Storage Duration: auto, static, extern

Automatic (the default)

Local variables get fresh storage per call and die when the block exits:

int counter(void) {
    int n = 0;      // fresh every call
    n++;
    return n;       // always returns 1
}

Static: born once, lives forever

static on a local makes it initialize once and persist across calls:

int counter(void) {
    static int n = 0;   // initialized once
    n++;
    return n;           // 1, 2, 3, ...
}

Same scope rules as a local (invisible outside the function), different lifetime. static on a file-scope function or variable instead means "private to this file" — module 18 uses that for internal helpers.

File-scope (global) variables

Declared outside all functions; visible from their declaration to the end of the file, alive for the whole program:

int g_attempts = 0;     // every function below can see it

void attempt(void) { g_attempts++; }

Globals are readable state — but mutable global state couples functions together in ways that break testing and reuse. The course rule: pass what a function needs as parameters; reach for a global only for genuinely program-wide constants.

extern (recognition level)

extern int g; says "g exists, defined elsewhere" — the linker connects the uses to the one definition. You need this at module 18; here, just read it.