Storage Classes

• auto — the default for local variables: created when the block starts, destroyed when it ends.
• static (inside a function) — keeps its value between calls:

int counter(void) {
    static int count = 0;   // initialised once
    return ++count;
}

• static (at file level) — visible only inside that .c file.
• extern — declares a global variable defined in another file.
• register — a hint to keep a variable in a CPU register (modern compilers ignore it).