Static variable

See also: Static (keyword)

In computer programming, a static variable is a variable that has been allocated statically—whose lifetime or "extent" extends across the entire run of the program. This is in contrast to the more ephemeral automatic variables (local variables are generally automatic), whose storage is allocated and deallocated on the call stack; and in contrast to objects whose storage is dynamically allocated in heap memory.

When the program (executable or library) is loaded into memory, static variables are stored in the data segment of the program's address space (if initialized), or the BSS segment (if uninitialized), and are stored in corresponding sections of object files prior to loading.

The static keyword is used in C and related languages both for static variables and other concepts.

Scope

See also: Variable (computer science) § Scope and extent

In terms of scope and extent, static variables have extent the entire run of the program, but may have more limited scope. A basic distinction is between a static global variable, which has global scope and thus is in context throughout the program, and a static local variable, which has local scope. A static local variable is different from local variable. It is initialized only once no matter how many times that function in which it resides is called. It may be used as a count variable. A static variable may also have module scope or some variant, such as internal linkage in C, which is a form of file scope or module scope.

In object-oriented programming, there is also the concept of a static member variable, which is a "class variable" of a statically defined class – a member variable of a given class which is shared across all instances (objects), and is accessible as a member variable of these objects. Note however that a class variable of a dynamically defined class, in languages where classes can be defined at run time, is allocated when the class is defined and is not static.

Example

An example of static local variable in C:

#include <stdio.h>

void func() {
	static int x = 0; 
	/* x is initialized only once across five calls of func() and
	  the variable will get incremented five 
	  times after these calls. The final value of x will be 5. */
	x++;
	printf("%d\n", x); // outputs the value of x
}

int main() { //int argc, char *argv[] inside the main is optional in the particular program
	func(); // prints 1
	func(); // prints 2
	func(); // prints 3
	func(); // prints 4
	func(); // prints 5
	return 0;
}

See also

• Thread-local storage

References

• The C++ Programming Language (special edition) by Bjarne Stroustrup (Addison Wesley, 2000; ISBN 0-201-70073-5)