Thread: static global

static local var is visible only in the function where it is declared
static "global" var is visible in any function of the current source file

A bit further expanation. A static variable defined in a function is different
from a normal variable defined in a function because a static variable will not be
freed at the end of the function. This allows its reference or pointer to be returned from
the function and it will have the same value the next time you call the function.

Code:

int& something()
{
  static int myVar1; //will not be freed at end of function
  int myVar2;  //will be freed at end of function so you cannot return its reference.
  
  printf("%i\n", myVar1); //The first time the function is called this may print junk. The next time it should print 32
  myVar1 = 32;
  myVar2 = 32;
  return myVar1;
}

In other words its like a global variable that can only be accessed inside the functions
scope and through its return value.

A global variable is defined outside of a function, class, structure, etc.

Code:

int myGlobal = 32;

void something()
{
  printf("%i\n", myGlobal); //prints 32
}

I dont think it matters if you put the word static in front of a global variable because
it should always be static (static, as in it wont be freed until the end of the program).

EDIT: As mentioned below, static does in fact affect global variables. It limits its scope to the C file it is defined in, meaning you can not use that variable in another C file, even if you use "extern".

Originally Posted by cs32

Is there a difference in where the two are stored in memory?

in general - no, but it is upto compiler.

returning pointer or ref to the static var could rise some problems in Multi-threading envirounment, so should be avoided

Originally Posted by 39ster

I dont think it matters if you put the word static in front of a global variable because
it should always be static (static, as in it wont be freed until the end of the program).

As mentioned, static before the global makes its linkage internal. That is, you can't access it from other .c files. If you don't add "static" before the global, then its linkage will be external and it can be accessed from other .c files.
Note that the opposite applies to C++! You have to specifically declare a global variable as extern to make it external linkage.

Originally Posted by vart

returning pointer or ref to the static var...

Not to mention, global variables!

What I meant is,

Code:

int x = 5;

External linkage in C. Can be accessed from multiple .c files.
In C++, however, it's internal linkage so trying to access it from another .cpp file will yield a compile error and redefining it will yield a linking error.

Code:

static int x = 5;

Internal linkage in C. Cannot be accessed from other .c files.
I don't know what the effect is in C++, though. I don't know if it will cause external linkage in this case.

Code:

extern int x;

If we add this in C++, THEN it's external linkage.

Well, I'm not a C programmer, but try this:
Do something like:

int x = 5;
In one .c file.
Then print the value in another .c file with extern:
extern int x;
It will work.

Now try adding
static int x = 5;
and then
extern int x;
Oops, linking error.

The only thing I know is that the same example works in C++ and causes an error as well
Yeah, someone has explained the linkage before, but I don't think I'm an expert at it.