c++ - Is extern "C" only required on the function declaration?

Question

I wrote a C++ function that I need to call from a C program. To make it callable from C, I specified extern "C" on the function declaration. I then compiled the C++ code, but the compiler (Dignus Systems/C++) generated a mangled name for the function. So, it apparently did not honor the extern "C".

To resolve this, I added extern "C" to the function definition. After this, the compiler generated a function name that is callable from C.

Technically, the extern "C" only needs to be specified on the function declaration. Is this right? (The C++ FAQ has a good example of this.) Should you also specify it on the function definition?

Here's an example to demonstrate this:

/* ---------- */
/* "foo.h"    */
/* ---------- */

#ifdef __cplusplus
extern "C" {
#endif

/* Function declaration */
void foo(int);

#ifdef __cplusplus
}
#endif

/* ---------- */
/* "foo.cpp"  */
/* ---------- */

#include "foo.h"

/* Function definition */
extern "C"               // <---- Is this needed?
void foo(int i) {
  // do something...
}

My issue may be the result of incorrectly coding something, or I may have found a compiler bug. In any case, I wanted to consult stackoverflow to make sure I know which is technically the "right" way.

Answer 1

The 'extern "C"' should not be required on the function defintion as long as the declaration has it and is already seen in the compilation of the definition. The standard specifically states (7.5/5 Linkage specifications):

A function can be declared without a linkage specification after an explicit linkage specification has been seen; the linkage explicitly specified in the earlier declaration is not affected by such a function declaration.

However, I generally do put the 'extern "C"' on the definition as well, because it is in fact a function with extern "C" linkage. A lot of people hate when unnecessary, redundant stuff is on declarations (like putting virtual on method overrides), but I'm not one of them.