Non-static declaration after static declaration

Question

Consider the following 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 normal in the first example, but not in the second?

As for scope variables (global scope), they have an external relationship and a static duration if no keywords are specified.

thanks

AFAIK, the duration of communication and storage for functions is slightly different.

EDIT: I tried to compile using gcc 4.5.2 -Wall -pedantic -std = c99

More details: http://c-faq.com/decl/static.jd.html You can see that the first example works there, but the second does not. However, I do not see what makes them so different.

Answer 1

The answer to your first question is in section 6.2.2 of standard C:

4 For an identifier declared using the extern storage class specifier in the area in which the preliminary declaration of this identifier is visible, if an internal or external relationship is specified in the previous declaration, the binding of the identifier to the later declaration is the same as the link specified in the previous declaration. If the previous announcement was not visible, or if the link was not specified in the previous announcement, then the identifier has an external link.

Thus, the connection a is internal.

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

5 If the identifier declaration for a function does not have a storage class specifier, its binding is defined exactly as if it were declared extern storage class specifier. If the identifier declaration for an object has a file size and there is no storage class specifier, its relationship is external.

Since a is an object, not a function, declaring int a; without a storage class specifier gives a external connection. The same section says the following:

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

Since in the second example a appears with both internal and external communication, this paragraph starts. One (especially useful) manifestation of undefined behavior is the error that your compiler produces.

All your examples can be understood by the following rules:

• int a; always announces a with external communication;
• static int a; always declares a internal connection;
• extern int a; declares a through any kind of connected link or external link if it does not;
• Two a declarations in the same area with different relationships give undefined behavior.

Answer 2

I get the same error when I forget "{" instead of ";" at the end of the declaration.

Example:

 extern void *HASHMP_get(struct HASHMP_wf_s *hmwf_ptr, Uint8 *key); extern void *HASHMP_remove(struct HASHMP_wf_s *hmwf_ptr, Uint8 *key) { extern Uint16 HASHMP_clear(struct HASHMP_wf_s *hmwf_ptr);