Compiling extern "c" code with array parameters in gcc

Question

I'm trying to include some C function in a C++ program. I get some issue with a function declaration that contains array as parameter.

extern "C" {
    double Test( int nVar1, double f[nVar1] ) {
        return f[nVar1-1];
    }
}

int main() {
    cout << "!!!Hello World!!!" << endl; 
    double f[2] = {1.0,2.0};
    Test(2, f);
    return 0;
 }

I get following errors

'f' was not declared in this scope
'nVar1' was not declared in this scope
use of parameter outside function body before ']' token

I use Atollic 8.0 (GCC and C -std=gnu11)

Any help will welcome

Thank-you

While it's a valid C99 function, it's not valid C++ (what you are compiling as). — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Oct 19, 2017 at 8:24
Just for my personal curiosity, why do you want to do this ? why not using c++ container ? — Garf365
– Garf365, Commented Oct 19, 2017 at 8:46
I would like to implement unit testing on legacy embedded C code. I use CppUnit test with output in a console. I'm facing to such prototypes: int ComputedF(int nPoints, int nFunc, double x[], double f[nPoints][nFunc], double df[nPoints][nFunc]) — André Lenoir
– André Lenoir, Commented Oct 19, 2017 at 19:33

Garf365 · Accepted Answer · 2017-10-19 11:53:15Z

6

extern "C" isn't for compiling c inside cpp source. It's only to use ABI of C inside cpp source/header: mangling, calling conventions, exception handling... (thanks to Ajay Brahmakshatriya)

By mangling, I want to say the internal unique name of function used by compiler/linker. C mangling is really different to c++ mangling, and so not compatible. To find C function inside c++, you have to say to compiler/linker under which internal unique name the function is known.

extern "C" only switches which ABI has to be used, including mangling used to create internal unique name and how function has to be called, not to switches compilation mode.

If you really want to compile c code, you have to put your code inside a c source file and compile it separately. And use extern "C"to declare function in cpp environment, to allow c++ code to use it. BUT declaration of function has to be compatible with c++, and double Test( int nVar1, double f[nVar1] ) isn't.

function.c, compile with gcc -c:

double Test( int nVar1, double f[] ) {
    return f[nVar1-1];
}

function.h, compatible c and c++:

#ifndef _FUNCTION_H
#define _FUNCTION_H

#ifdef __cplusplus
extern "C" {
#endif

double Test( int nVar1, double f[] );

#ifdef __cplusplus
}
#endif

#endif

main.cpp, compile with g++ -c:

#include "function.h"

int main() {
    cout << "!!!Hello World!!!" << endl; 
    double f[2] = {1.0,2.0};
    Test(2, f);
    return 0;
 }

And finally, link everything with g++ linker:

g++ function.o main.o -o my_program

See example here:

edited Oct 19, 2017 at 11:53

answered Oct 19, 2017 at 8:27

Garf365

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3 Comments

Ajay Brahmakshatriya Over a year ago

It is not just the mangling, but also calling conventions, exception handling that is done C stlye. ABI would be the correct term in that case.

Andrew Henle Over a year ago

C mangling is really different to c++ mangling I'd say that "really different" is quite an understatement. There's no real C++-style name mangling in C at all, because there's no overloading. There may be some ABI-based name changes where the symbol isn't the same as the name of the function in the source code, but that's not what I think of when dealing with C++ name mangling.

André Lenoir Over a year ago

BUT declaration of function have to be compatible with c++ Clear thank-you

Anton Malyshev · Accepted Answer · 2017-10-19 08:53:25Z

0

The problem is with array size. It cannot be variable, should be const. If you need to pass an array of variable size then just pass the pointer to its first element: double Test( int nVar1, double * f) {

answered Oct 19, 2017 at 8:53

Anton Malyshev

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3 Comments

André Lenoir Over a year ago

This type of declaration is working since C99 link I would like to use an existing code without changing it

Anton Malyshev Over a year ago

See nothing like function(int array[count], int count) there, only function(int array[], int count) which is not the same

André Lenoir Over a year ago

link From C99, C language supports variable sized arrays to be passed simply by specifying the variable dimensions

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