I am porting a C++ project from an old Borland compiler to VisualStudio 2008. This project uses a third party DLL that I don't have the source code for, so I am unable to recompile or modify it. The header file for the DLL defines functions along the lines of:
extern "C" {
void __stdcall Init(int a, int b);
}
However when I try to link to this DLL, VisualStudio says that it cannot find the function _Init#8. When I look at the DLL I find that the function name is _Init, and not _Init#8 (it seems that the DLL to predates Microsoft adding #8 to stdcall name mangling).
My question is: How can I call funcions in this DLL? Does VisualStudio simply not support these old DLLs, or is there a flag/setting that I'm missing. (I am aware that I could use the LoadLibrary/GetProcAddress functions to dynamically call the functions at run-time, but would prefer to not to.)
I didn't notice anything that would tell me whether you are doing 32 or 64 bit coding. MSDN has some information on how to create an import library based on a dll that you have no source for. This article is for making a 32 bit import library. CHEERS!
Related
I have some DLLs that I want to use in a FORTRAN Project in VISUAL STUDIO but I can't find how.
Here is a simple code I'm using to find out how.
Using visual studio I created a DLL from this
subroutine printing
!DEC$ ATTRIBUTES DLLEXPORT::printing
print*,"dll naimi created"
end subroutine printing
I added the link of the DLL to project>properties>Linker>General>Additional Library directories
Main program:
program Console11
implicit none
call printing
end program Console11
ERROR : Error 1 error LNK2019: unresolved external symbol _PRINTING referenced in function _MAIN__.
other solutions related to this suggest using the .lib created while generating the DLL, but in my real case I only have the DLLs without their .lib.
So how to use a DLL ... ?
You appear to be trying to use a DLL as an input file to the linker.
(You also appear to be trying to specify a file for a linker option that takes a directory. To specify an additional input file for the linker, either add the file to the project just like you would attach a source file, or use the "Linker > Input > Additional dependencies" project property. The property you mention in your post then tells the linker where (which directories) to search for those additional dependencies.)
Unlike the unix convention, you do not link against DLLs when building executables and other DLLs on Windows. The DLL typically does not contain the necessary information for the linker - instead that information is contained in an import library (which is just a variation of a typical .lib static library) or equivalent.
If you were able to successfully build a DLL, then you will probably find the import library for that DLL in the same directory as the DLL. Supply that import library as an additional dependency for projects that require the DLL.
When you link an EXE or other DLL using an import library on Windows, the target DLL is automatically loaded by the operating system when your executable code is loaded. This is called load time dynamic linking.
If you do not have the import library for a DLL, then your choices are:
Ask the person who built the DLL for the import library.
Reference the DLL using run time dynamic linking, rather than load time. This means that you use the Windows API functions LoadLibrary, GetProcAddress and friends in your program to explicitly tell the operating system to load a particular DLL and to obtain the address of a function pointer. If you are using Intel Fortran, then complete examples of this are installed with the compiler - see in the file "C:\Program Files (x86)\IntelSWTools\samples_2016\en\compiler_f\psxe\DLL.zip" or similar.
Generate an import library from the minimum information in the DLL, plus other information about the DLL that you may have. One approach to this is to write a module definition file (.def) for the DLL, and then use the LIB utility to turn that def file into an import library. See How to make a .lib file when have a .dll file and a header file for an example.
I'm around trying to create an interoperable dll written in C # for use in Microsoft NAV 2016, for the development of a fiscal printer driver.
In my C # project I called to other functions dll (delivered by the company Bematech, in 32-bit and unmanaged).
Calls to the functions of the dll unmanaged (and tested from an .EXE) I make as follows:
[DllImport ( "BemaFi32.dll")]
public static extern int Bematech_FI_ProgramaAlicuota (Aliquot string, int ICMS_ISS);
When I try to run it from the NAV2016 I get the following error: .A call to "MyAssembly" failed with this message: You are trying to load a program with an incorrect format. Expection from HRESULT: 0x8007000B
I guess the error because the dll is not managed or 32-bit version (if that's the problem becomes more difficult because apparently there is no 64-bit version). Not how to solve the problem.
NAV cannot work with unmanaged code.
The only way is to write a wrapper dll which can be called from NAV.
Microsoft is doing the same with the Office SDK for example...
Cheers!
The problem was not the embedded dll unmanaged, was that the new dll to be 32 bit was not compatible with the client NAV, the solution was to change the 32bit client as follows:
Rename the file to Microsoft.Dynamics.Nav.Client.X64.exe Microsoft.Dynamics.Nav.Client.exe, and then a Microsoft.Dynamics.Nav.Client.X86.exe rename the file to Microsoft.Dynamics.Nav.Client. exe.
and the C / AL change in the variable declared the call to the dll
Property Value
Yes RunOnClient
I am playing with DLLs to get a better understanding of them. So I created a simple dll (with load-time dynamic linking) which has functions to Add, Sub and Mul. In the header file for the dll I used __declspec(dllexport) for the function declaration.
For the executable, I added the .lib created after compiling the dll to the properties (for linking). After that I directly called the function Add without using __declspec(dllimport). The program worked. I then changed the function calling to __declspec(dllimport) (Add) and the program worked again.
I am not able to understand what the need of __declspec(dllimport) is? I have not yet coded a run-time linking DLL but from the examples I have seen, dllimport is not needed in that case as well.
Thanks for your assistance.
I'm new to vc++. I've just built a software and it generated a .dll and a .lib. I need to use functions from this in my code. Do I need to link to both .lib and .dll to build my code? What project properties do I have to alter to do this linking?
Actually, you need only the .dll file. It contains all the necessary code and data to run it's functions. It also contains a table that links the symbolic names of the functions (e.g. the function PrintMe), their ordinals (the number of that function in the DLL) and their addresses in the DLL.
If you want to use only the DLL, you have to "manually" get the symbols resolved:
Let's say you want to use the function PrintMe of the DLL. What you had to do is to resolve it's name (PrintMe) or it's ordinal (PrintMe is the 1st function of the DLL) to it's address. For this, you could use LoadLibrary, GetModuleHandle and GetProcAdress from the Win32 API (aka Windows SDK). Additionally, this method allows you to load the DLL at runtime (see below).
The easier way is to use the MSVC(++) features __declspec(dllexport) and __declspec(dllimport), e.g.
// your DLL
__declspec(dllexport) void PrintMe()
{
printf("Hello World!");
}
// you project to use the DLL
__declspec(dllimport) void PrintMe();
The first one (dllexport) tells the compiler to export the function. The second one (dllimport) is the interesting one: It creates all the necessary code to be able to use the function from the DLL.
For this, you need the .lib file in your project (which wants to use the DLL). The .lib file contains information for the linker to resolve the symbol name (PrintMe) to its address in the DLL. Since the .lib is statically bound, the linker can make use of it - the DLL on the contrary is bound at runtime / loading time, so the linker cannot use it. (Yes, the information in the .lib file is redundant.). Note: You cannot change the whole DLL when using this method w/o rebuilding your project with the new .lib file. Some structure changes affect the addresses of the functions in the DLL, see this SO answer.
One last difference between using the Win32 API (LoadLibrary...) and the MSVC method via __declspec is the loading of the DLL. When you use LoadLibrary, the DLL is loaded at runtime, of course (so you can catch exceptions when it cannot be found and so on). The other method loads the DLL at loading time, so you program will terminate (will not run) when Windows cannot find the DLL.
When you create a project in VS, you can activate the "export symbols" checkbox at the end of a wizard (Win32 project). That gives you some examples of exported symbols. Additionally, it introduces a macro plus a preprocessor defition plus some directives that are very useful:
// DLL header
#ifdef _YOUR_DLL_EXPORTS
#define YOUR_DLL_API __declspec(dllexport)
#else
#define YOUR_DLL_API __declspec(dllimport)
#endif
YOUR_DLL_API PrintMe();
You now can use this header file to build you DLL as your DLL project has that _YOUR_DLL_EXPORTS definition (see project properties page, C++, preprocessor). The project that uses the DLL can use this header, too, but then must not have such a name defined. When you include the header file in the project in which you want to use the DLL, the macro is resolved to __declspec(dllimport). This instructs the linker to look for this function (which is found in the .lib file) and create all the necessary code to load the DLL at runtime and resolve the symbol name.
I'm just looking for a simple, concise explanation of the difference between these two. MSDN doesn't go into a hell of a lot of detail here.
__declspec( dllexport ) - The class or function so tagged will be exported from the DLL it is built in. If you're building a DLL and you want an API, you'll need to use this or a separate .DEF file that defines the exports (MSDN). This is handy because it keeps the definition in one place, but the .DEF file provides more options.
__declspec( dllimport ) - The class or function so tagged will be imported from a DLL. This is not actually required - you need an import library anyway to make the linker happy. But when properly marked with dllimport, the compiler and linker have enough information to optimize the call; without it, you get normal static linking to a stub function in the import library, which adds unnecessary indirection. ONT1 ONT2
__declspec(dllexport) tells the linker that you want this object to be made available for other DLL's to import. It is used when creating a DLL that others can link to.
__declspec(dllimport) imports the implementation from a DLL so your application can use it.
I'm only a novice C/C++ developer, so perhaps someone's got a better explanation than I.
Two different use cases:
1) You are defining a class implementation within a dll. You want another program to use the class. Here you use dllexport as you are creating a class that you wish the dll to expose.
2) You are using a function provided by a dll. You include a header supplied with the dll. Here the header uses dllimport to bring in the implementation to be used by the current program.
Often the same header file is used in both cases and a macro defined. The build configuration defines the macro to be import or export depending which it needs.
Dllexport is used to mark a function as exported. You implement the function in your DLL and export it so it becomes available to anyone using your DLL.
Dllimport is the opposite: it marks a function as being imported from a DLL. In this case you only declare the function's signature and link your code with the library.