Static Variables in C

In C programming, a static variable is declared using static keyword and have the property of retaining their value between multiple function calls. It is initialized only once and is not destroyed when the function returns a value. It extends the lifetime of the variable till the end of the program.

Example:

1 2 

Explanation: The above program prints 1 2 because static variables count is only initialized once. In the first call to fun(), count in increased to 1 which is printed. In the second call, normal local variable would have value 1 as it would have been created and initialized again, but count variable is 2 because it was not destroyed with the function. That is why it was able to retain its value between multiple function calls.

Syntax

The static variable can be simply declared by prefixing the static keyword before the normal variable declaration.

static dataType variableName;

Examples of static Variables

The following examples demonstrate the use of static variables in C programs:

Return Address of Local Variable from a Function

As static variables live till the end of the program, we can use them to return data by reference from a function in C:

123

Explanation: In this program, fun() returns a pointer to the static local variable local_var, which retains its value even after the function exits. Unlike normal local variables, which are destroyed when function exits.

Control Recursion Depth

As these variables are only initialized once and retain their value, they can be used to control the recursion in case of infinite recursive calls.

Output

(infinite recursion)

The above program is an example where a specific value of n (here all negative values) leads to infinite recursion. A static variable can be used to control the recursion depth in this case and avoid the stack overflow.

-1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 

Explanation: In this program, the static variable depth retains its value across recursive calls, allowing the function to track the recursion depth. It ensures the recursion terminates when depth > 10, preventing excessive recursive calls and potential stack overflow.

Global Static Variables

Static variables are generally declared inside functions, but they can also be declared outside functions (at the global scope). Global static variables have their scope limited to the file they are defined in (internal linkage). It means that they cannot be accessed outside the current translation unit (C source file).

Other than that, there is no effect of static on global variables in C.

Example:

5
10

Interesting Facts

Following are some interesting facts about static variables:

• Static variables (like global variables) are initialized as 0 if not initialized explicitly. See this for more details. 
• In C, static variables can only be initialized using constant literals. See this for more details.

Local vs Static Local Variables

The static local variables and local variables behave differently from each other. Following table lists the primary differences between them: