Question

So, while being schooled by James Kanze and Loki Astari about C linkage, I was wondering about this:

extern "C" int foo1 (void (*)());
extern "C" { int foo2 (void (*)()); }

After my schooling, I think it must be that foo1 only takes a function pointer with C++ linkage, while foo2 only takes a function pointer with C linkage. Is my understanding correct? Are there specific references in the C++ standard that explain the differences in my example above?

Edit: To make it easier for everyone to follow along here’s a pastebin with the relevant part from the C++ 11 draft standard.

Answer 1

foo1 takes a pointer to a C function as shown in [dcl.link] 7.5p4

In a linkage-specification, the specified language linkage applies to
the function types of all function declarators, function names with
external linkage, and variable names with external linkage declared
within the linkage-specification. [Example:

extern "C" void f1(void(*pf)(int));
                                                                 // the name f1 and its function type have C language
                                                                 // linkage; pf is a pointer to a C function

The example applies directly to foo1 and the added emphasis highlights what I think is the reason. The function’s parameter lists contains a function declarator for a parameter, and all function declarators are affected by the linkage specification. This applies to both braced and non-braced linkage specifications.

Some differences when not using braces are that names are automatically extern and explicit use of a storage specifier is prohibited.

extern "C" int i; // not a definition

int main() {
    i = 1; // error, no definition
}

extern "C" static void g(); // error

As an example of where this difference matters, consider a header containing the following:

extern "C" int a;
extern "C" double b;
extern "C" char c;

Someone might be tempted to change this to:

extern "C" {
    int a;
    double b;
    char c;
}

But that would be incorrect because that converts the declarations into definitions. Instead the correct code using extern "C" {} is:

extern "C" {
    extern int a;
    extern double b;
    extern char c;
}