I'm using g_data_input_stream_read_upto() to read a NULL terminated string, and before wastefully reallocating the string to add a NULL pointed, I checked the code in GLib and found that it already adds the NULL-pointer:

  data_until = g_malloc (found_pos + 1);

  res = g_input_stream_read (G_INPUT_STREAM (stream),
                             data_until,
                             found_pos,
                             NULL, NULL);
  if (length)
    *length = (gsize)found_pos;
  g_warn_if_fail (res == found_pos);
  data_until[found_pos] = 0;

I think g_data_input_stream_read_upto_async() does this too, but it's hard to follow the code.

Anyway, I think this should be documented so people don't waste their time copying the string to NULL terminate it.