Scope and Lifetime of Variables in C

Scope determines where a variable can be accessed. Lifetime determines how long a variable exists in memory.

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Types of Scope

1. Block Scope (Local)

Variables declared inside {} are accessible only within that block.

void func() {
    int x = 10;     /* block scope */
    {
        int y = 20; /* inner block scope */
        printf("%d %d\n", x, y);  /* OK */
    }
    /* printf("%d", y); — ERROR: y not accessible here */
}

2. Function Scope

Labels used with goto have function scope — visible anywhere in the function.

3. File Scope (Global)

Variables declared outside all functions are accessible throughout the file.

int globalCount = 0;  /* file scope */

void increment() { globalCount++; }
void display()   { printf("%d\n", globalCount); }

4. Program Scope (extern)

Variables declared extern can be shared across multiple files.

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Lifetime Categories

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Variable Shadowing

A local variable can shadow a global variable with the same name.

int x = 100;   /* global */

void demo() {
    int x = 200;   /* local — shadows global */
    printf("%d\n", x);  /* 200, not 100 */
}

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Static Local Variables

Static local variables retain their value between function calls.

void counter() {
    static int count = 0;  /* initialised once */
    count++;
    printf("Count: %d\n", count);
}

int main() {
    counter();  /* Count: 1 */
    counter();  /* Count: 2 */
    counter();  /* Count: 3 */
}

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Best Practices

• Prefer local variables over globals to reduce side effects
• Keep variable scope as narrow as possible
• Use meaningful names to avoid confusion when scopes overlap
• Avoid global variables; if needed, use static to limit them to one file