#if COMPILING_DLL
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT __declspec(dllimport)
#endif
How / where do I define COMPILING_DLL ?
Seen here:
what does __declspec(dllimport) really mean?
Sounds like I can't use load-time dynamic linking at all if I can't use the same header?
One another option:
Use the default defined macro local to the project.
You can see the default defined macros local to the project in the below location:
Properties -> C/C++ -> Preprocessor -> Preprocessor Definition.
Example:
Suppose your Project Name is: MyDLL
Default Macro Local to that project: MYDLL_EXPORTS
#ifdef MYDLL_EXPORTS
/*Enabled as "export" while compiling the dll project*/
#define DLLEXPORT __declspec(dllexport)
#else
/*Enabled as "import" in the Client side for using already created dll file*/
#define DLLEXPORT __declspec(dllimport)
#endif
Best place to define COMPILING_DLL=1 is command line of compiler. If you use Visual Studio IDE then it is in Project properties ... C/C++ ... Preprocessor ... Preprocessor Definitions.
__declspec(dllimport) is Microsoft specific extension to C++. Microsoft has excellent online documentation.
In the DLL project, you add a #define (either in a header file or in the project properties) for COMPILING_DLL. As this will not be set for any other project (especially if you name it something better than COMPILING_DLL) then the #if directive will work properly.
You (actually Visual Studio in ideal cases) defines the COMPILING_DLL as an argument to the compiler when you build the DLL. So, it will default to __declspec(dllexport). On the other end, when you USE the DLL's header file, you don't define this, so DLLEXPORT will be evaluated by default to __declspec(dllimport).
If you use CMake to generate your build configuration, you should be able to use
the macro <projectname>_EXPORTS the way you want to use COMPILING_DLL, where projectname was defined with the CMake command project(projectname):
A preprocessor macro, <target_name>_EXPORTS is defined when a shared library compilation is detected.
source
I tested and it works on Windows using the Ninja generator with compiler MSVC from Visual Studio 2015 Express.
Related: CMake adds -Dlibname_EXPORTS compile definition
You can't define function body that way in the header file. It is prohibited by __declspec(dllimport). This specifier can only be specified on function declaration, not definition.
You have to move the function body to a source file.
in header file:
extern DLLEXPORT void test2();
In .cpp file:
void test2()
{
// ...
}
As folks said, don't forget to add COMPILING_DLL to the project preprocessor definitions.
Actually, the real problem is the preprocessor directive.
You should use #ifdef and not #if to test if the variable is really defined (and we don't care about the defined value or if there is any).
NOTE: I know this thread is 1-year old but it still may be useful for somebody who have this problem in the future.
Related
I had a question about DLL building / linking in Visual Studio 2005 and later. Basically my understanding and experience is this:
To build a DLL, I specify the project properties to build a DLL, and then I but __declspec(dllexport) in front of any functions or members that I want to publically expose from the DLL. Building the project will result in a DLL, a Lib, and a header file that can be deployed as say an API or something.
On the other end, to have your other compiled executable application dynamically link to the DLL and use its functions, you simply need to have your executable project include the header files and link with the small lib file that was created when the DLL was built. As long and the compiled application can find the DLL, everything will work.
That has been my experience and that is also how the Microsoft DLL building tutorial described everything on MSDN. I am wondering: is this standard practice? When would you ever need to use __declspec(dllimport) anywhere? Am I missing something?
Thanks!
Yes you would use __declspec(dllimport) and you generally have a macro that controls whether a source file either exports (if it's part of your DLL) or imports (if it's part of the using-executable) symbols.
In your DLL you can set a manifest constant to the build settings of some sort, say 'BUILDING_MY_DLL' and then create the macro like this within your header file:
#ifdef BUILDING_MY_DLL
#define MY_DLL_EXPORT __declspec(dllexport)
#else
#define MY_DLL_EXPORT __declspec(dllimport)
#endif
and then decorate your exported functions like this:
MY_DLL_EXPORT int func(int y);
You can also export entire classes this way too:
class MY_DLL_EXPORT InterestingClass
{
...
};
I had a question about DLL building / linking in Visual Studio 2005 and later. Basically my understanding and experience is this:
To build a DLL, I specify the project properties to build a DLL, and then I but __declspec(dllexport) in front of any functions or members that I want to publically expose from the DLL. Building the project will result in a DLL, a Lib, and a header file that can be deployed as say an API or something.
On the other end, to have your other compiled executable application dynamically link to the DLL and use its functions, you simply need to have your executable project include the header files and link with the small lib file that was created when the DLL was built. As long and the compiled application can find the DLL, everything will work.
That has been my experience and that is also how the Microsoft DLL building tutorial described everything on MSDN. I am wondering: is this standard practice? When would you ever need to use __declspec(dllimport) anywhere? Am I missing something?
Thanks!
Yes you would use __declspec(dllimport) and you generally have a macro that controls whether a source file either exports (if it's part of your DLL) or imports (if it's part of the using-executable) symbols.
In your DLL you can set a manifest constant to the build settings of some sort, say 'BUILDING_MY_DLL' and then create the macro like this within your header file:
#ifdef BUILDING_MY_DLL
#define MY_DLL_EXPORT __declspec(dllexport)
#else
#define MY_DLL_EXPORT __declspec(dllimport)
#endif
and then decorate your exported functions like this:
MY_DLL_EXPORT int func(int y);
You can also export entire classes this way too:
class MY_DLL_EXPORT InterestingClass
{
...
};
I am creating a c++ dll to be imported in vb.net.
I have found that the best way to organize the imports and exports in the header file of the dll, is something of the following type:
#ifndef MY_DLL_EXPORTS
#define MY_DLL_EXPORT __declspec(dllexport)
#else
#define MY_DLL_EXPORT __declspec(dllimport)
#endif
#ifdef __cplusplus
extern "C"
{
#endif
MY_DLL_EXPORT BOOL my_function(uint32_t x);
#ifdef __cplusplus
}
#endif
My question is, where are the __cplusplus and MY_DLL_EXPORTS supposed to be defined ?
I have not done this before and I can't seem to find a place in the code for it. I read something about putting these definitions in the
Project Properties -> Configuration Properties -> C/C++ -> Preprocessor -> Preprocessor Definitions
Do I put them in the DLL's project properties ? and if i do, on using this DLL, will they always be defined so the if statement will always go to the dllimport ?
Also, is __cplusplus already defined or must I define it ?
__cplusplus is defined for you by a C++ compiler, but NOT by a C compiler. This is how you can determine if your code is being compiled by a C or C++ compiler. That's why this is used to guard the extern "C" construct. A C compiler would just omit it (since it emits C linkage already).
The other constants can be either defined on the compiler's command line (eg. in the menu you listed), or in the code somewhere:
#define MY_DLL_EXPORTS
Best place to define COMPILING_DLL is command line of compiler. If you use Visual Studio IDE then it is in Project properties ... C/C++ ... Preprocessor ... Preprocessor Definitions.
__declspec(dllimport) is Microsoft specific extension to C++. Microsoft has excellent online documentation.
#if COMPILING_DLL
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT __declspec(dllimport)
#endif
How / where do I define COMPILING_DLL ?
Seen here:
what does __declspec(dllimport) really mean?
Sounds like I can't use load-time dynamic linking at all if I can't use the same header?
One another option:
Use the default defined macro local to the project.
You can see the default defined macros local to the project in the below location:
Properties -> C/C++ -> Preprocessor -> Preprocessor Definition.
Example:
Suppose your Project Name is: MyDLL
Default Macro Local to that project: MYDLL_EXPORTS
#ifdef MYDLL_EXPORTS
/*Enabled as "export" while compiling the dll project*/
#define DLLEXPORT __declspec(dllexport)
#else
/*Enabled as "import" in the Client side for using already created dll file*/
#define DLLEXPORT __declspec(dllimport)
#endif
Best place to define COMPILING_DLL=1 is command line of compiler. If you use Visual Studio IDE then it is in Project properties ... C/C++ ... Preprocessor ... Preprocessor Definitions.
__declspec(dllimport) is Microsoft specific extension to C++. Microsoft has excellent online documentation.
In the DLL project, you add a #define (either in a header file or in the project properties) for COMPILING_DLL. As this will not be set for any other project (especially if you name it something better than COMPILING_DLL) then the #if directive will work properly.
You (actually Visual Studio in ideal cases) defines the COMPILING_DLL as an argument to the compiler when you build the DLL. So, it will default to __declspec(dllexport). On the other end, when you USE the DLL's header file, you don't define this, so DLLEXPORT will be evaluated by default to __declspec(dllimport).
If you use CMake to generate your build configuration, you should be able to use
the macro <projectname>_EXPORTS the way you want to use COMPILING_DLL, where projectname was defined with the CMake command project(projectname):
A preprocessor macro, <target_name>_EXPORTS is defined when a shared library compilation is detected.
source
I tested and it works on Windows using the Ninja generator with compiler MSVC from Visual Studio 2015 Express.
Related: CMake adds -Dlibname_EXPORTS compile definition
You can't define function body that way in the header file. It is prohibited by __declspec(dllimport). This specifier can only be specified on function declaration, not definition.
You have to move the function body to a source file.
in header file:
extern DLLEXPORT void test2();
In .cpp file:
void test2()
{
// ...
}
As folks said, don't forget to add COMPILING_DLL to the project preprocessor definitions.
Actually, the real problem is the preprocessor directive.
You should use #ifdef and not #if to test if the variable is really defined (and we don't care about the defined value or if there is any).
NOTE: I know this thread is 1-year old but it still may be useful for somebody who have this problem in the future.
I have a dll with a class that define some methods and variables inside it. I marked it as
__declspec(dllexport)
and i imported the .h header inside a win32 application project in the same solution. I can use the functions but when I try to compile the project I have a lot of errors about external symbols not resolved. Why?
Please read about the standard way of using macros for this very common task here: http://wiki.tcl.tk/8721
The basic idea is that you define a macro, say MY_API like so:
#ifdef BUILD_MYAPI
# define MY_API __declspec(dllexport)
#else
# define MY_API __declspec(dllimport)
#endif
When you declare a function or a class in the header file you do this:
void MY_API myApiFunction(int x);
When you build your own dll which declares the body of the function, you add the definition of BUILD_MYAPI to the build. This makes all declerations to be dllexport
when you include the header from some other dll you don't add BUILD_MYAPI so the decelerations are dllimport
When compiling with visual studio, you can add a macro definition to the compilation without changing the source from project properties -> C/C++ -> Preprocesson -> Preprocessor definitions
For the application where you want to import that class, you will need to mark the class as
__declspec(dllimport)
Instead of dllexport.
You must also make sure to link with the import library of the DLL (a .lib file).