#define SZSIZE 8192

SV* LZW_zread( ... ) {
  SV* retval;
  u_char bp, end_bp;
  u_int offset;

  ...

  retval = NEWSV( ..., SZSIZE );
  SvPOK_only( retval );
  bp = (u_char*)SvPVX( retval );
  end_bp = SZSIZE + bp;

  ...

  while ( ... ) {
    if ( bp == end_bp ) {
      /* When bp is == end_bp then the buffer must be grown. bp is currently
         just off the end of the buffer and any operations on it is unsafe. */
      offset = bp - PvPVX(retval);
      sv_grow(sv, offset + SZSIZE);
      bp = PvPVX(retval) + offset;
      end_bp = SZSIZE + bp;
    }

    ...

    *bp++ = ...;
  }

  ...

  SvCUR_set(sv, bp - SvPVX(retval));
  return retval;
}