GNU C Library (libc) Programming Guide

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12.21.2 Obstack Streams

You can open an output stream that puts it data in an obstack. See 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 (see 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 (see 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 (see 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);
     }