C++ by-reference argument and C linkage

I have encountered a working (with XLC8 and MSFT9 compilers) piece of code, containing a C++ file with a function defined with C linkage and a reference argument. This bugs me, as references are C++ only. The function in question is called from C code, where it is declared as taking a pointer argument to the same type in place of the reference argument.

Simplified example:

C++ file:

extern "C" void f(int &i)
{
    i++;
}

C file:

void f(int *);

int main()
{
    int a = 2;
    f(&a);
    printf("%d\n", a);  /* Prints 3 */
}

Now, the word on the street is that most C++ compilers, under the hood, implement references just like a pointer. Is it like that and just pure luck the reason this code works or does it say somewhere in the C++ specification what the result is when you define a function with a reference argument and C linkage? I haven't been able to find any information on this.

Solution

My copy of n3000.pdf (from here), has this to say in section 7.5—Linkage specifications:

9. Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved.

Since C and C++ are different languages, this means that you can't rely on this "feature" of common compilers.

Stronger is note 5 in the same section (emphasis mine):

If two declarations declare functions with the same name and parameter-type-list (8.3.5) to be members of the same namespace or declare objects with the same name to be members of the same namespace and the declarations give the names different language linkages, the program is ill-formed; no diagnostic is required if the declarations appear in different translation units.

So, I would say that what you did is not guaranteed to work according to the standard, and the compiler is not required to print a diagnostic for the example you have given because the declarations are in different translation units.

FYI, it "works for me" with gcc and g++ version 4.2.1 on Snow Leopard.