in ftell(), given:

if (fp->_flags & __SWR)
    __sflush(fp); /* may adjust seek offset on append stream */

it seems like this is dead code because _p should always end up equal
to _bf._base because we just flushed:

} else if (fp->_flags & __SWR && fp->_p != NULL) {
    /*
     * Writing.  Any buffered characters cause the
     * position to be greater than that in the
     * underlying object.
     */
    pos += fp->_p - fp->_bf._base;
}

or am i missing something?