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Unable to call C++ function from a C code

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I'm new to mixing C & C++ code. Understood the need of extern & __cplusplus directives after reading some SO links & online reading. Not sure why am I getting the error. Did i miss something ?

C++ header: cppexh.h

#include <iostream>

#ifdef __cplusplus
extern "C" {
#endif

void Callme(int a);

#ifdef __cplusplus
}
#endif

C++ Source

#include "cppexh.h"

void Callme(int a)
{
    std::cout << "Val is " << a << std::endl;
}

C Code calling C++ function

#include <stdio.h>
#include "cppexh.h"

int main()
{
    CallMe(2);
    retun 1;
}

But I'm getting compilation errors:

1>------ Rebuild All started: Project: CProject, Configuration: Debug Win32 ------
1>  Ccode.c
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtgmath.h(214): warning C4602: #pragma pop_macro : 'new' no previous #pragma push_macro for this identifier
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtgmath.h(215): warning C4193: #pragma warning(pop) : no matching '#pragma warning(push)'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtgmath.h(216): warning C4161: #pragma pack(pop...) : more pops than pushes
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(23): error C2061: syntax error : identifier 'abs'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(23): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(23): error C2061: syntax error : identifier 'acos'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(23): error C2061: syntax error : identifier 'asin'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(24): error C2061: syntax error : identifier 'atan'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(24): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(24): error C2061: syntax error : identifier 'atan2'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(24): error C2061: syntax error : identifier 'ceil'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(25): error C2061: syntax error : identifier 'cos'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(25): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(25): error C2061: syntax error : identifier 'cosh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(25): error C2061: syntax error : identifier 'exp'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(26): error C2061: syntax error : identifier 'fabs'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(26): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(26): error C2061: syntax error : identifier 'floor'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(26): error C2061: syntax error : identifier 'fmod'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(27): error C2061: syntax error : identifier 'frexp'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(27): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(27): error C2061: syntax error : identifier 'ldexp'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(27): error C2061: syntax error : identifier 'log'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(28): error C2061: syntax error : identifier 'log10'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(28): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(28): error C2061: syntax error : identifier 'modf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(28): error C2061: syntax error : identifier 'pow'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(29): error C2061: syntax error : identifier 'sin'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(29): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(29): error C2061: syntax error : identifier 'sinh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(29): error C2061: syntax error : identifier 'sqrt'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(30): error C2061: syntax error : identifier 'tan'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(30): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(30): error C2061: syntax error : identifier 'tanh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(32): error C2061: syntax error : identifier 'acosf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(32): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(32): error C2061: syntax error : identifier 'asinf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(33): error C2061: syntax error : identifier 'atanf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(33): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(33): error C2061: syntax error : identifier 'atan2f'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(33): error C2061: syntax error : identifier 'ceilf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(34): error C2061: syntax error : identifier 'cosf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(34): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(34): error C2061: syntax error : identifier 'coshf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(34): error C2061: syntax error : identifier 'expf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(35): error C2061: syntax error : identifier 'fabsf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(35): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(35): error C2061: syntax error : identifier 'floorf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(35): error C2061: syntax error : identifier 'fmodf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(36): error C2061: syntax error : identifier 'frexpf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(36): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(36): error C2061: syntax error : identifier 'ldexpf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(36): error C2061: syntax error : identifier 'logf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(37): error C2061: syntax error : identifier 'log10f'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(37): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(37): error C2061: syntax error : identifier 'modff'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(37): error C2061: syntax error : identifier 'powf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(38): error C2061: syntax error : identifier 'sinf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(38): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(38): error C2061: syntax error : identifier 'sinhf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(38): error C2061: syntax error : identifier 'sqrtf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(39): error C2061: syntax error : identifier 'tanf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(39): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(39): error C2061: syntax error : identifier 'tanhf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(41): error C2061: syntax error : identifier 'acosl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(41): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(41): error C2061: syntax error : identifier 'asinl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(42): error C2061: syntax error : identifier 'atanl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(42): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(42): error C2061: syntax error : identifier 'atan2l'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(42): error C2061: syntax error : identifier 'ceill'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(43): error C2061: syntax error : identifier 'cosl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(43): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(43): error C2061: syntax error : identifier 'coshl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(43): error C2061: syntax error : identifier 'expl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(44): error C2061: syntax error : identifier 'fabsl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(44): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(44): error C2061: syntax error : identifier 'floorl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(44): error C2061: syntax error : identifier 'fmodl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(45): error C2061: syntax error : identifier 'frexpl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(45): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(45): error C2061: syntax error : identifier 'ldexpl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(45): error C2061: syntax error : identifier 'logl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(46): error C2061: syntax error : identifier 'log10l'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(46): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(46): error C2061: syntax error : identifier 'modfl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(46): error C2061: syntax error : identifier 'powl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(47): error C2061: syntax error : identifier 'sinl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(47): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(47): error C2061: syntax error : identifier 'sinhl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(47): error C2061: syntax error : identifier 'sqrtl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(48): error C2061: syntax error : identifier 'tanl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(48): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(48): error C2061: syntax error : identifier 'tanhl'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(50): error C2054: expected '(' to follow 'using'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(50): error C2061: syntax error : identifier 'using'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(52): error C2061: syntax error : identifier 'acosh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(52): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(52): error C2061: syntax error : identifier 'asinh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(52): error C2061: syntax error : identifier 'atanh'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(53): error C2061: syntax error : identifier 'cbrt'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(53): error C2059: syntax error : ';'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(53): error C2061: syntax error : identifier 'erf'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(53): error C2061: syntax error : identifier 'erfc'
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\cmath(53): fatal error C1003: error count exceeds 100; stopping compilation
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

VERSION 2 OF THE CODE

EDITED THE CODE WHICH SOLVED THE COMPILATION ISSUES BUT THE LINK ERROR STILL EXISTS

#ifdef __cplusplus
#include <iostream>
#endif

#ifdef __cplusplus
extern "C" {
#endif

void Callme(int a);

#ifdef __cplusplus
}
#endif

MY C++ CODE IS A STATIC LIBRARY.

LINK ERROR

1>------ Rebuild All started: Project: CProject, Configuration: Debug Win32 ------
1>  Ccode.c
1>c:\users\ngk\documents\ccg\vsprojects\expapp\cproject\ccode.c(6): warning C4013: 'CallMe' undefined; assuming extern returning int
1>Ccode.obj : error LNK2019: unresolved external symbol _CallMe referenced in function _main
1>C:\Users\ngk\Documents\CCG\VSprojects\ExpApp\Debug\CProject.exe : fatal error LNK1120: 1 unresolved externals
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========
c++ c extern linkage extern-c
Original Q&A
5

There are 5 answers

0
abraxascarab On

I've had the lovely misfortune of having to combine a main C program calling homegrown C++.

As for your first problem (as others have stated), move the #include <iostream> into your C++ file.

As for your Linker problem:

This may (or may not) be a similar thing that had me spinning my wheels. But it's something to keep in mind because it's insanely silly.

Assuming you are building your C++ code in a DLL, and separately compiling your C code in an exe that calls the DLL library:

In Visual Studio's Unmanaged C, the DLL's don't get cleaned out of the "Release" folder the way they do in Managed C++. You must physically move your new DLL into the "Release" folder of your application program each time you make changes to your DLL.

I learned how to deal with updating DLL's in Managed C++ where all you have to do (in your application program folder) is clear out your old DLL "library subfolder" and copy in the new Release of the changed DLL's into that empty "library subfolder". When you "Clean and Rebuild" the main application that calls those DLL's, the Visual Studio Managed C++ will delete the old DLL's out of the "Release" folder and copy the new DLL's from the "library subfolder" into the applications "Release" folder. It nicely keeps your DLL's up to date.

In Unmanaged C, the "Clean and Rebuild" does not delete your old DLL's out of the application's "Release" folder. And it doesn't copy the new DLL's over from your "library subfolder" either. So even though I thought I was placing my new DLL's in a folder that my program could find them, they were not being copied like they are under Managed C++.

This basically means that any new changes you are making to your C++ DLL... if you are only copying them over to a "library subfolder" in your C project, the new improved code won't be copied to the Release folder like managed C++ does. You also have to copy those new DLL's over to the Release folder.

Which also means... if you had your initial DLL compiled and linked and later added the Callme() function, if you didn't physically copy that DLL to your Release folder, it'll never see your new code. It'll still be looking at your old version and will give you the error your are now seeing.

Here are some tools that can help you track down the dates of the versions your programs are really trying to call. These will let you know if your linker is grabbing an outdated version of your code:

link /dump /exports CPPSource.dll | grep subCreate

link /dump /exports CSource.exe | grep subCreate

Putting your correct file names in for the .dll and .exe

If you are not using DLL's for your C++ library, you can kindly ignore my stab at a solution for you. (Or you can consider using DLL's for your pure C++ code and call/link to it in your pure C application/project.) I've found keeping C++ code completely separate in a DLL from the main C code project is a way to save you from many headaches. (Especially if you are mixing unmanaged code and managed code.)

Hope that helps :)

0
Kaveh Vahedipour On

You cannot compile C++ code using a C compiler.

0
Jesper Juhl On

Your C++ function needs to be marked as extern "C" to avoid name mangling and follow C calling convention. If you don't do that it won't be (easily) callable from C.

The above also means you can't use default argument values nor overloading (and more) for such functions that need to be callable from C.

And you need to compile the C++ code with a C++ compiler, of course.

11
user0042 On

Move 

#include <iostream>

into the c++ source file.

The C compiler cannot deal with the C++ code in this header if it's included from the main() translation unit.

Also take care to use the exact same function names and parameter types for declaration, definition and calling. Both compilers are case sensitive.

CallMe(int) isn't the same as Callme(int).

As for your edit:

Make sure you add the static library correctly to your project.

3
user5329483 On

Simply a stupid Typo! You're declaring and defining Callme() and calling CallMe()

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