A `static` local variable in C keeps its value between function calls

A local variable declared `static` inside a function has local scope (only visible inside that function) but static storage duration: it is allocated once for the lifetime of the programme and retains its value between calls. This is useful for counters and memoisation inside a function without exposing the state to the rest of the codebase. It is very different from `static` at file scope, which restricts linkage to the current translation unit.

/* I count how many times this function has been called without a global */
int get_next_id(void) {
    static int counter = 0;   /* initialised once; value persists across calls */
    return ++counter;
}

int a = get_next_id();  /* a = 1 */
int b = get_next_id();  /* b = 2 */
int c = get_next_id();  /* c = 3 */

/* Thread safety: incrementing a static local is not atomic.
   Use _Atomic or a mutex if called from multiple threads. */