Wrap code in C into C++ one under Linux

Hello all!
my.c

Code:

---

#define EXPORT __attribute__((visibility("default")))
EXPORT int foo(void);
int foo(void)
{
    extern int _new();
    return _new();
}

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my.cpp

Code:

---

extern "C" {
#define EXPORT __attribute__((visibility("default")))       
        EXPORT int foocpp(void)
        {
            extern int foo();
            return foo();
        }
}

---

Then

Quote:

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g++ -shared -fpic -o lib.so my.cpp my.c asm.s

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where asm.s contains _new.
I want to call my foocpp using PInvoke from C#:

Code:

---

using System.Runtime.InteropServices;
class Program{
[DllImport("lib.so")] public static extern int foocpp ();
 static void Main(string[] args)
        {
            int code = foocpp();
System.Console.WriteLine(code);
}
}

---

But on start using

Quote:

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dotnet run

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I have an error

Quote:

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symbol lookup error: undefined symbol: foo

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Please, help.

I think that by invoking g++ to compile my.c, my.c is treated as C++ source rather than C source, so name mangling etc comes into effect without extern "C":

Quote:

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Originally Posted by gcc 9.3 manual

g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats ‘.c’, ‘.h’ and ‘.i’ files as C++ source files instead of C source files unless -x is used.

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(Compiling C++ Programs)