Below is a comparison of the difference between scope visibility and lifetime of variables in C:

Aspect

Scope of a Variable

Lifetime of a Variable

Definition

Scope refers to the region of the program where the variable is visible and can be accessed.

Lifetime refers to the duration for which the variable exists in memory during program execution.

Focus

Focuses on where the variable can be accessed.

Focuses on how long the variable occupies memory.

Types

Local scope (function/block), global scope, function scope, file scope.

Automatic (local), static, global, dynamic (heap).

Start

The scope of a variable starts when it is declared.

The lifetime of a variable starts when it is allocated in memory.

End

The scope ends when the block or function where the variable is declared ends.

The lifetime ends when the variable is deallocated or the program terminates.

Example (Local)

A local variable in a function is only accessible inside that function.

A local variable is created when the function is called and destroyed when the function ends.

Example (Global)

A global variable is accessible throughout the entire program.

A global variable persists for the entire program’s duration.

Influence on Access

Scope determines which part of the program can access the variable.

Lifetime determines when the variable is available in memory and can be accessed