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)
Related
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 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.
using Visual C++ 2008, I am having an "Unresolved external symbol" even if with the IDE I can correctly see both declaration and definition of the function:
The error:
hook_file.obj : error LNK2001: unresolved external symbol "int __cdecl is_directory_objattr(struct _OBJECT_ATTRIBUTES const *)" (?is_directory_objattr##YAHPBU_OBJECT_ATTRIBUTES###Z)
Relevant code in hook_file.cpp:
#include "misc.h"
void handle_new_file(HANDLE file_handle, const OBJECT_ATTRIBUTES *obj)
{
if(is_directory_objattr(obj) == 0 ) {
// etc.
Declaration in misc.h file:
BOOL is_directory_objattr(const OBJECT_ATTRIBUTES *obj);
Definition in misc.cpp file:
#include "misc.h"
BOOL is_directory_objattr(const OBJECT_ATTRIBUTES *obj)
{ //function body here }
I really don't get what the linker has to complain here.
The header with function declaration is included by the file which
calls the function.
The cpp file with the function definition included header file with declaration.
Declaration and definitions are identical.
All files are listed among project files.
Any idea whats wrong?
Thanks!
Have you only include one time your header file?
in misc.h
#ifndef MISC
#define MISC
...
#endif MISC ?
What's wrong with this unresolved externals? I'm trying to implement this into InitInstance of my MFC application. But I'm getting this line error from the debugger.
LNK2019: unresolved external symbol "extern "C" int __stdcall EnumProcesses(unsigned long *,unsigned long,unsigned long *)" (?EnumProcesses##$$J212YGHPAKK0#Z) referenced in function "public: virtual int __thiscall COkApp::InitInstance(void)" (?InitInstance#COpenEyeProcessApp##$$FUAEHXZ)
Here is the OkApp files :
OkApp.h
//...
#include <psapi.h>
//...
OkApp.cpp
#include "OkApp.h"
//...
BOOL COkApp::InitInstance()
{
//...
if (!EnumProcesses(aProcesses, sizeof(aProcesses), &cbNeeded))
{
return 1;
}
//...
}
Knowing that my project is coded into Unicode project, how do I to fix this, please?
Any brilliant idea, please?
You need Psapi.lib to be linked.
#pragma comment(lib, "Psapi.lib")
It looks like EnumProcesses is declared extern "C" in the header, but not in the source file. Both have to be consistent. Either change both to extern "C" (if you intend to use EnumProcesses from within C), or remove the extern "C" in the header.
EDIT: Or you just forgot to add the library containing EnumProcesses when calling the linker.
I'm using VS2008 WinCE7. I'm facing linking error during build.
The file system structure is
menu.c - ./menu.c
eboot.h - ./eboot.h
file_1.cpp - ./dir1/file_1.cpp
where . represents current directory
menu.c
#include <eboot.h>
static VOID OALWriteToEMMC(OAL_BLMENU_ITEM *pMenu);
VOID OALWriteToEMMC(OAL_BLMENU_ITEM *pMenu)
{
OALTestEMMC();
}
file_1.cpp
#include <eboot.h>
VOID OALTestEMMC();
VOID OALTestEMMC()
{
//some code
}
eboot.h
VOID OALTestEMMC();
I'm getting the error
menu.obj : error LNK2019: unresolved external symbol OALTestEMMC referenced in function OALWriteToEMMC
Please guide me how to solve it.
EDIT1:
menu.c
#ifdef __cplusplus
extern "C" VOID OALTestEMMC();
#endif
and removed the declaration in eboot.h and added it in file_1.cpp
But, the error persists.
It seems that you are compiling the function definition in C++ but the use of that function in C. Without making the function extern "C" in C++, the compiler will mangle the name, and generate a symbol that won't match the declaration used in the C code.
If you want to mix C and C++, make sure that the declarations in C++ are marked extern "C" so that the compiler won't mangle the names and will use the C calling conventions. Alternatively, compile everything in C++ (or in C)
David RodrÃguez's original answer is correct. You are invoking a C++ function (OALTestEMMC) from a C source file. As David mentioned, the C++ compiler will mangle the function name. In order to suppress the name mangling so that the code in menu.c can invoke it, place the 'extern "C"' qualifier on the OALTestEMMC function (in file_1.cpp):
extern "C" VOID OALTestEMMC();
extern "C" VOID OALTestEMMC() {
// stuff
}