14 Jan Storage Classes in C
In C, storage classes define the scope, visibility, and lifetime of variables or functions within a program. The following are the four main storage classes in C:
- auto
- register
- static
- extern
Let us discuss them one by one with examples:
auto storage class
- The default storage class for local variables.
- Scope: Local to the block or function where it’s defined.
- Lifetime: Only during the execution of the block or function.
register storage class
- Suggests that the variable be stored in a CPU register instead of RAM for faster access.
- Scope: Local to the block or function where it’s defined.
- Lifetime: Only during the execution of the block or function.
- Note: The compiler can ignore this suggestion.
static storage class
- For local variables, it extends the lifetime of the variable to the entire runtime of the program, but the scope remains local to the block or function where it’s defined.
- For global variables and functions, it restricts the visibility to the file where it’s defined.
- Scope: Local for block or function, file-level for global.
- Lifetime: Entire runtime of the program.
extern storage class
- Used to declare a global variable or function that is defined in another file.
- Scope: Global, across multiple files.
- Lifetime: Entire runtime of the program.
- Important for sharing variables and functions across different files in a multi-file program.
Example: Storage Classes in C
Let us see an example of implementing storage classes in C:
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#include <stdio.h> int globalVar = 0; // Default storage class for global variables is extern void function() { auto int localVar = 0; // auto storage class register int regVar = 1; // register storage class static int staticVar = 2; // static storage class printf("localVar: %d, regVar: %d, staticVar: %d, globalVar: %d\n", localVar, regVar, staticVar, globalVar); localVar++; regVar++; staticVar++; globalVar++; } int main() { for (int i = 0; i < 3; i++) { function(); } return 0; } |
Output
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localVar: 0, regVar: 1, staticVar: 2, globalVar: 0 localVar: 0, regVar: 1, staticVar: 3, globalVar: 1 localVar: 0, regVar: 1, staticVar: 4, globalVar: 2 |
In this example:
- localVar is a local variable with auto storage class, so it gets reinitialized every time the function is called.
- regVar is a register variable, intended for quick access, and it also gets reinitialized every time.
- staticVar retains its value between function calls because of the static storage class.
- globalVar is a global variable with extern storage class, and it retains its value across the entire program.
Above, you saw how staticVar and globalVar behave differently compared to localVar and regVar.
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