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| <h4 class="subsection">12.21.2 Obstack Streams</h4> |
| |
| <p>You can open an output stream that puts it data in an obstack. |
| See <a href="Obstacks.html#Obstacks">Obstacks</a>. |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: FILE * <b>open_obstack_stream</b> (<var>struct obstack *obstack</var>)<var><a name="index-open_005fobstack_005fstream-1159"></a></var><br> |
| <blockquote><p>This function opens a stream for writing data into the obstack <var>obstack</var>. |
| This starts an object in the obstack and makes it grow as data is |
| written (see <a href="Growing-Objects.html#Growing-Objects">Growing Objects</a>). |
| <!-- @end group Doubly invalid because not nested right. --> |
| |
| <p>Calling <code>fflush</code> on this stream updates the current size of the |
| object to match the amount of data that has been written. After a call |
| to <code>fflush</code>, you can examine the object temporarily. |
| |
| <p>You can move the file position of an obstack stream with <code>fseek</code> or |
| <code>fseeko</code> (see <a href="File-Positioning.html#File-Positioning">File Positioning</a>). Moving the file position past |
| the end of the data written fills the intervening space with zeros. |
| |
| <p>To make the object permanent, update the obstack with <code>fflush</code>, and |
| then use <code>obstack_finish</code> to finalize the object and get its address. |
| The following write to the stream starts a new object in the obstack, |
| and later writes add to that object until you do another <code>fflush</code> |
| and <code>obstack_finish</code>. |
| |
| <p>But how do you find out how long the object is? You can get the length |
| in bytes by calling <code>obstack_object_size</code> (see <a href="Status-of-an-Obstack.html#Status-of-an-Obstack">Status of an Obstack</a>), or you can null-terminate the object like this: |
| |
| <pre class="smallexample"> obstack_1grow (<var>obstack</var>, 0); |
| </pre> |
| <p>Whichever one you do, you must do it <em>before</em> calling |
| <code>obstack_finish</code>. (You can do both if you wish.) |
| </p></blockquote></div> |
| |
| <p>Here is a sample function that uses <code>open_obstack_stream</code>: |
| |
| <pre class="smallexample"> char * |
| make_message_string (const char *a, int b) |
| { |
| FILE *stream = open_obstack_stream (&message_obstack); |
| output_task (stream); |
| fprintf (stream, ": "); |
| fprintf (stream, a, b); |
| fprintf (stream, "\n"); |
| fclose (stream); |
| obstack_1grow (&message_obstack, 0); |
| return obstack_finish (&message_obstack); |
| } |
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