使用 dllexport 从 DLL 导出函数

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最佳答案

If you want plain C exports, use a C project not C++. C++ DLLs rely on name-mangling for all the C++isms (namespaces etc...). You can compile your code as C by going into your project settings under C/C++->Advanced, there is an option "Compile As" which corresponds to the compiler switches /TP and /TC.

If you still want to use C++ to write the internals of your lib but export some functions unmangled for use outside C++, see the second section below.

Exporting/Importing DLL Libs in VC++

What you really want to do is define a conditional macro in a header that will be included in all of the source files in your DLL project:

#ifdef LIBRARY_EXPORTS
#    define LIBRARY_API __declspec(dllexport)
#else
#    define LIBRARY_API __declspec(dllimport)
#endif

Then on a function that you want to be exported you use LIBRARY_API:

LIBRARY_API int GetCoolInteger();

In your library build project create a define LIBRARY_EXPORTS this will cause your functions to be exported for your DLL build.

Since LIBRARY_EXPORTS will not be defined in a project consuming the DLL, when that project includes the header file of your library all of the functions will be imported instead.

If your library is to be cross-platform you can define LIBRARY_API as nothing when not on Windows:

#ifdef _WIN32
#    ifdef LIBRARY_EXPORTS
#        define LIBRARY_API __declspec(dllexport)
#    else
#        define LIBRARY_API __declspec(dllimport)
#    endif
#elif
#    define LIBRARY_API
#endif

When using dllexport/dllimport you do not need to use DEF files, if you use DEF files you do not need to use dllexport/dllimport. The two methods accomplish the same task different ways, I believe that dllexport/dllimport is the recommended method out of the two.

Exporting unmangled functions from a C++ DLL for LoadLibrary/PInvoke

If you need this to use LoadLibrary and GetProcAddress, or maybe importing from another language (i.e PInvoke from .NET, or FFI in Python/R etc) you can use extern "C" inline with your dllexport to tell the C++ compiler not to mangle the names. And since we are using GetProcAddress instead of dllimport we don't need to do the ifdef dance from above, just a simple dllexport:

The Code:

#define EXTERN_DLL_EXPORT extern "C" __declspec(dllexport)


EXTERN_DLL_EXPORT int getEngineVersion() {
return 1;
}


EXTERN_DLL_EXPORT void registerPlugin(Kernel &K) {
K.getGraphicsServer().addGraphicsDriver(
auto_ptr<GraphicsServer::GraphicsDriver>(new OpenGLGraphicsDriver())
);
}

And here's what the exports look like with Dumpbin /exports:

  Dump of file opengl_plugin.dll


File Type: DLL


Section contains the following exports for opengl_plugin.dll


00000000 characteristics
49866068 time date stamp Sun Feb 01 19:54:32 2009
0.00 version
1 ordinal base
2 number of functions
2 number of names


ordinal hint RVA      name


1    0 0001110E getEngineVersion = @ILT+265(_getEngineVersion)
2    1 00011028 registerPlugin = @ILT+35(_registerPlugin)

So this code works fine:

m_hDLL = ::LoadLibrary(T"opengl_plugin.dll");


m_pfnGetEngineVersion = reinterpret_cast<fnGetEngineVersion *>(
::GetProcAddress(m_hDLL, "getEngineVersion")
);
m_pfnRegisterPlugin = reinterpret_cast<fnRegisterPlugin *>(
::GetProcAddress(m_hDLL, "registerPlugin")
);

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I think _naked might get what you want, but it also prevents the compiler from generating the stack management code for the function. extern "C" causes C style name decoration. Remove that and that should get rid of your _'s. The linker doesn't add the underscores, the compiler does. stdcall causes the argument stack size to be appended.

For more, see: http://en.wikipedia.org/wiki/X86_calling_conventions http://www.codeproject.com/KB/cpp/calling_conventions_demystified.aspx

The bigger question is why do you want to do that? What's wrong with the mangled names?

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I had exactly the same problem, my solution was to use module definition file (.def) instead of __declspec(dllexport) to define exports(http://msdn.microsoft.com/en-us/library/d91k01sh.aspx). I have no idea why this works, but it does

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For C++ :

I just faced the same issue and I think it is worth mentioning a problem comes up when one use both __stdcall (or WINAPI) and extern "C":

As you know extern "C" removes the decoration so that instead of :

__declspec(dllexport) int Test(void)                        --> dumpbin : ?Test@@YaHXZ

you obtain a symbol name undecorated:

extern "C" __declspec(dllexport) int Test(void)             --> dumpbin : Test

However the _stdcall ( = macro WINAPI, that changes the calling convention) also decorates names so that if we use both we obtain :

   extern "C" __declspec(dllexport) int WINAPI Test(void)   --> dumpbin : _Test@0

and the benefit of extern "C" is lost because the symbol is decorated (with _ @bytes)

Note that this only occurs for x86 architecture because the __stdcall convention is ignored on x64 (msdn : on x64 architectures, by convention, arguments are passed in registers when possible, and subsequent arguments are passed on the stack.).

This is particularly tricky if you are targeting both x86 and x64 platforms.

Two solutions

Use a definition file. But this forces you to maintain the state of the def file.
the simplest way : define the macro (see msdn) :

#define EXPORT comment(linker, "/EXPORT:" __FUNCTION__ "=" __FUNCDNAME__)

and then include the following pragma in the function body:

#pragma EXPORT

Full Example :

 int WINAPI Test(void)
{
#pragma EXPORT
return 1;
}

This will export the function undecorated for both x86 and x64 targets while preserving the __stdcall convention for x86. The __declspec(dllexport) is not required in this case.