Scope and Visibility of Variables

The scope of a variable determines where in the program that variable is accessible. In C, we generally deal with two main types of scope.

11.1 Local Scope

A variable declared inside a function or a block { } is local to that block.

• Visibility: Only visible within the defining block.
• Lifetime: Created when the block is entered and destroyed when the block is exited.
• Precedence: If a local variable has the same name as a global variable, the local variable takes precedence.

void myFunction() {
    int x = 5; // Local variable
    printf("%d", x);
}
// printf("%d", x); // Error! x is unknown here.

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11.2 Global Scope

A variable declared outside all functions is global.

• Visibility: Visible to all functions following its declaration in that file.
• Lifetime: Exists for the entire duration of the program.
• Usage: Useful for data that needs to be accessed by many functions.

int globalCount = 0; // Global variable

void increment() { globalCount++; }
void decrement() { globalCount--; }

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11.3 Block Scope

Variables can also be localized to specific structures like loops or if statements.

for (int i = 0; i < 10; i++) { // 'i' exists only inside this for loop
    printf("%d", i);
}
// printf("%d", i); // Error! 'i' is out of scope.

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11.4 Formal Parameters

Variables in a function definition (arguments) are also treated as local variables within that function.

void add(int a, int b) { // 'a' and 'b' are local to add()
    printf("%d", a + b);
}

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Example: Scope Precedence

#include <stdio.h>

int x = 50; // Global

int main() {
    int x = 10; // Local
    printf("X in main: %d\n", x); // Prints 10 (local shadows global)
    return 0;
}

Refer to the Lecture Slides for visual diagrams explaining the Symbol Table and memory segments.