What is the difference between extern and non-extern variable declarations?

extern only declares a variable (no storage) — it promises the definition lives in another file; without extern you define it, allocating storage.

// file1.c
// DEFINITION: allocates storage
int global = 42;

// file2.c
// DECLARATION: uses storage from file1.c
extern int global;
void foo() {
    // Works!
    printf("%d", global);
}

Without extern:

// file1.c
// Definition
int global = 42;

// file2.c
// Another definition! (weak/tentative)
int global;
// May work (Rule 2/3) but dangerous

Best practices:

Why extern matters:

// Without extern, each file gets its own copy!
// This is a common source of subtle bugs

// myheader.h
// BAD: defines in every file that includes it!
int counter;

// myheader.h (correct)
// Good: declaration only
extern int counter;
// counter.c
// Single definition
int counter = 0;