Question

Consider these examples:

static int a;
extern int a; //OK -- what linkage does the a have now?

static int a;
int a; //ERROR

extern int a;
static int a; //ERROR

int a;
static int a; //ERROR

extern int a;
int a; //OK as expected

int a;
extern int a; //OK as expected

Why was it OK in the first example but not in the second?

As far as file-scope variables (global-scope) are concerned, these have external linkage and a static duration when no keyword is specified.

Thank you

AFAIK, linkage and storage duration for functions is a bit different.

EDIT:
I’ve tried compiling using gcc 4.5.2 -Wall -pedantic –std=c99

More on: http://c-faq.com/decl/static.jd.html You can see that the 1st example works there too but 2nd doesn’t. However, I don’t see what makes them so different.

Answer 1

The answer to your first question is found in §6.2.2 of the C standard:

4 For an identifier declared with the storage-class specifier extern
in a scope in which a prior declaration of that identifier is visible,
if the prior declaration specifies internal or external linkage, the
linkage of the identifier at the later declaration is the same as the
linkage specified at the prior declaration. If no prior declaration is
visible, or if the prior declaration specifies no linkage, then the
identifier has external linkage.

So the linkage of a is internal.

For your second question, the second sentence of the immediately following paragraph is apropos:

5 If the declaration of an identifier for a function has no
storage-class specifier, its linkage is determined exactly as if it
were declared with the storage-class specifier extern. If the
declaration of an identifier for an object has file scope and no
storage-class specifier, its linkage is external.

Because a is an object, not a function, the declaration int a; with no storage-class specifier gives a external linkage. The same section then has this to say:

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

Since, in your second example, a appears with both internal and external linkage, this paragraph is triggered. One (particularly helpful) manifestation of undefined behaviour is the error that your compiler is producing.

All of your examples can be understood by these rules:

1. int a; always declares a with external linkage;
2. static int a; always declares a with internal linkage;
3. extern int a; declares a with whatever linkage it already had, or external linkage if it had none;
4. Two declarations of a in the same scope with different linkage give undefined behaviour.