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(libc.info)Obstack Chunks

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Obstack Chunks
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   Obstacks work by allocating space for themselves in large chunks, and
then parceling out space in the chunks to satisfy your requests.  Chunks
are normally 4096 bytes long unless you specify a different chunk size.
The chunk size includes 8 bytes of overhead that are not actually used
for storing objects.  Regardless of the specified size, longer chunks
will be allocated when necessary for long objects.

   The obstack library allocates chunks by calling the function
`obstack_chunk_alloc', which you must define.  When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling `obstack_chunk_free', which you must
also define.

   These two must be defined (as macros) or declared (as functions) in
each source file that uses `obstack_init' (Note: Creating Obstacks).
Most often they are defined as macros like this:

     #define obstack_chunk_alloc malloc
     #define obstack_chunk_free free

   Note that these are simple macros (no arguments).  Macro definitions
with arguments will not work!  It is necessary that
`obstack_chunk_alloc' or `obstack_chunk_free', alone, expand into a
function name if it is not itself a function name.

   If you allocate chunks with `malloc', the chunk size should be a
power of 2.  The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet
used.

 - Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     This returns the chunk size of the given obstack.

   Since this macro expands to an lvalue, you can specify a new chunk
size by assigning it a new value.  Doing so does not affect the chunks
already allocated, but will change the size of chunks allocated for
that particular obstack in the future.  It is unlikely to be useful to
make the chunk size smaller, but making it larger might improve
efficiency if you are allocating many objects whose size is comparable
to the chunk size.  Here is how to do so cleanly:

     if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE)
       obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE;