I have a C++ Visual Studio 2013 console application which is supposed to make use of a DLL MyDLLlib.dll which I have written. MyDLLlib is written in C. One of the functions is called Get_Version. The prototype is
const char *Get_Version();
I put this at the top of the source files to make use of the prototype:
extern "C"{
#include "MyDLLlib.h"
}
If in the function is called in the main as this
printf("version %s\n",Get_Version());
then it works.
However if I add a class with some static methods and a static method makes a call to Get_Version()
const char * ret = Get_Version();
then I get a link error:
Error 1 error LNK2019: unresolved external symbol
"__declspec(dllimport) char * __cdecl Get_Version(void)" (__imp_?Get_Version##YAPADXZ)
referenced in function "private: static class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl ServiceDispatch::decoder_Get_Version(class StringBuffer &)"
(?decoder_Get_Version#ServiceDispatch##CA?AV?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##AAVStringBuffer###Z)
D:\devt\CplusPlus\VSTrials\Link_to_MyDLLlib\Link_to_MyDllLib\ServiceDispatch.obj Link_to_MyDLLlib``
I am using the same include.
Any clue as to what I might be doing wrong?
If you have CLASS_DECLSPEC defined always as __declspec(dllimport), this will not work for sure. Look at this sample:
DLL_header.h
#if defined( _BUILD_DLL )
# define DLLAPI __declspec(dllexport) //Export when building DLL
#else
# define DLLAPI __declspec(dllimport) //Import when using in other project
#endif
DLLAPI const char *Get_Version();
DLL_source.cpp
#include "Header.h"
const char *Get_Version()
{
return "1.1.0.4";
}
Build DLL with _BUILD_DLL defined.
Main.cpp
#include "DLL_header.h"
int main()
{
printf("%s\n", Get_Version());
return 0;
}
Build this, with _BUILD_DLL not defined.
In your case, it could be problem with extern "C" - you include header inside extern "C", which declares Get_Version() as having __cdecl linkage. But linker is searching for
__imp_?Get_Version##YAPADXZ
Which is a mangled (C++) name. Is your DLL a C or C++ project? If your DLL is build as C project (not C++), put extern "C" on Get_Version()'s declaration with this #ifdef:
#ifdef __cplusplus
extern "C" {
#endif
DLLAPI const char *Get_Version();
#ifdef __cplusplus
}
#endif
Either way, remove extern "C" from around the #include. Also, check if .lib file for this DLL is attached to project as dependency.
Related
I have a sample.c file where a non-static function is defined
Source: sample.c
#if defined(__cplusplus)
extern "C" {
#endif
int get_sample_value()
{
return 1000;
}
#if defined(__cplusplus)
}
#endif
There is pure C++ SDK project, sample_sdk which basically generates a couple of static-libs, where get_sample_value() function is used within one of the source files as follows:
Source: sample_sdk_source.cpp
extern int get_sample_value();
static void do_processing()
{
int sample_value = get_sample_value();
process( sample_value );
}
Above-mentioned sample.c will be compiled in another C++/CLI GUI application, SampleViewer where sample_sdk libs are included within this application.
But when compiling the SampleViewer we are getting the following error:
libsample-sdk-x86.lib(sample_sdk_source.obj) : error LNK2019:
unresolved external symbol "int __cdecl get_sample_value()"
(?get_sample_value##YAPBUint##XZ) referenced in function "public:
static void __cdecl do_processing()" (?do_processing##SAXXZ)
I also tried to use the same function from SampleViewer::main.cpp file, but the same error exists.
Is there any issue when accessing function defined in C file as extern from C++/CLI environment?
The linker error says it all:
Your extern int get_sample_value(); declaration in C++ sets up an undefined symbol for the mangled name ?get_sample_value##YAPBUint##XZ
Your definition in sample.c defines a symbol with a non-mangled name (_get_sample_value).
To solve this, either mark your declaration in C++ with extern "C" as well, or better yet: move the declaration into a header that both sample.c and sample_sdk_source.cpp can include (with the #if defined(__cplusplus) guard)
I implement a C++ function as DLL with vs2013. But I have linker error (error LNK2019: unresolved external symbol _main referenced in function ___tmainCRTStartup) building to the dll. I have proper setup and not sure what is wrong. The code is very simple, but I don't know why it can't be built successfully.
Demoone.h
#ifndef _Demo_H_
#define _Demo_H_
#ifdef LIBDLL
#define LIBDLL extern "C" _declspec(dllimport)
#else
#define LIBDLL extern "C" _declspec(dllexport)
#endif
LIBDLL int Add(int plus1, int plus2);
#endif
Demoone.cpp
#include "Demoone.h"
int Add(int a, int b)
{
return (a + b);
}
update：
I modified the header file as bellow
#ifndef _Demo_H_
#define _Demo_H_
extern "C" int Add (int a , int b);
#endif
and add an def file
LIBRARY "Dllmaketwo"
EXPORTS
Add # 1
The same linker error (error LNK2019: unresolved external symbol _main referenced in function ___tmainCRTStartup) also appeared.
If you want to use load-time linking in the project that uses the DLL you need to link against the .lib produced by the DLL project build.
You can do that with:
#pragma comment(lib, "dllproject.lib")
Or by adding the .lib to the additional dependencies line in the project settings under Linker->Input. You may also need to mess with the library search path, either in the VC++ Directories page or in the Linker->General page.
#ifndef _Demo_H_
#define _Demo_H_
#ifdef LIBDLLIMPORT //Changed here
#define LIBDLL extern "C" _declspec(dllimport)
#else
#define LIBDLL extern "C" _declspec(dllexport)
#endif
LIBDLL int Add(int plus1, int plus2);
#endif
You should not use a same name in #ifdef and #define,
Did you setup you project correctly? And you should not write a `main()' function in it.
I have created an MSVS2015 project (MFC/C++) which
links with a static C library. When I build the
projects I get the following error:
error LNK2019:
unresolved external symbol "void __cdecl testLinkerError(void)" (?testLinkerError##YAXXZ) referenced
in function "void __cdecl test1(void)" (?test1##YAXXZ) [PATH_REMOVED]\test.obj [NAME_OF_MFC_PROJECT]
For debugging purposes, I have created 3 files:
test.cpp
linkertest.c
linkertest.h
test.cpp has been added to the MFC project and it looks like this:
#include "linkertest.h"
void test1(void)
{
testLinkerError();
}
The header file linkertest.h looks like this:
#pragma once
#ifdef _cplusplus
extern "C" {
#endif
void testLinkerError(void);
#ifdef _cplusplus
}
#endif
And linkertest.c looks like this:
#include "linkertest.h"
void testLinkerError(void) {
int x = 5;
int y = 7;
int z = x + y;
}
I am quite sure that this is some kind of name mangling issue, but I can't figure out how to solve it.
The calling convention in both projects is set to __cdecl
based on the compiler error, you compiler tried to look up ?testLinkerError##YAXXZ which is a C++ function name. so which means on your import part (not your DLL export) you did not use extern "C"....
and one more thing, The name __cplusplus (two underscores) is defined to the value 201402L when compiling a C++ translation unit. please check your code make sure _cplusplus (one underscore) is defined.
I created a DLL project and successfully built it. I then tried to use the DLL in another Project, TEST, and I am getting the following error.
Error 1 error LNK2001: unresolved external symbol "public: void __thiscall SnoMessage::setRawMessageName(class ATL::CStringT<wchar_t,class StrTraitMFC_DLL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >)" (?setRawMessageName#SnoMessage##QAEXV?$CStringT#_WV?$StrTraitMFC_DLL#_WV?$ChTraitsCRT#_W#ATL#####ATL###Z)
I added the required lib in the linker properties, and I also added the header files in the TEST include directory. So the function is being recognized, but it keeps giving those errors. The DLL is comprised of the following files
SnoMessage.h
#pragma once
#include "StdAfx.h"
class SnoMessage
{
public:
__declspec(dllexport) SnoMessage(void);
__declspec(dllexport) ~SnoMessage(void);
__declspec(dllexport) void setRawMessageName(CString messageName);
__declspec(dllexport) void setRawMessageType(CString messageType);
__declspec(dllexport) void setRawMessageAttributes(std::map<CString,CString> attributes);
__declspec(dllexport) CString getRawMessageName();
__declspec(dllexport) CString getRawMessageType();
__declspec(dllexport) std::map<CString,CString> getRawMessageAttributes();
private:
CString messageName;
CString messageType;
std::map<CString,CString> attributes;
};
SnoMessage.cpp
#include "stdafx.h"
#include "SnoMessage.h"
SnoMessage::SnoMessage(void)
{
}
SnoMessage::~SnoMessage(void)
{
}
void SnoMessage::setRawMessageName(CString messageName){
this->messageName = messageName;
}
void SnoMessage::setRawMessageType(CString messageType){
this->messageType = messageType;
}
void SnoMessage::setRawMessageAttributes(std::map<CString,CString> attributes){
this->attributes = attributes;
}
CString SnoMessage::getRawMessageName(){
return messageName;
}
CString SnoMessage::getRawMessageType(){
return messageType;
}
std::map<CString,CString> SnoMessage::getRawMessageAttributes(){
return attributes;
}
And in test I am doing the following:
test.cpp
// test.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "SnoMessage.h"
int _tmain(int argc, _TCHAR* argv[])
{
SnoMessage *msg = new SnoMessage();
msg->setRawMessageName("TEST");
return 0;
}
Let me know if you need more info, thanks.
In your dll define this in some header you want to use for your export defs...
MyExports.h
#ifdef SNOMESSAGE_EXPORTS
#define SNOMESSAGE_API __declspec(dllexport)
#else
#define SNOMESSAGE_API __declspec(dllimport)
#endif
Now in your dll you just define SNOMESSAGE_EXPORTS, then when your dll is compiled your class and methods will be visible to the exe. But when you include those same headers in the exe the Macro will import them instead of export.
//In the DLL this is == to export, in the executable this is import. Problem solved.
class SNOMESSAGE_API SnoMessage
{
public:
//...
};
You no longer need to export each member, just the class.
I would mark the whole class as exported, not just its member functions. Also, following the advice of this conversation, you need to specify __declspec(dllecport) or __declspec(dllimport) based on whether you are including the header in the DLL or the code that uses the DLL; and define the guarding macro in the DLL project.
When you compile the DLL you should have __declspec(dllexport), but when you compile exe you should have __declspec(dllimport). The easiest way to do this is to have a #define somewhere that has different value when "in DLL" and "out of DLL". Also do export the whole class instead of individual methods.
There is a case when dll compile use C call but exe use standard call, the link in x64 has no problem, but when using win32 will show this link error 2001. For that situation just use C call for both dll and exe for win32 platform (https://learn.microsoft.com/en-us/cpp/error-messages/tool-errors/name-decoration?view=msvc-160).
I have a 3rd party library that's written in C. It exports all of its functions to a DLL.
I have the .h file, and I'm trying to load the DLL from my C++ program.
The first thing I tried was surrounding the parts where I #include the 3rd party lib in
#ifdef __cplusplus
extern "C" {
#endif
and, at the end
#ifdef __cplusplus
} // extern "C"
#endif
But the problem there was, all of the DLL file function linkage looked like this in their header files:
a_function = (void *)GetProcAddress(dll, "a_function");
While really a_function had type int (*a_function) (int *). Apparently MSVC++ compiler doesn't like this, while MSVC compiler does not seem to mind.
So I went through (brutal torture) and fixed them all to the pattern
typedef int (*_x_a_function) (int *); // using _a_function will not work, C uses it!
_x_a_function a_function ;
Then, to link it to the DLL code, in main():
a_function = (_x_a_function)GetProcAddress(dll, "a_function");
This SEEMS to make the compiler MUCH, MUCH happier, but it STILL complains with this final set of 143 errors, each saying for each of the DLL link attempts:
error LNK2005: _x_a_function already defined in main.obj main.obj
Multiple symbol definition errors.. sounds like a job for extern! SO I went and made ALL the function pointer declarations as follows:
function_pointers.h
typedef int (*_x_a_function) (int *);
extern _x_a_function a_function ;
And in a cpp file:
function_pointers.cpp
#include "function_pointers.h"
_x_a_function a_function ;
ALL fine and dandy.. except for linker errors now of the form:
error LNK2001: unresolved external symbol _a_function main.obj
Main.cpp includes "function_pointers.h", so it should know where to find each of the functions..
I am bamboozled. Does any one have any pointers to get me functional? (Pardon the pun..)
Linker errors like that suggest you've defined all the functions in function_pointers.cpp, but forgotten to add it to the project/makefile.
Either that, or you've forgotten to "extern C" the functions in function_pointers.cpp too.
I believe that if you declared the typedefs and/or the prototype as extern "C", you must remember to extern "C" the definition too.
When you link C functions the prototypes will get a leading _ in front of them by default so
when you do a typedef using the same name
typedef int (*_a_function) (int *);
_a_function a_function
you'll get issues because there is already a function in the dll named _a_function.
Usually you declare a function in yourlibrary.h like extern "C" __declspec(dllexport) int __cdecl factorial(int); then create that function in yourlibrary.c:
extern "C" __declspec(dllexport) int __cdecl factorial(int x) {
if(x == 0)
return 1;
else
return x * factorial(x - 1);
}
After compiling your DLL you get your .dll and .lib files. The latter is used when you want to import your functions to the project. You put #include "yourlibrary.h" and #pragma comment(lib, "yourlibrary.lib") in your project, after this you can use int factorial(int) in you application.