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(libc.info)Obstack Streams

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Obstack Streams
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   You can open an output stream that puts it data in an obstack.
Note: Obstacks.

 - Function: FILE * open_obstack_stream (struct obstack *OBSTACK)
     This function opens a stream for writing data into the obstack
     OBSTACK.  This starts an object in the obstack and makes it grow
     as data is written (Note: Growing Objects).

     Calling `fflush' on this stream updates the current size of the
     object to match the amount of data that has been written.  After a
     call to `fflush', you can examine the object temporarily.

     You can move the file position of an obstack stream with `fseek' or
     `fseeko' (Note: File Positioning).  Moving the file position past
     the end of the data written fills the intervening space with zeros.

     To make the object permanent, update the obstack with `fflush', and
     then use `obstack_finish' 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 `fflush' and `obstack_finish'.

     But how do you find out how long the object is?  You can get the
     length in bytes by calling `obstack_object_size' (Note: Status of
     an Obstack), or you can null-terminate the object like this:

          obstack_1grow (OBSTACK, 0);

     Whichever one you do, you must do it _before_ calling
     `obstack_finish'.  (You can do both if you wish.)

   Here is a sample function that uses `open_obstack_stream':

     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);
     }