Explain Variable Scope in C with example

Variable Scope in C

Variable scope in C refers to the context in which a variable is defined and accessible. Understanding variable scope helps you manage where variables can be used and modified in your programs. There are four primary types of scope in C:

Local Variable Scope in C

Definition:

A variable has local scope if it is declared inside a function or a block (enclosed by {}).

Characteristics:

  • Access: The variable is only accessible within the function or block where it is declared.
  • Lifetime: The variable is created when the function or block starts and destroyed when it ends.

Example:

void function() {
    int localVar = 10; // localVar is accessible only within this function
    printf("%d\n", localVar); // This works
}

Global Variable Scope in C

Definition:

A variable has global scope if it is declared outside of all functions, typically at the top of the file.

Characteristics:

  • Access: The variable is accessible from any function within the same file and, with the extern keyword, across different files.
  • Lifetime: The variable exists for the entire duration of the program.

Example:

int globalVar = 20; // globalVar is accessible throughout the file and other files if declared with extern

void function() {
    printf("%d\n", globalVar); // This works
}

Block Scope

Definition: A variable has block scope if it is declared within a specific block of code, such as a loop or an if statement.

Characteristics:

  • Access: The variable is only accessible within that specific block.
  • Lifetime: The variable exists only for the duration of the block.

Example:

void function() {
    if (1) {
        int blockVar = 30; // blockVar is only accessible within this if block
        printf("%d\n", blockVar); // This works
    }
    // printf("%d\n", blockVar); // Error: blockVar is not accessible here
}

Function Variable Scope in C

Definition: Function scope applies to labels used with goto statements, which have a unique kind of scope.

Characteristics:

  • Access: Labels are only accessible within the same function where they are declared.

Example:

void function() {
    goto label; // Can jump to the label if defined in the same function
    
    label:
    printf("Jumped to label!\n"); // This works
}

Scope Resolution

When there are multiple variables with the same name, C uses a process called scope resolution to determine which variable to refer to:

  • Innermost Scope: C looks for the variable in the most immediate (innermost) scope
  • Outer Scopes: If not found, C looks in outer scopes, moving outward until it finds the variable or reaches the global scope.

Example:

int var = 50; // Global scope

void function() {
    int var = 10; // Local scope
    {
        int var = 20; // Block scope
        printf("%d\n", var); // Prints 20, the innermost variable
    }
    printf("%d\n", var); // Prints 10, the local variable
}
printf("%d\n", var); // Prints 50, the global variable

Summary Table

ScopeAccessLifetime
LocalWithin the function or blockDuration of function/block
GlobalThroughout the file (and other files with extern)Duration of the program
BlockWithin the specific blockDuration of the block
FunctionWithin the function (for goto statements)N/A

By understanding these scopes, you can manage variable visibility and lifetime effectively in your C programs.

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