static global variable and static extern global variable

Hello all,


What is the difference between static global variable and static extern global variable in c ?

PLese Help me..!!


Thanks,
Gunjan

"static" can be used to limit the scope of global variable to only the file it is declared in. I don't know what "static extern" does, the two seem contradictory. Intuition says it should cause an error.

Static and extern don't go together. You have to choose one.

What about the use of 'static' inside a function. The variables thus declared maintain their values across function calls. So in that sense, they are "global", as long as the function itself is callable from any module.

Quote:

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Originally Posted by nonoob

What about the use of 'static' inside a function. The variables thus declared maintain their values across function calls. So in that sense, they are "global", as long as the function itself is callable from any module.

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I think "global" is not quite what you're looking for. Their lifespan is the entire run-time, not just the duration of the function call. True, they are usually located in the data or BSS section like global variables, but they don't have global scope, and thus can't be made available to other translation units (.c files), even with extern. Their name is only valid within that function.

EDIT: according to section 6.2.2, paragraph 7 of the standard: "If, within a translation unit, the same identifier appears with both internal and external
linkage, the behavior is undefined."

Correct. Sorry... I confused scope (direct syntactical accessibility) with ultimate accessibility (can get to the data in some way from any other translation unit). I was just thinking of BSS from a compiler's point of view. My use of the word 'global' was somewhat loose.

Thanks to all..

But i read over internet that "Static storage class can be specified for automatic as well as external variables such as:
static extern varx; "

Still i m confuse what is the purpose of using two storage class at a time as static limit the scope of global variable.

There is no purpose...

Unless you read that in the standard (which you didn't), it doesn't really matter what it says "on the internet". That person doesn't know what they're talking about. There are two distinct concepts here, linkage and storage duration. I blame (at least in part) the creators of the C language for this common misunderstanding, because they reused the static keyword for two completely different purposes. But such is life...so here's the difference:

Linkage basically describes where you can see the variable/function. Using 'static' for global variables and functions means they're only visible to that translation unit (synonym for a .c file). The other option is 'extern', which is a way of describing the type of some variable/function, but stating that you will find the details (function body or storage space for said variable) in another .c file. I hope this makes things like "static extern ..." seem ridiculous. The standard says this behavior is "undefinied", which means anything at all, or nothing, may happen. Don't trust code like this.

Storage duration describes when there is valid space to put stuff in said variable. By default, variables local to a function have automatic storage, meaning that space is allocated when entering the function and deallocated when the function exits. Global variables, on the other hand, have space allocated throughout the duration of the program. When using the 'static' keyword with a local variable, you are stating that it's lifespan is the duration of the program. There are other complexities that go along with this, primarily that all instances of a function share a single instances of that variable, which gives problems with recursive or re-entrant functions.

Thanks a lot..