I have Math.h and Math.lib. How I create dynamic link library from this files?
You can create project for Dynamic Link Library (DLL) and add Math.lib as input library in project properties. Also you classes in Math.h should be exported (use __declspec(dllexport) and __declspec(dllimport) for that).
#pragma once
#ifdef MATH_DLL
#define EXPORT_CLASS __declspec(dllexport)
#else
#define EXPORT_CLASS __declspec(dllimport)
#endif
class EXPORT_CLASS Math {
public:
Math( double y );
int DoSomething( int x );
};
Macro MATH_DLL should be defined in preprocessor definitions in project properties.
Related
Hi I'm little bit confused with dllexport.When I use __declspec( dllexport ) for example in class
#define DllExport __declspec( dllexport )
class DllExport C {
int i;
virtual int func( void ) { return 1; }
};
do I export class C to dll file or do I export C class from dll file?
When compiling the DLL you have to write __declspec(dllexport) as you did. This tells the compiler you want it to be exported. When using the DLL you want __declspec(dllimport) in your included files. The compiler then knows that this functions and classes are in a DLL-file and need to be imported. Because you don't want to change your header-files that much, you should define a macro e.g. BUILD_DLL.
#ifdef BUILD_DLL
#define DLL_PORTING __declspec(dllexport)
#else
#define DLL_PORTING __declspec(dllimport)
#endif
Now you write in example.h:
class DLL_PORTING example_class { … };
In your .exe file just include the header files you need and everything will work.
I have a set of functions written in C that I need to be able to call from another project written in C++. The C code is essentially some functions that do some calculations on a large data set. I didn't write them - all I want to do is allow my C++ project to be able to call those functions. My solution was to create a DLL for the C code and link it to my C++ project.
In order to make the DLL, I structured myCproj.h (the header in the C project, not C++ project) like so:
#ifdef __cplusplus
extern "C" {
#endif
struct __declspec(dllexport) neededStruct {
int a;
//I need to be able to initialize this struct in my C++ project.
}
__declspec(dllexport) void neededFunc( struct neededStruct *input ) {}
//I need to be able to call this function from my C++ project and feed
//it my local instance of neededStruct.
#ifdef __cplusplus
}
#endif
The src file, myCproj.c, was not changed at all. The function definitions do not have __declspec(dllexport)in front of them, nor is extern "C" inserted anywhere. The code compiles without error and produces myCproj.dll and myCproj.lib.
I then tell my C++ project in VS where to find myCproj.lib and myCproj.h accordingly and copy the DLL over to the directory where my C++ executable lives. To use the DLL, I gave myCPPproj.cpp the following addition:
#define DLLImport __declspec(dllimport)
struct DLLImport neededStruct input;
input.a = 0;
extern "C" DLLImport void neededFunc( &input );
However, I get error EO335 'linkage specification is not allowed' on that last line. What am I doing wrong?
It is preferable to use the same header for both the library and using code.
As mentioned, it is usually done by a conditional define, like the following:
MyLibrary.h:
#if defined(MYLIBRARY_API)
#define MYLIBRARY_EXPORTS __declspec(dllexport)
#else
#define MYLIBRARY_EXPORTS __declspec(dllimport)
#endif
#if defined(__cplusplus)
extern "C" {
#endif
MYLIBRARY_API bool MyLibFunc();
#if defined(__cplusplus)
#endif
MyLibrary.c:
#include "MyLibrary.h"
void MyLibFunc()
{
....
}
App.cpp:
#include <MyLibrary.h>
int main()
{
MyLibFunc();
}
The symbol MYLIBRARY_API will be defined for the library project (usually as a /D on the compiler command line). And if you use visual studio that is pretty much exactly what you get when creating a dll project with exports.
To be clear:
I'm aware that the below example demonstrates a dll-dependancy, i.e. one library is not self-containe, but depends on another library to function.
Let's say I'm creating a runtime library, Utility.dll, which contains various useful functions of general nature.
I create a header file Utility.h to be included in other files which need to use Utility.dll.
The header file looks like
#ifndef _UTILITY_H
#define _UTILITY_H
#ifdef BUILD_DLL
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif
DLL_EXPORT void foo();
DLL_EXPORT void foo2();
....
#endif
When I compile the source code file Utility.cpp into machine code (into Utility.dll) I make sure BUILD_DLL is defined so DLL_EXPORT gets replaced with __declspec(dllexport). This makes the functions be exported to the .dll file.
Whenever I include the header Utility.h and link with the import library (Utility.lib for MS VS, libUtility.a for g++) and do not define BUILD_DLL, the function declarations in Utility.h begin with __declspec(dllimport) instead, telling the compiler that the functions are imported from a .dll (so to speak).
Now, let's say I'm also building another library, MyLibrary.dll, which wants to use some of the useful functions in Utility.dll. Similarily, I would create MyLibrary.h as
#ifndef _MYLIBRARY_H
#define _MYLIBRARY_H
#ifdef BUILD_DLL
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif
DLL_EXPORT void myLibraryFunc1();
....
#endif
When I compile MyLibrary.cpp into MyLibrary.dll I'm including Utility.h and also linking against the Utility import library.
This leads us to my question:
Since I define BUILD_DLL also when I compile MyLibrary.dll, this means that the function declarations in Utility.h also will read
__declspec(dllexport) void foo();
__declspec(dllexport) void foo2();
....
Not
__declspec(dllimport) void foo();
__declspec(dllimport) void foo2();
Don't we want it to be __declspec(dllimport) for the function declarations in Utility.h when we compile MyLibrary.dll, and __declspec(dllexport) for the function declarations in MyLibrary.h?
This is precisely the reason why you normally don't name such macros BUILD_DLL, but BUILD_UTILITY and BUILD_MYLIBRARY or similar. Likewise, the declspec macro should not be DLL_EXPORT, but UTILITY_EXPORT and MYLIBRARY_EXPORT (or perhaps UTILITY_API and MYLIBRARY_API).
I have a base class (QIndicator) and I want to implement derived classes in DLLs. The DLL project in Visual Studio 2012 for a sample derived class has the following code:
header file with base class
#ifndef _DLL_COMMON_INDICATOR_
#define _DLL_COMMON_INDICATOR_
// define the DLL storage specifier macro
#if defined DLL_EXPORT
#define DECLDIR __declspec(dllexport)
#else
#define DECLDIR __declspec(dllimport)
#endif
class QIndicator
{
private:
int x;
int y;
};
extern "C"
{
// declare the factory function for exporting a pointer to QIndicator
DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}
#endif
source file with derived class
#define DLL_EXPORT
#include "indicator.h"
class QIndicatorDer : public QIndicator
{
public:
QIndicatorDer (void) : QIndicator(){};
~QIndicatorDer (void){};
private:
// list of QIndicatorDer parameters
int x2;
int y2;
};
extern "C"
{
DECLDIR QIndicator * __stdcall getIndicatorPtr(void)
{
return new QIndicatorDer();
};
}
The problem I have is that upon successful build, the produced DLL file does not contain the exported getIndicatorPtr function (as shown by DependencyWalker). I checked whether the dllexport keyword gets propagated properly into the declaration of getIndicatorPtr and it does.
Another interesting problem is that I already have another derived class like this, in another DLL project, that I created some months ago. This older project is basically the same and everything works well there. I checked all properties of both the old and the current projects, and they seem identical. So I ran out of ideas, why I can't get getIndicatorPtr to export.
Any help is much appreciated,
Daniel
That's because it's not being exported. Why?
__declspec specifier should only be placed in the declaration of a function, not it's definition. Also, avoid something like #define DLL_EXPORT. Preprocessor definitions should either defined in project properties (MSVC) or command line option (-D in GCC, for example).
Look at you code:
Header
extern "C"
{
DECLDIR QIndicator * __stdcall getIndicatorPtr(void);
}
When compiler parses this header, is sees DECLDIR as dllimport (because you define DLL_EXPORT in .cpp). Then in .cpp, it suddenly appears as dllexport. Which one is used? The first one.
So, leave your header (it's fine), but change your source:
//#define DLL_EXPORT -> remove this!
#include "indicator.h"
class QIndicatorDer : public QIndicator
{
//...
};
extern "C"
{
/* DECLDIR -> and this! */ QIndicator * __stdcall getIndicatorPtr(void)
{
return new QIndicatorDer();
};
}
Then, go to project properties (I assume you use Visual Studio) and then C/C++ -> Preprocessor -> Preprocessor Definitions and add there DLL_EXPORT=1.
That should work.
I am using Visual Studio 2012 and I creared dll without using MFC, it generating the dll. But when I specify to generate a lib file in specific directory using the following option:-
go to proeperties page->Advanced->Import Library
../../../lib/myapp.lib
It is not genearting the lib file in the specified folder. Please help me .
Thanks & Regards
Vikas
Just resolved a similar problem. Visual Studio does not create a .lib file without instructing which objects to expose in dll. You need to create a win exports header like this:
#ifndef BLABLABLA
#define BLABLABLA
#ifdef MYAPPLIB_EXPORTS
#define MYAPPLIB_API __declspec(dllexport)
#elif defined(MYAPPLIB_EXPORTS_STATIC)
#define MYAPPLIB_API
#else
#define MYAPPLIB_API __declspec(dllimport)
#endif
#endif // !BLABLABLA
Then, you need to declare MYAPPLIB_EXPORTS as a preprocessor macro. After including this new header file in other API headers, for every class or method which you want to expose in your API, you can add MYAPPLIB_API to their declarations like:
class MYAPPLIB_API MyClass{ ... };
calculatelibrary.h
/*
By default, the New Project template for a DLL adds PROJECTNAME_EXPORTS to the defined preprocessor symbols for the DLL project.
In this example, CALCULATELIBRARY_EXPORTS is defined when your calculateLibrary DLL project is built.
When the CALCULATELIBRARY_EXPORTS symbol is defined, the CALCULATELIBRARY_API symbol sets the __declspec(dllexport) modifier on the member function declarations in this code.
This modifier tells the compiler and linker to export the function or variable from the DLL so that it can be used by other applications.
When CALCULATELIBRARY_EXPORTS is undefined—for example, when the header file is included by a client application—CALCULATELIBRARY_API defines the __declspec(dllimport) modifier on the member function declarations. This modifier optimizes the import of the function in an application. For more information
*/
#ifndef _calculate_library_h
#define _calculate_library_h
#ifdef CALCULATELIBRARY_EXPORTS
#define CALCULATELIBRARY_API _declspec(dllexport)
#else
#define CALCULATELIBRARY_API _declspec(dllimport)
#endif
namespace calculatelibrary
{
class CALCULATELIBRARY_API clsCalculateLibrary{
// Returns a + b
double Add(double a, double b);
};
}
#endif _calculate_library_h
// CalculateLibrary.cpp : Defines the exported functions for the DLL application.
//
#include "stdafx.h"
#include "CalculateLibrary.h"
namespace calculatelibrary
{
double clsCalculateLibrary::Add(double a, double b)
{
return a + b;
}
}
See the following: you may need to declare /EXPORTS (http://msdn.microsoft.com/en-us/library/7k30y2k5.aspx)
http://msdn.microsoft.com/en-us/library/67wc07b9.aspx see "REMARKS"