user6898756 user6898756 - 2 months ago 9
C Question

External, internal and no linkage or why this does not work?

According to C standard:


In the set of translation units and libraries that constitutes an entire program, each
declaration of a particular identifier with
external linkage
denotes the same object or
function. Within one translation unit, each declaration of an identifier with
internal
linkage

denotes the same object or function. Each declaration of an identifier with
no
linkage

denotes a unique entity.


In my example we have three separate declarations with each identifier having a different linkage.So why this doesn't work?

static int a; //a_Internal

int main(void) {
int a; //a_Local
{
extern int a; //a_External
}
return 0;
}


Error:


In function 'main':
Line 9: error: variable previously declared 'static' redeclared 'extern'


Why does compiler insist that I'm redeclaring instead of trying to access external object in another file?

Valid C++ example for reference:

static void f();
static int i = 0; // #1
void g() {
extern void f(); // internal linkage
int i; // #2 i has no linkage
{
extern void f(); // internal linkage
extern int i; // #3 external linkage
}
}

usr usr
Answer

§6.2.2, 7 says:

If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.

So, your program has undefined behaviour.

§6.2.2, 4 says that

extern int a; //a_External

has external linkage because the prior declaration visible in the scope int a; //a_Local has no linkage. But

static int a; //a_Internal

declares a with internal linkage. Hence, it's undefined per §6.2.2, 7.