The Naming Conventions of C: A struct and a function

Question

While exploring the facilities for handling system signal requests in the sigaction.h header, I noticed that a structure and a function returning an int were named sigaction.

Even though it seems semantically correct, since the compiler should be able to deduce between the two definitions, why are the duplicate definitions of sigaction valid C syntax?

Answer 1

In C, struct tags live in a separate namespace from the other names. The struct is called struct sigaction, while the function is simply sigaction.

Answer 2

The compiler is able to separate structure tags (also unions and enumerations) because they're following the keywords struct/union/enum, respectively (C11 §6.2.3p1).

Then, declarations are required to be unique within "the same name space", according to §6.7p3.

Since structure tags and function identifiers (which are ordinary identifiers) aren't in the same name space, the "clash" is OK.

When it comes to usage, you can't do:

typedef struct _test {
    int a;
} test;

void test(void) {

}

The compiler will inform you:

test.c:5:6: error: 'test' redeclared as different kind of symbol
 void test(void) {
      ^
test.c:3:3: note: previous declaration of 'test' was here
 } test;