arduino....

foo.c

int foo(int a, int b)
{
    return a+b;                                                                                                          
}

foo.h

#ifdef __cplusplus
extern "C" {
#endif // __cplusplus

int foo(int a, int b);

#ifdef __cplusplus
} // extern "C"
#endif // __cplusplus

#ifdef __cplusplus
int foo(int a, int b = 10);
#endif // __cplusplus

main.cpp

#include <cstdio>
#include "foo.h"
                                                                                                                         
int main(int argc, char *argv[]){
    printf("%d, %d\n", foo(1, 2), foo(20));
    return 0;
}