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| <h5 class="subsubsection">3.2.4.7 Extra Fast Growing Objects</h5> |
| |
| <p><a name="index-efficiency-and-obstacks-325"></a> |
| The usual functions for growing objects incur overhead for checking |
| whether there is room for the new growth in the current chunk. If you |
| are frequently constructing objects in small steps of growth, this |
| overhead can be significant. |
| |
| <p>You can reduce the overhead by using special “fast growth” |
| functions that grow the object without checking. In order to have a |
| robust program, you must do the checking yourself. If you do this checking |
| in the simplest way each time you are about to add data to the object, you |
| have not saved anything, because that is what the ordinary growth |
| functions do. But if you can arrange to check less often, or check |
| more efficiently, then you make the program faster. |
| |
| <p>The function <code>obstack_room</code> returns the amount of room available |
| in the current chunk. It is declared as follows: |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>obstack_room</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005froom-326"></a></var><br> |
| <blockquote><p>This returns the number of bytes that can be added safely to the current |
| growing object (or to an object about to be started) in obstack |
| <var>obstack</var> using the fast growth functions. |
| </p></blockquote></div> |
| |
| <p>While you know there is room, you can use these fast growth functions |
| for adding data to a growing object: |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_1grow_fast</b> (<var>struct obstack *obstack-ptr, char c</var>)<var><a name="index-obstack_005f1grow_005ffast-327"></a></var><br> |
| <blockquote><p>The function <code>obstack_1grow_fast</code> adds one byte containing the |
| character <var>c</var> to the growing object in obstack <var>obstack-ptr</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_ptr_grow_fast</b> (<var>struct obstack *obstack-ptr, void *data</var>)<var><a name="index-obstack_005fptr_005fgrow_005ffast-328"></a></var><br> |
| <blockquote><p>The function <code>obstack_ptr_grow_fast</code> adds <code>sizeof (void *)</code> |
| bytes containing the value of <var>data</var> to the growing object in |
| obstack <var>obstack-ptr</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_int_grow_fast</b> (<var>struct obstack *obstack-ptr, int data</var>)<var><a name="index-obstack_005fint_005fgrow_005ffast-329"></a></var><br> |
| <blockquote><p>The function <code>obstack_int_grow_fast</code> adds <code>sizeof (int)</code> bytes |
| containing the value of <var>data</var> to the growing object in obstack |
| <var>obstack-ptr</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_blank_fast</b> (<var>struct obstack *obstack-ptr, int size</var>)<var><a name="index-obstack_005fblank_005ffast-330"></a></var><br> |
| <blockquote><p>The function <code>obstack_blank_fast</code> adds <var>size</var> bytes to the |
| growing object in obstack <var>obstack-ptr</var> without initializing them. |
| </p></blockquote></div> |
| |
| <p>When you check for space using <code>obstack_room</code> and there is not |
| enough room for what you want to add, the fast growth functions |
| are not safe. In this case, simply use the corresponding ordinary |
| growth function instead. Very soon this will copy the object to a |
| new chunk; then there will be lots of room available again. |
| |
| <p>So, each time you use an ordinary growth function, check afterward for |
| sufficient space using <code>obstack_room</code>. Once the object is copied |
| to a new chunk, there will be plenty of space again, so the program will |
| start using the fast growth functions again. |
| |
| <p>Here is an example: |
| |
| <pre class="smallexample"> void |
| add_string (struct obstack *obstack, const char *ptr, int len) |
| { |
| while (len > 0) |
| { |
| int room = obstack_room (obstack); |
| if (room == 0) |
| { |
| /* <span class="roman">Not enough room. Add one character slowly,</span> |
| <span class="roman">which may copy to a new chunk and make room.</span> */ |
| obstack_1grow (obstack, *ptr++); |
| len--; |
| } |
| else |
| { |
| if (room > len) |
| room = len; |
| /* <span class="roman">Add fast as much as we have room for.</span> */ |
| len -= room; |
| while (room-- > 0) |
| obstack_1grow_fast (obstack, *ptr++); |
| } |
| } |
| } |
| </pre> |
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