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| <h5 class="subsubsection">3.2.4.6 Growing Objects</h5> |
| |
| <p><a name="index-growing-objects-_0028in-obstacks_0029-314"></a><a name="index-changing-the-size-of-a-block-_0028obstacks_0029-315"></a> |
| Because memory in obstack chunks is used sequentially, it is possible to |
| build up an object step by step, adding one or more bytes at a time to the |
| end of the object. With this technique, you do not need to know how much |
| data you will put in the object until you come to the end of it. We call |
| this the technique of <dfn>growing objects</dfn>. The special functions |
| for adding data to the growing object are described in this section. |
| |
| <p>You don't need to do anything special when you start to grow an object. |
| Using one of the functions to add data to the object automatically |
| starts it. However, it is necessary to say explicitly when the object is |
| finished. This is done with the function <code>obstack_finish</code>. |
| |
| <p>The actual address of the object thus built up is not known until the |
| object is finished. Until then, it always remains possible that you will |
| add so much data that the object must be copied into a new chunk. |
| |
| <p>While the obstack is in use for a growing object, you cannot use it for |
| ordinary allocation of another object. If you try to do so, the space |
| already added to the growing object will become part of the other object. |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_blank</b> (<var>struct obstack *obstack-ptr, int size</var>)<var><a name="index-obstack_005fblank-316"></a></var><br> |
| <blockquote><p>The most basic function for adding to a growing object is |
| <code>obstack_blank</code>, which adds space without initializing it. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_grow</b> (<var>struct obstack *obstack-ptr, void *data, int size</var>)<var><a name="index-obstack_005fgrow-317"></a></var><br> |
| <blockquote><p>To add a block of initialized space, use <code>obstack_grow</code>, which is |
| the growing-object analogue of <code>obstack_copy</code>. It adds <var>size</var> |
| bytes of data to the growing object, copying the contents from |
| <var>data</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_grow0</b> (<var>struct obstack *obstack-ptr, void *data, int size</var>)<var><a name="index-obstack_005fgrow0-318"></a></var><br> |
| <blockquote><p>This is the growing-object analogue of <code>obstack_copy0</code>. It adds |
| <var>size</var> bytes copied from <var>data</var>, followed by an additional null |
| character. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_1grow</b> (<var>struct obstack *obstack-ptr, char c</var>)<var><a name="index-obstack_005f1grow-319"></a></var><br> |
| <blockquote><p>To add one character at a time, use the function <code>obstack_1grow</code>. |
| It adds a single byte containing <var>c</var> to the growing object. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_ptr_grow</b> (<var>struct obstack *obstack-ptr, void *data</var>)<var><a name="index-obstack_005fptr_005fgrow-320"></a></var><br> |
| <blockquote><p>Adding the value of a pointer one can use the function |
| <code>obstack_ptr_grow</code>. It adds <code>sizeof (void *)</code> bytes |
| containing the value of <var>data</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void <b>obstack_int_grow</b> (<var>struct obstack *obstack-ptr, int data</var>)<var><a name="index-obstack_005fint_005fgrow-321"></a></var><br> |
| <blockquote><p>A single value of type <code>int</code> can be added by using the |
| <code>obstack_int_grow</code> function. It adds <code>sizeof (int)</code> bytes to |
| the growing object and initializes them with the value of <var>data</var>. |
| </p></blockquote></div> |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: void * <b>obstack_finish</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005ffinish-322"></a></var><br> |
| <blockquote><p>When you are finished growing the object, use the function |
| <code>obstack_finish</code> to close it off and return its final address. |
| |
| <p>Once you have finished the object, the obstack is available for ordinary |
| allocation or for growing another object. |
| |
| <p>This function can return a null pointer under the same conditions as |
| <code>obstack_alloc</code> (see <a href="Allocation-in-an-Obstack.html#Allocation-in-an-Obstack">Allocation in an Obstack</a>). |
| </p></blockquote></div> |
| |
| <p>When you build an object by growing it, you will probably need to know |
| afterward how long it became. You need not keep track of this as you grow |
| the object, because you can find out the length from the obstack just |
| before finishing the object with the function <code>obstack_object_size</code>, |
| declared as follows: |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>obstack_object_size</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005fobject_005fsize-323"></a></var><br> |
| <blockquote><p>This function returns the current size of the growing object, in bytes. |
| Remember to call this function <em>before</em> finishing the object. |
| After it is finished, <code>obstack_object_size</code> will return zero. |
| </p></blockquote></div> |
| |
| <p>If you have started growing an object and wish to cancel it, you should |
| finish it and then free it, like this: |
| |
| <pre class="smallexample"> obstack_free (obstack_ptr, obstack_finish (obstack_ptr)); |
| </pre> |
| <p class="noindent">This has no effect if no object was growing. |
| |
| <p><a name="index-shrinking-objects-324"></a>You can use <code>obstack_blank</code> with a negative size argument to make |
| the current object smaller. Just don't try to shrink it beyond zero |
| length—there's no telling what will happen if you do that. |
| |
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