Lesson 24: Scope Rules: Local and Global Variables

Scope defines the region of the program where a declared identifier (variable, function, etc.) can be accessed.

1. Local Scope (Automatic Variables)

• Variables declared inside a function, block ({}), or as function parameters.
• Their scope is limited to that block.
• They are created when the block is entered and destroyed when the block is exited.

c void example_func() { int x = 10; // Local to example_func if (x > 5) { int y = 20; // Local to the if block printf("y is %d\n", y); } // y is destroyed here // printf("y is %d\n", y); // ERROR: y is out of scope }

2. Global Scope (External Variables)

• Variables declared outside of any function.
• Their scope extends from the point of declaration to the end of the file.
• They can be accessed and modified by any function in the program (and potentially other files).

c int GLOBAL_COUNTER = 0; // Global variable

void increment() { GLOBAL_COUNTER++; // Accessible here }

int main() { increment(); printf("Counter: %d\n", GLOBAL_COUNTER); // Accessible here return 0; }

Storage Classes (Brief Mention)

Storage class specifiers like auto, extern, static, and register control the scope, lifetime, and linkage of variables. We will focus on static later, which allows a variable to retain its value between function calls, even if defined locally.