Variable scope

Learn the scope of local and global variables.

Variable scope

A variable's scope is the region of code where it is visible. In C, each { } block creates its own scope. A variable declared inside a block is not visible outside:

{
    int x = 5;
}
printf("%d", x);  // error

This is called block scope. Variables declared outside are visible inside; inner declarations can shadow outer ones.

Local and global

C has two main kinds of scope:

Local: declared inside a function or block; visible only there
Global: declared outside all functions; visible everywhere

A local variable with the same name can shadow a global one. This causes confusion, so avoid reusing the same name.

Exercise Use a global variable

Define a global int counter with value 0. Inside main, do counter = counter + 1; and print it with printf using %d.

Show example solution

#include <stdio.h>

int counter = 0;

int main(void) {
    counter = counter + 1;
    printf("%d", counter);
    return 0;
}

Test yourself

A variable declared inside a block
- Is visible throughout the program
- Is visible only at the top of the file
- Is visible only inside that block
- Is not visible anywhere
Show answer

Is visible only inside that block — A variable declared inside a block is visible only inside that block.

Inner block opening

if (x > 0) ____
    int y = 1;
}

Show answer

if (x > 0) {
    int y = 1;
}

Match the scope kinds

Local

Inside function
Global

Outside function
Block

Curly braces
Shadowing

Inner covers outer

Next lesson: typedef

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