alloca-1.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

/* Verify that alloca storage is sufficiently aligned.  */
/* ??? May fail if BIGGEST_ALIGNMENT > STACK_BOUNDARY.  Which, I guess
   can only happen on !STRICT_ALIGNMENT targets.  */

typedef __SIZE_TYPE__ size_t;

struct dummy { int x __attribute__((aligned)); };
#define BIGGEST_ALIGNMENT __alignof__(struct dummy)

_Bool foo(void)
{
  char *p = __builtin_alloca(32);
  return ((size_t)p & (BIGGEST_ALIGNMENT - 1)) == 0;
}

int main()
{
  if (!foo())
    abort ();
  return 0;
}