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| <h5 class="subsubsection">3.2.4.10 Obstack Chunks</h5> |
| |
| <p><a name="index-efficiency-of-chunks-338"></a><a name="index-chunks-339"></a> |
| 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. |
| |
| <p>The obstack library allocates chunks by calling the function |
| <code>obstack_chunk_alloc</code>, 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 <code>obstack_chunk_free</code>, which you |
| must also define. |
| |
| <p>These two must be defined (as macros) or declared (as functions) in each |
| source file that uses <code>obstack_init</code> (see <a href="Creating-Obstacks.html#Creating-Obstacks">Creating Obstacks</a>). |
| Most often they are defined as macros like this: |
| |
| <pre class="smallexample"> #define obstack_chunk_alloc malloc |
| #define obstack_chunk_free free |
| </pre> |
| <p>Note that these are simple macros (no arguments). Macro definitions with |
| arguments will not work! It is necessary that <code>obstack_chunk_alloc</code> |
| or <code>obstack_chunk_free</code>, alone, expand into a function name if it is |
| not itself a function name. |
| |
| <p>If you allocate chunks with <code>malloc</code>, 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. |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>obstack_chunk_size</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005fchunk_005fsize-340"></a></var><br> |
| <blockquote><p>This returns the chunk size of the given obstack. |
| </p></blockquote></div> |
| |
| <p>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: |
| |
| <pre class="smallexample"> if (obstack_chunk_size (obstack_ptr) < <var>new-chunk-size</var>) |
| obstack_chunk_size (obstack_ptr) = <var>new-chunk-size</var>; |
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