function allocates size bytes of space in the stack
frame of the caller. This temporary space is automatically freed on
alloca() function returns a pointer to
the beginning of the allocated space.
pagesize(1), brk(2), malloc(3)
alloca() function is unsafe because it
cannot ensure that the pointer returned points to a valid and usable block
of memory. The allocation made may exceed the bounds of the stack, or even
go further into other objects in memory, and
alloca() cannot determine such an error. Avoid
alloca() with large unbounded allocations.