D.1.12 Variable Length Arrays (VLA):

6.7.5.2 Array declarators

VLAs are allocated on the stack as if by calling the alloca function. Their lifetime, regardless of their scope, is the same as any data allocated on the stack by calling alloca; until the function returns. The space allocated is freed when the stack is released upon returning from the function in which the VLA is allocated.

Not all constraints are yet enforced for variable length arrays. Constraint violations lead to undefined results.

#include <stdio.h>
void foo(int);

int main(void) {
  foo(4);
  return(0);
}

void foo (int n) {
  int i;
  int a[n];
  for (i = 0; i < n; i++)
    a[i] = n-i;
  for (i = n-1; i >= 0; i--)
    printf("a[%d] = %d\n", i, a[i]);
}

example% cc test.c
example% a.out
a[3] = 1
a[2] = 2
a[1] = 3
a[0] = 4