I have a similar setup as described below:
Project A
Windows DLL
Depends on Project B
Project B
Compiled as static library
Now, if a global function, e.g. createNewFoo is compiled in Project B, it is never exported into the final DLL, although it is declared with dllexport and the final application uses it as dllimport:
namespace example
{
class __declspec(dllexport) Foo
{
Foo();
};
__declspec(dllexport) Foo* createNewFoo();
}
}
Note that, the class Foo is always exported into the final DLL, its only global or static functions that are somehow discarded.
Furthermore, note that the same global function is always exported if it is compiled in Project A instead of Project B.
Is there any setting in Visual Studio 2010 that allows exporting of global or static functions from dependent projects? or this is a limitation of the tools?
Using a static library is a great way to reuse code. Rather than re-implementing the same routines in every app that requires the functionality, you write them one time in a static library and then reference it from the apps. Code linked from a static library becomes part of your app—you don’t have to install another file to use the code.
As you case, Lib B's function is linked and become a part of DLL A, if you want to export these function in DLL A, you should explicitly dllexport function in DLL A, and in function body, you can call Lib B's function to reuse code.
Related
I have a dll, which accesses some classes outside of its project (I'm using Visual Studio, so I have two projects). The thing is, in the header that the dll includes, which is outside of the dll's project, there are only bodies of functions, like this:
x.h
class x
{
void myFunc();
}
And in another cpp file, outside of the dll file:
#include "x.h"
x::myFunc()
{
//.....
}
The dll is only getting the bodies of the functions, so when I compile, I get lots of unresolved external symbols (I'm quite sure that this is the issue, because I tested with another class fully built in a .h file, in another project, and no errors). So how can I solve this mystery?
It is normal for the import headers to only have function signatures; the actual function bodies are already compiled into the DLL binary and are resolved at link time by linking into the actual DLL.
The first thing to try is to make sure you are actually linking to the said DLL. It isn't enough to just include the header, you also need to link to the binary. So in your project configuration, you need to add a link to (for example) the .lib file that gets created along-side the DLL when the DLL is compiled (if in MSVC). This lib file lets the linker know how to connect the function signatures you included via the import header to the actual implementations contained in the DLL. If you're on a different platform, the mechanics might be a little different, but the concepts will be similar.
Edits:
The next step is to make sure the binary is actually exporting the symbols you're trying to link against. Make sure that all interface signatures are being exported via __declspec(dll_export) prefixes. Normally this is wrapped up in an IFDEF so that the header is declared export while the DLL is being compiled, but not when that header is included in a client project. Next, you could use dumpbin to check the mangled export names, and see if there is anything unexpected.
Here's a modified version of your example that illustrates this style of export (note, I haven't tested if this compiles, apologies for any typos):
#ifdef BUILDING_MYDLL
#define MYDLL_API __declspec(dllexport)
#else
#define MYDLL_API __declspec(dllimport)
#endif
class MYDLL_API x
{
void myFunc();
}
You would then set your configuration to define BUILDING_MYDLL when compiling the dll, but not when compiling the executable. This way the functions are only marked export when compiling the library dll. Now you can mark your public API functions with MYDLL_API and they should get exported during build.
Please note that dll_export, dll_import, and declspec are all very MSVC-specific constructs. Other compilers/runtimes handle symbol export in different ways.
There's multiple ways to link DLL into your app on Windows, check out this existing question/answer:
https://stackoverflow.com/a/2060508/1701823
I am interested in using my static lib to create a dll (implicitly linking). which means I need to (in vs2008)
create a dll project that should generate the following:
header file (which have export function declarations. These are simple wrappers to actual functions in the static lib using __declspec(dllexport) which are in the .cpp )
import lib which will be made as a result of creating the dll
the actual dll which is created.
I have made a test program that will utilize the above dll(including the import lib/header files) to test it.
in this I have included all the three items. now the exe compiles/links without issue.
however in the main.cpp when i call the exported functions (with the associated __declspec(dllimport) call it never seems to execute. I am uncertain why this is?
Its almost like the even though the exe can see the exported function in in the dll...the dll cannot call on the code that is in the static lib?
i just cannot answer why my exe can't see the code in the static lib? do i need an archiver/librarian for vs2008 to include all those obj files as part of the import lib?
I am at a loss and am not sure how to test this?
other than just making my static lib directly into a dll. I wanted to try this method. I know i am missing something...i have read all over the place and i am just stuck. There were some threads here that had some people posting something similar but i can't seem to get it. please be as detailed as possible as I am new to this. thanks again.
update 1:
ok so currently i added the extern line to the function prototype and now it sees the exported function from the dll. however, now the only issue left is that:
i can't invoke the function that this exported function (aka wrapper) is trying to call. which happens to be in the static library. how should my exe get visibility to that static library function. I know it can be done because I think there was one other person on this board who was able to make this work.
update 2: my setup is exactly like this questioner...
How to force inclusion of an object file in a static library when linking into executable?
but i am not using explicit linking. i am using implicit linking. my real issue is how to call a static lib function in my dll wrapper which is exported to the exe?
If the app and DLLs are MFC app/dlls, then make sure that the application and all dlls are either "Debug" versions or "release" versions and not a mix.
I'm building a DLL from a group of static libraries and I'm having a problem where only parts of classes are exported.
What I'm doing is declaring all symbols I want to export with a preprocessor definition like:
#if defined(MYPROJ_BUILD_DLL)
//Build as a DLL
# define MY_API __declspec(dllexport)
#elif defined(MYPROJ_USE_DLL)
//Use as a DLL
# define MY_API __declspec(dllimport)
#else
//Build or use as a static lib
# define MY_API
#endif
For example:
class MY_API Foo{
...
}
I then build static library with MYPROJ_BUILD_DLL & MYPROJ_USE_DLL undefined causing a static library to be built.
In another build I create a DLL from these static libraries. So I define MYPROJ_BUILD_DLL causing all symbols I want to export to be attributed with __declspec(dllexport) (this is done by including all static library headers in the DLL-project source file).
Edit:
Note on unrefenced symbols: Linker options Keep Unreferenced Data (/OPT:NOREF) and Do Not Remove Redundant COMDATs (/OPT:NOICF) is set so that no unreferenced symbols will be removed.
Ok, so now to the problem. When I use this new DLL I get unresolved externals because not all symbols of a class is exported. For example in a class like this:
class MY_API Foo{
public:
Foo(char const* );
int bar();
private:
Foo( char const*, char const* );
};
Only Foo::Foo( char const*, char const*); and int Foo::bar(); is exported. How can that be? I can understand if the entire class was missing, due to e.g. I forgot to include the header in the DLL-build. But it's only partial missing.
Also, say if Foo::Foo( char const*) was not implemented; then the DLL build would have unresolved external errors. But the build is fine (I also double checked for declarations without implementation).
Note: The combined size of the static libraries I'm combining is in the region of 30MB, and the resulting DLL is 1.2MB.
I'm using Visual Studio 9.0 (2008) to build everything. And Depends to check for exported symbols.
Edit:
For the ones who wonder why I don't just build a DLL from each of the static libraries: I can't because they cross-reference each other (that's why I need to group them together in one a single DLL). I know, it's horrible I can't really understand the logic behind it.
Remember that when you link against a static LIB, by default the linker is only pulling in the functions, classes, and data that the client (which in this case is your DLL) actually references.
So what happens is this:
You build your static LIB, and that's fine. This LIB is 100% valid.
You now build your static DLL around the original binary LIB. It pulls in only the stuff that it actually references. It doesn't pull in the entire class definition which is what it needs to do. This DLL, though it builds, is invalid.
A client then uses the DLL, and expects to see a complete binary definition for the exported class, because that's what the client sees in the accompanying header file.
However the class has only been partially imported, and this is why you're getting those link errors.
To fix:
If you're able to, don't build your DLL off your static LIB. Build your DLL off the original source code. And build your static LIB off the original source code.
Otherwise you can possibly fiddle with linker settings but I strongly recommend option 1 above. Note that OPT:NOREF won't work here as OPT:NOREF has no effect on static LIBs.
What wont't fix it:
High-level linker tweaks like OPT:NOREF, anything involving COMDATs, etc. If you want those functions present, you have to make sure they're referenced, either by referencing them, or by telling the linker explicitly, "hey, this Symbol X is referenced".
As a side note:
Anytime I build a DLL or LIB, I build both a DLL and a LIB, using exactly the technique you're using. Having a single code base that can generate either a DLL or a LIB by toggling a setting is ideal. But building a DLL off of a (binary) static LIB when you own the source code for both... I'm having a hard time imagining when such a scenario would be necessary.
The problem is surely that you use the already-built static .lib in your DLL project. That cannot work, you have to rebuild the .lib so that the functions get the __declspec(dllexport) declarator and will be exported by the linker.
At that point, it just isn't all that useful anymore to create the DLL compatible version of the .lib in the first place. Just create two projects, one that creates the static .lib, another that creates the DLL. Technically it is possible to still use the static .lib in your DLL project but you'll have to export the functions with a .def file. That can be high maintenance if the library has a lot of exports.
This is probably years too late, but OP's complaint was that a private method (the destructor) was not exported from a dllexport'd class. Isn't that normal? No external user is allowed to call a private method.
Been a while since I have programmed in C++, so the whole export/import idea slipped off my mind.
Can you explain me why to use __declspec(dllexport) & import thingy if it looks like I can use classes from other libraries without those.
I have created a solution in VC++ 2005, added the console applicaiton project and two dll libraries projects. Then create ClassA in a LibA, ClassB in LibB project.
Once I have included ClassA.h & ClassB.h into my console app source code, and has linked it with a LibA.lib and LibB.lib I was able to create and use instances of ClassA and ClassB in a console applicaiton. So basically I was able to use classes without exporting/importing them using __declspec.
Can you explain me - what I am missing here.
Once I have included ClassA.h & ClassB.h into my console app source code, and has linked it with a LibA.lib and LibB.lib I was able to create and use instances of ClassA and ClassB in a console applicaiton.
This sounds like you have used static linking. This works without the __declspec(dllexport) in the same way as linking with the object files of your classes directly.
If you want to use dynamic (run-time) linking with a DLL, you have to use either the mentioned declaration or a DEF-file specifying the exported functions. DLLs contain an exports table listing the functions exposed to other executables. All other functions remain internal to your DLL.
Perhaps you are confused coming from the Linux world, where the situation is the other way round: All symbols are visible externally by default.
You would use __declspec(dllexport) if you wanted to provide symbols in your dll for other dlls/exes to access.
You would use __declspec(dllimport) if you wanted to access symbols in your dll/exe provided by another dll.
Not necessary if you are linking against a static .lib.
If you are including .h files and linking to .lib files then you can drop the DLL declarations. Why do you need a dynamic link library if you only need static linking?
The export declaration marks the function as available for exporting. The declaration you are using may be a macro for "extern" and "pascal" It's been years since I've done this but I think DLLs function calls have a different ordering of pushing params on the stack and the allocation of the return result is done differently (the pascal flag). The extern declaration helps the linker make the function available when you link the library.
You may have missed the step of linking the DLL - the linker will take classA.lib and turn it into classA.dll ( you may need to setup a setupA.def file to define the DLL library). Same applies to ClassB
I need in a DLL to use a class, defined in an executable (DLL and executable are compiled by the same compiler). But I don't want the source code of this class definition to be available to DLL, only declaration.
One possible way to do it is to make all the necessary class methods to be virtual (so that DLL linker will not need these methods' definitions). The disadvantages of this approach:
I cannot create objects of exported classes
in DLL code using new (a have to
create additional functions in
executable's code).
I have to make all these methods virtual,
even if otherwise they don't need to
be virtual.
There is a way to export a class from a DLL to an executable using Microsoft's __declspec(dllexport) storage-class extended attribute.
Is there a way to export a class from executable to DLL using the same technique?
My old Borland C 6 compiler does not allow me to create import library during the build of executable project. (So, when compiling the DLL, linker gives me unresolved external error messages for all imported non-virtual class methods.) Is it a limitation of this very compiler, or maybe I'm missing something important?
So far as I know, it is ok to use MS VS's dllexport to export a class or function from a exe and use it in a DLL. and it runs cool if your DLL and Exe execute in one process.
You could put the class in a second DLL if you really didn't want it in the first one.
I'm having a hard time understanding your reasoning for not just putting the class in the DLL though.
ETA: Did some more digging and found this link that explains how to generate an import library from an EXE project in Visual Studio 2008. As for how to export them, it looks like you just use the regular __declspec(dllexport).
OK, new answer in light of new information here. If you can't generate an export library for your EXE with your compiler, and you really really have to do it this way, here's a creative, hacky and generally not recommended solution:
Step 1: Create a C wrapper API for your class, kinda like this (probably wont compile, but you get the idea):
// Yes, need some 32 bit/64 bit checks here
#define MYHANDLE unsigned int
__declspec(dllexport) MYHANDLE MyClassNewInstance() {
MyClass* ptr = new MyClass();
return (MYHANDLE)ptr;
}
__delspec(dllexport) MyClassDoSomething( MYHANDLE handle, int parm ) {
MyClass* ptr = (MyClass*)handle;
ptr->DoSomething(parm);
}
etc..
Step 2: To actually get the C functions from the EXE for use in the DLL, use the Win32 API functions GetModuleHandle() and GetProcAddress().
Step 3: Create a proxy class in your DLL. The methods in the proxy class do nothing but call their counterpart C functions from the EXE.
This would keep the "real" implementation of your class out of the DLL. It's a hack, but it would probably work.