An obstack is a pool of memory containing a stack of objects. You
can create any number of separate obstacks, and then allocate objects in
specified obstacks. Within each obstack, the last object allocated must
always be the first one freed, but distinct obstacks are independent of
each other.
Aside from this one constraint of order of freeing, obstacks are totally
general: an obstack can contain any number of objects of any size. They
are implemented with macros, so allocation is usually very fast as long as
the objects are usually small. And the only space overhead per object is
the padding needed to start each object on a suitable boundary.