I am using QT 5.5.0 within QtCreator to build a C++ application (referenced to as "the application" from now on) which I am trying to link against dynamic link C library ("the library"). All methods which originate from this library cannot be found by the linker when building the application, example output for one method:
error: LNK2019: unresolved external symbol "__declspec(dllimport) int __cdecl rlwe_FFT_CTX_init(struct fft_ctx *)" (__imp_?rlwe_FFT_CTX_init##YAHPAUfft_ctx###Z) referenced in function [...]
The library is imported by the following statement in the .pro file:
LIBS += "-L$$_PRO_FILE_PWD_/../../Libraries/rlwekex/Build/rlwekex/Release/Win32/" -lrlwekex
I expect this to work, since qmake is running fine and the linker will output another error if I change this to a non-existent library name. Both the application and the library are 32 bit.
The .lib and .dll files for the library have been built by myself using Visual Studio 2013. The same header files are used for building the library and as includes inside the application. A compile switch will mark methods as __declspec(dllexport) or __declspec(dllimport) using the following code:
# ifdef RLWEKEX_DLL_EXPORT
# define RLWEKEX_EXPORT __declspec(dllexport)
# else
# define RLWEKEX_EXPORT __declspec(dllimport)
# endif
The macros are correctly evaluated in both the library (export) as well as the application (import). The signature of the example method would be as follows:
RLWEKEX_EXPORT int rlwe_FFT_CTX_init(FFT_CTX *ctx);
Using dumpbin I can verify that the method is actually exported to the .lib - corresponding /HEADERS entry:
Version : 0
Machine : 14C (x86)
TimeDateStamp: 563B4D17 Thu Nov 05 13:35:35 2015
SizeOfData : 0000001F
DLL name : rlwekex.dll
Symbol name : _rlwe_FFT_CTX_init
Type : code
Name type : no prefix
Hint : 2
Name : rlwe_FFT_CTX_init
I have another Visual Studio 2013 project which is a test suite (written in C) for this particular library, which is linking against it just fine! QtCreator and Visual Studio 2013 are running on the same machine and are both using the same compiler toolkit (msvc).
Now of course my question: What could be the reason that I can link against the library in the VS2013 test suite, but not in the QT project? Any help would be appreciated!
C++ mangles names of functions while C does not.
The header file for the library needs to contain, near the beginning of the file, something like:
#ifdef __cplusplus
extern "C" {
#endif
and near the end of the header file
#ifdef __cplusplus
}
#endif
Note: the above lines are between the 'include guards', not to replace them
.
Related
I'm writing a DLL for an existing application. The DLL is designed to interface to commercial software who's functionality can be enhanced by adding a user defined feature with a DLL.
My DLL compiles and the application can utilize the features, but I want to debug it. It is difficult for me to attach to the process and debug when the commercial software is calling the DLL. Instead, I'd like to build separate code to test the DLL.
All the examples I can find w/DLL's are for C# or C++. I can't quite understand how to do this in C. I seem to be stuck at importing the DLL or referencing the library in the linker.
I've inserted:
__declspec(dllimport) int UserDefinedSurface4(USER_DATA *UD, FIXED_DATA4 *FD);
but I get:
LNK2019 unresolved external symbol "__declspec(dllimport) int __cdecl UserDefinedSurface4(struct USER_DATA *,struct FIXED_DATA4 *)" (__imp_?UserDefinedSurface4##YAHPEAUUSER_DATA##PEAUFIXED_DATA4###Z)
Following advice for C# and C++, I've added a path to the DLL in my project using: Properties->Linker->Input->Additional Dependencies with an explicit path: "D:......\mydll.lib" (that seems kind of brittle... is this right or should I use a relative path or ...?)
Furthermore, I also inserted:
#pragma comment(lib, "D:\\...\\mydll.lib")
and I have included the project with the DLL in my references. What's missing?
Your dll import looks fine.
Check the name of the exported function. Good tool for this task is "Dependency Walker". If your dll compiled as C++, function name can be mangled. To prevent name mangling your export definition in DLL project should be:
extern "C" __declspec(dllexport) int UserDefinedSurface4(USER_DATA *UD, FIXED_DATA4 *FD);
In your application specify Linker -> Additional Library Directories to your library directory and in Linker -> Input specify additional dependency mydll.lib. After that check Linker -> Command Line parameter, it should contains:
"mydll.lib" /LIBPATH:"d:\yourlibrarydir\"
The short question is: Which directive adds no prefix to an imported symbol?
I have a library that is 100% C (no C++ and not C compiled with a C++ compiler) It's C top to bottom. (I have to mention that because sometimes folks confuse how Visual Studio works when compiling C using the C++ compiler with C using a C compiler.)
The library was compiled with CMAKE and it outputted a DLL with a symbol table that looks like this:
dumpbin.exe /exports mydll.dll
...stuff...
1 0 00001087 mylib_get_x
..etc..
There's a static lib file too.
dumpbin.exe /exports mydll.lib
...stuff...
mylib_get_x
Notice how there are no weird prefixes? (i.e. no imp on the symbols) This makes the DLL extremely useful when trying to import the functions into things like the FFI in LuaJIT and other compilers/environments.
And I have an app that uses "mylib_get_x()"
/* myfile.c */
... stuff ...
int x;
x = mylib_get_x();
The result is: unresolved external symbol _mylib_get_x referenced in function ...
If I prefix the mylib_get_x() with any of these:
__declspec(dllimport)
__cdecl
__stdcall
_cdecl
extern
extern "C" __declspec(dllimport)
<nothing>
It doesn't matter. It won't work. They add prefixes. Which one adds nothing?
When it was compiled, it had __declspec(dllexport) on it, but it's a 3rd party library, so somehow they got it to avoid the extra prefixes.
I can link in the supplied .lib file that was generated. Same result. (see the dumpbin above that shows the symbol table from the .lib file without prefixes) I can create a manual .def file, generate a lib file from it and link to that. Same result.
I would love it if someone could explain what prefixes go with what directive.
Which directive adds no prefix to the symbol?
I cannot implicitly link DLL to C console application.
I use Visual Studio 2008.
I created empty DLL project "Library" which contains only one file main.c:
__declspec(dllexport) int get_value()
{
return 123;
}
I also created empty console project "CallingProgram" with file main.c:
__declspec(dllimport) int get_value();
void main()
{
int result = get_value();
}
I added "Library.lib" to "Linker\Input\Additional Dependencies".
And still I have this error:
error LNK2019: unresolved external symbol __imp__get_value referenced in function _main
I tested created DLL with LoadLibrary/GetProcAddress - it works fine.
I checked Library.dll using DumpBin and it also looks good:
Microsoft (R) COFF/PE Dumper Version 9.00.30729.01
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file library.dll
File Type: DLL
Section contains the following exports for Library.dll
00000000 characteristics
5340072C time date stamp Sat Apr 05 17:37:48 2014
0.00 version
1 ordinal base
1 number of functions
1 number of names
ordinal hint RVA name
1 0 00011109 get_value = #ILT+260(_get_value)
Summary
1000 .data
1000 .idata
2000 .rdata
1000 .reloc
1000 .rsrc
4000 .text
10000 .textbss
Please help me understand what is missing!
1 0 00011109 get_value
The symbol does not have its normal decoration. It would normally be _get_value, all cdecl functions get a leading underscore. And using __declspec(dllexport) also provides the __imp_get_value export. It is a function pointer that optimizes the binding.
But that did not happen, you must have used a .def file in your library project. Which renames exported functions. Which is okay, but now your __declspec(dllimport) is incompatible, the DLL no longer exports the __imp_ function pointer. The linker complains because it cannot find it the import library.
Fix this either by removing the .def file from your library project (best) or by deleting the __declspec(dllimport) attribute from the declaration in your exe project. Writing a .h file that declares the exported functions in the DLL is also highly recommended.
Just to be clear - I have searched the depths of the internet and back for information on how to do this
I'm looking for assistance setting up pthread_Win32 to work with Visual Studio 2005. I'm programming in C, and I have a number of multithreaded assignments to write using pthread.h. However, since pthread is native to unix, I have to write all of my code, ftp it, and then ssh to my class' remote unix system to run it. It makes development take so much longer, and it's incredibly inefficient. I'd love (more than anything) to be able to get this working on my win32 machine, so I can develop in visual studio as I've been doing for quite some time.
I've installed the pthread.lib file and pthread.h file into the respective lib/header directories, where all of the other files are. The DLL on the other hand (the actual library), I've placed in c:\windows\system32. I've tried to add the DLL as a dependency (right click project -> references -> Add new reference), but as others have stated, all I get is a blank dialogue box with no option to add any DLL files or anything. It seems to recognize the header file, but I get these errors when I compile:
1>Linking...
1>main.obj : error LNK2019: unresolved external symbol _imp_pthread_join referenced in function _main
1>main.obj : error LNK2019: unresolved external symbol _imp_pthread_create referenced in function _main
1>main.obj : error LNK2019: unresolved external symbol _imp_pthread_exit referenced in function _fcount
From my research, I've determined that this is a problem with the DLL, and I'm assuming it can't find the definitions of the functions I've referenced in my code. I've searched high and low and I can't seem to figure out any way to overcome this problem. I've added the directories of the lib/header files to my linker, just in-case, but that didn't solve the issue. I need to do something, in visual studio, to specify that I need pthreadVC2.dll as a project dependency - and it seems to be impossible (and extremely frustrating) at this point.
Any words of wisdom?
Thank you very much
I have been through this problem recently.
It appears that the __imp__ prefix is prepended to the function name in case pthread.h is included in dynamic linking mode.
Simply add the PTW32_STATIC_LIB define to your project or your source code before including pthread.h should solve the issue.
#define PTW32_STATIC_LIB
#include <pthread.h>
Although, I am not completely over as Visual Studio now trys to link with the _[FuncName] instead of [FuncName]
In visual studio, function seems to be declared differently wether you are going to link them statically (.lib) or dynamically (.dll).
To define a function you will link dynamically :
__declspec (dllimport) int myFunc(int myArgs) ;
To define function you are exporting for dynamic linking :
__declspec (dllexport) int myFunc(int myArgs) ;
And the simpliest, to define a function you will link statically:
int myFunc(int myArgs) ;
[EDIT]
I am going on my investigations and went through this on MS help center.
It seems that to avoid the _[FuncName] effect it is required to define a static linked library function by the following:
int __cdecl myFunc(int MyArgs) ;
Have you added pthreadVC.lib (or whichever particular lib you need) to the project property:
Linker/Input/Additional Dependencies
It's not enough to just have the lib file in a particular directory, the linker needs to be told to use it as an input.
Just adding pthreadVC2.lib to linker list is not suffiecient.
You also have to add addtional lib like pthreadVCE2.lib and pthreadVSE2.lib.
I am facing same issue but then I resolved it through adding these files.
I desperatly need your help.
Im trying to compile statically the poppler library (specially for qt4) on windows with the visual c++ 2008 compiler. To achieve this task I needed to compile a bunch of other libraries as dependencies for poppler statically too. When I finally generate the static version of poppler I got a linking error when building my app:
error LNK2019: unresolved external symbol "__declspec(dllimport)...
I already added the new include path and linked the poppler-qt4.lib but i get the error anyways.
Searching for a solution I found this discussion here in stackoverflow
How to link a static library in Visual C++ 2008?
whit this information I looked on the include files of the libraries (dependencies of poppler like zlib, libpng, cairo, ...) and I found, in various cases, that they don't have a preprocessor directive to especify the static version of the lib.
Example static directive (openjpeg.h):
#if defined(OPJ_STATIC) || !(defined(_WIN32) || defined(WIN32) || defined(__WIN32__))
# define OPJ_API
# define OPJ_CALLCONV
#else
# define OPJ_CALLCONV __stdcall
# ifdef OPJ_EXPORTS
# define OPJ_API __declspec(dllexport)
# else
# define OPJ_API __declspec(dllimport)
# endif /* OPJ_EXPORTS */
#endif /* !OPJ_STATIC || !WIN32 */
Example without static directive (jconfig.h from jpeg lib):
#if defined(_WIN32)
#if defined(libjpeg_EXPORTS)
#define JPEG_EXPORT __declspec(dllexport)
#else
#define JPEG_EXPORT __declspec(dllimport)
#endif
#else
#define JPEG_EXPORT
#endif
My question is: Is not enough to change the properties of the project from dynamic to static so I have to change this headers too?, and if this is true, where can I define this new directives for making a difference between static or dynamic compilation?
Thanks in advance.
First please note Windows does not have any dynamic linkage at all. Surprise! Instead, it uses thunks. So what happens is: if you make a symbol dllexport, it has its actual name, the same name as if it were not dllexport. However it is marked in the object file for exporting.
If you say dllimport, on the other hand, the name is changed, in C roughly by prepending __imp_ to the name, more nasty in C++.
Now, when you link a DLL, you get a DLL (of course) but you also get a LIB file. That is a static link library. Which is the only kind the linker can handle. For each symbol exported from the DLL there is a dllimport symbol in that LIB file, in particular with __imp_ prefix or whatever for C++.
So now in a program or DLL you want to link to that DLL you made you link instead against the import LIB. The import LIB routines are thunks that patch up to the actual load time addresses from the DLL.
So now, if you try to do ordinary static linkage against a LIB file made by LIB.EXE by simply combining OBJ files which contained some dllexport, it will fail if the reference is a dllimport. Because you're refering to __imp_function() when the library actually contains plain function().
So with static linkage, you have to drop the dllimport. AFAIK dllexport is irrelevant. Note that this applies to the client of the library, not the library itself.
What does that mean? Well it is perfectly fine to statically link to a library which in turn dynamically links to another library. In fact by default static links on Windows dynamically link to the C runtime and OS DLLs. So the rule is: the client must chose the method of linking to a library, the provider should provide both versions. But take care they have different names!! (Otherwise the LINK making the DLL will make fred.LIB and the LIB will also make fred.LIB)
If you are changing the project properties from dynamic to static linkage as specified in the openjpeg.h you have to specify preprocessor that can use the static linkage..so in addition to changing the property from dynamic to static, add the preprocessor OPJ_STATIC...
For example:
#if defined(_WIN32)
#if defined(OPJ_STATIC)
# define OPJ_CALLCONV __stdcall
#el if defined(libjpeg_EXPORTS)
#define JPEG_EXPORT __declspec(dllexport)
#else
#define JPEG_EXPORT __declspec(dllimport)
#endif
#else
#define JPEG_EXPORT
#endif