Each source file in which you plan to use the obstack functions
must include the header file obstack.h, like this:
Also, if the source file uses the macro obstack_init, it must
declare or define two functions or macros that will be called by the
obstack library. One, obstack_chunk_alloc, is used to allocate
the chunks of memory into which objects are packed. The other,
obstack_chunk_free, is used to return chunks when the objects in
them are freed. These macros should appear before any use of obstacks
in the source file.
Usually these are defined to use malloc via the intermediary
xmalloc (see Unconstrained Allocation). This is done with
the following pair of macro definitions:
Though the memory you get using obstacks really comes from malloc,
using obstacks is faster because malloc is called less often, for
larger blocks of memory. See Obstack Chunks, for full details.
At run time, before the program can use a struct obstack object
as an obstack, it must initialize the obstack by calling
— Function: int obstack_init (struct obstack *obstack-ptr)
Initialize obstack obstack-ptr for allocation of objects. This
function calls the obstack's obstack_chunk_alloc function. If
allocation of memory fails, the function pointed to by
obstack_alloc_failed_handler is called. The obstack_init
function always returns 1 (Compatibility notice: Former versions of
obstack returned 0 if allocation failed).
Here are two examples of how to allocate the space for an obstack and
initialize it. First, an obstack that is a static variable:
The value of this variable is a pointer to a function that
obstack uses when obstack_chunk_alloc fails to allocate
memory. The default action is to print a message and abort.
You should supply a function that either calls exit
(see Program Termination) or longjmp (see Non-Local Exits) and doesn't return.