.
Any ideas appreciated. Thanks.

Finally managed to get to the bottom of this - it was caused by a .lib that didn't get checked-in to match the DLL from source control - maybe built with a different compiler version.
Sorry for any wasted time.
... if all else fails, try rebuilding your library dependencies.

Related

Some functions/methods not exported into static library - Visual Studio 2019 C++

Apologies if this is an ill formed question. But it is something I don't even know where to start debugging.
I have a library, in which I keep general purpose code. i build this as a static library to include in my projects. The library and the projects are written in C++ and built in Visual Studio 2019.
I have just made some modifications to some code (which happens to be for dealing with times and dates) and suddenly most of my functions and methods within the .cpp file in question are no longer exported into my static library.
I am now getting unresolved symbols linker errors when I try to use these functions/methods. I have run
dumpbin /symbols /exports mylibrary.lib > exports.txt
and searching this output, shows that only a handful of symbols associated with my function/method definitions are listed. Symbols are missing for most of my methods, including those for which I have linker errors.
My questions are - have I done enough to show that my functions are not being exported - i.e. is dumpbin /symbols /exports the correct command to use? And secondly, if I have performed that check correctly, how is it possible that I have only exported some of my methods/functions and are there any steps I can take to fix it or debug further? I did not even realise this was possible when building a static lib.
Please note I am building a static lib, not an import lib or a dll.
Edit:
One example of a method not being exported is
constexpr int sci::UtcTime::getYear() const
{
return m_cTime.tm_year+1900;
}
where sci is a namespace, UtcTime is my time related class and m_cTime is a std::tm member variable of UtcTime.
I have tried cleaning and rebuilding everything and even rebooting my computer in desperation.
Edit 2:
Removing constexpr seems to result in the symbol being exported and resolves the linker problems. Why would that be the case?
As mentioned in my comment, constexpr is implying inline which causes compiler to not emit the symbol at all.
A constexpr specifier used in a function or static member variable (since C++17) declaration implies inline.
Your edit is also empirical proof that that's the case. I would say it is working as expected. The choice is now yours about whether you want the function to be exported or be constexpr qualified.
As for why you have started seeing this now, VC++ has been undergoing compliance related changes for the past 5 years or so, and generally each release is more compliant with the latest language standard at the time of release than previous releases.

DLL needs symbols (a class) from the app it will link to

I cannot get a DLL to link, that needs a class exported from the app this DLL will be used with. Visual Studio 2008, Windows 7.
I had a small sample DLL compiling (the default MSFT DLL project actually), and the app can use LoadLibrary() and GetProcAddress() on it correctly. I'm experienced with dllimport/dllexport. Using dumpbin /exports was critical to discover the mangled names to use with GetProcAddress() (MSFT's default empty DLL includes a sample function without extern "C") but this is a solved problem.
The next step was to have the app export a class that the DLL will need to subclass. I do the dllimport/dllexport #define's in the reverse sense from usual: a special symbol when the app is compiled tags the class with dllexport and non-app code (such as the DLL) using that header, without the special symbol, gets a dllimport spec. dumpbin /exports on the .exe file shows exactly this class's (mangled) symbols being exported, and no others, as expected.
The next step was to have the DLL include the header and create an object of the exported object's type (as a baby-step towards actually subclassing it). Compiles fine, but linker shows error:
DynTest.obj : error LNK2019: unresolved external symbol "public: __thiscall Test::Test(class Toast*,double)" (??0Test##QAE#PAVToast##N#Z) referenced in function "public: __thiscall CDynTest::CDynTest(void)" (??0CDynTest##QAE#XZ)
OK, that didn't surprise me, as I know on WIN32 I need to supply DLL's at link time to make a DLL, unlike my usual Unix. Since Windows seems to treat DLLs and executables a bit similarly, I tried adding the .exe at Properties->Configuration Properties->Linker->Input->Additional Dependencies. That gets an error that looks like LINK.EXE didn't auto-detect that it was being given an .exe:
T:\mypath\MyBinary.exe : fatal error LNK1107: invalid or corrupt file: cannot read at 0x348
I then tried adding instead the object file that defines this class... That seems to be understood by the linker, and is probably successfully satisfying the DLL's need to link, but now shows a myriad of other symbols this file depends upon.
So I've considered refactoring the app such that most of it is in a DLL or LIB, just so I can supply that as an "Additional Dependencies" to the DLL I'm actually worried about. But this seems to be draconian. Is that my only option?
Invoking dumpbin /exports gives you a list of mangled names of exe exports. You need to create a module definition file (.def) containing these names:
EXPORTS
#d3d_some_fancy_mangedled_method_1
#d3d_some_fancy_mangedled_method_2
...
Notice that it is basically dumpbin output with first columns removed.
Then you use lib tool to generate export library from module definition file:
LIB /DEF:prog.def /NAME:prog.exe /MACHINE:x86
Finally you link generated export library prog.lib into your application. /MACHINE option should match to your executable. Notice that you don't need to link or anyhow use program executable to link it, only export library is used.

When building a dll, can the lib be used for linking?

I wanted to create a C++ dll (to be used in a dot net application). Some functionality I needed was already implemented in another C++ dll.
The dll I was referencing was set up like this (including the comment):
extern "C"
{
__declspec(dllexport) BOOL SomeFunctionToBeUsedExternally();
}
// internal functions
BOOL OtherFunctions();
I need to use one of the OtherFunctions in my code.
So, I added the proper include in my own code, added dependencies on the lib created by the dll above, and used the method I needed. As a result, of course, I got another __declspec(dllexport)... function.
It refused to link though, I got an error about the OtherFunction.
I verified everything, looked online - nothing seems to solve my problem.
Then, I added a __declspec(dllexport) in front of the function I needed, and it works.
I don't understand though. I thought, the dllexport marked functions will be exported to the dll, but aren't all functions sent to the lib ?
Why do I have to export functions to the dll, if I am not linking against the dll but against the lib ?
No, the linker does not automatically export all identifiers. The dllexport attribute tells the linker which identifiers are exported. Without this you would be forced to either export every identifier in the DLL or specify which identifiers should not be exported. When the linker creates the DLL it also creates an import library and includes information about which identifiers are exported based on that attribute.
When you want to use a DLL you need link with the appropriate .lib file for good reason. The .lib file tells the linker which identifiers are exported, the name of the DLL they are in and other information. It is also possible to export identifiers from the DLL by ordinal instead of by name. In this case the linker still needs to match the identifier with the appropriate ordinal. This is only possible by having an accompanying library file that contains that information since it is not present in DLL's export table.
No, only exported functions end up in the .lib. As you can tell.
It is not a static link library, it the import library for the DLL. It is a very simple and very small file since it contains no code at all. Just a list of the exported functions. The linker needs it to resolve the external in the client code, it needs to know the name of the DLL and the actual exported function name or ordinal (could be different) so it can add the entry to client's import table. Import libraries having the same filename extension as static libraries was perhaps a bit unfortunate.

How do I link a DLL to my project? error LNK2019: unresolved external symbol

I have a file foo.h that has various declarations for functions. All of these functions are implemented in a file foo.dll. However, when I include the .h file and try to use any of the functions, I get the error:
bar.obj : error LNK2019: unresolved external symbol SomeFunction
so obviously the function implementations aren't being found.
What do I have to do to help the compiler find the definitions in the DLL and associate them with the .h file?
I've seen some stuff about __declspec(dllexport) and __declspec(dllimport) but I still can't figure out how to use them.
You should have received at least three files from the DLL owner. The DLL which you'll need at runtime, the .h file with the declarations of the exported functions, you already have that. And a .lib file, the import library for the DLL. Which the linker requires so it knows how to add the functions to the program's import table.
You are missing the step where you told the linker that it needs to link the .lib file. It needs to be added to the linker's Input + Additional Dependencies setting of your project. Or most easily done by writing the linker instruction in your source code:
#include "foo.h"
#pragma comment(lib, "foo.lib")
Which works for MSVC, not otherwise portable but linking never is. Copy the .lib file to your project directory or specify the full path.
I just had a similar problem. The solution turned out to be that the DLL was 64 bit, and the simple app using it was 32. I had forgotten to change it to x64 in the Configuration Manager.
You need to specify in front of function definitions __declspec(dllexport) keyword at the time of building the dll
You need to import or load the .dll file into process memory.
You need to acquire the address of function you want to use from that dll.
Some useful links to get started:: MSDN Documentation, SO, Random

Trouble compiling dll that accesses another dll

So, I have an interesting issue. I am working with a proprietary set of dlls that I ,obviously, don't have the source for. The goal is to write an intermediate dll that groups together a large series of funnction calls from the proprietary dlls. The problem I am having, when compiling with g++, is that I get errors for the original dlls along the lines of:
cannot export libname_NULL_THUNK_DATA. Symbol not found.
If I add a main and just compile to an executable everything works as expected. I'm using mingw for compilation. Thanks for any help.
In response to the first reply: Either I'm confused about what you're saying or I didn't word my question very well. I'm not explicitly trying to export anything from my wrapper I am just calling functions from their dlls. The problem is that I get errors that it can't export these specific symbols from the dll to my wrapper. The issue is that I'm not even entirely sure what these _NULL_THUNK_DATA symbols are for. I did a search and read somewhere that they shouldn't be exported because they're internal symbols that windows uses. I have tried using the --exclude-symbols directive to the linker but it didn't seem to do anything. I apologize if I'm completely misunderstanding what you're trying to say.
So, I think my issue was related to this. When just compiling a standard executable that uses a dll I was able to include the headers and directly call the functions for example:
#include :3rdparty.h
int main(){
dostuff(); // a function in the 3rdparty.dll
}
this would compile and run fine. I just needed to link the libraries in the g++ command.
When linking with the -shared flag I would get these errors (with main removed of course). I think it has something to do with the fact that by default g++ attempts to import all symbols from the dll. What I didn't understand is why this happens in the dll vs in an executable. I will try doing it using GetProcAddress(). Thank you!
it should be as easy as you think it should be.
eg:
your dll code needs:
void doStuff()
{
3rdparty.login();
3rdparty.dostuff();
3rdparty.logoff();
};
so far - so good, you've included the right headers .... (if you have them, if you don't then you need to import the library using LoadLibrary(), then create a function pointer to each exported dll entrypoint using GetProcAddress() and then call that function pointer)
You then link with the 3rd party lib and that's it. Occasionally you will have to wrap the definitions with 'extern "C"' in order to get the linkage name mangling correct.
As you say you're using g++, you can't be getting confused with __declspec(dllimport) which is a MS VC extension.
"Compiling" tells me that you're approaching this from the wrong end. Your DLL should not export its own wrapper functions, but directly refer to exports from other DLLs.
E.g. in a Windows Kernel32.DEF file, the following forward exists:
EXPORTS
...
HeapAlloc = NTDLL.RtlAllocHeap
There's no code for the HeapAlloc function.