Reference Manual - 0.9.25
IDirectFBSurface   DrawString

Draw an UTF-8 string at the specified position with the given color following the specified flags.

DrawString (
  IDirectFBSurface   * thiz,
  const char   * text,
  int   bytes,
  int   x,
  int   y,
  DFBSurfaceTextFlags   flags
);

If font was loaded with the DFFA_CHARMAP flag, the string specifies UTF-8 encoded raw glyph indices.

Bytes specifies the number of bytes to take from the string or -1 for the complete NULL-terminated string. You need to set a font using the SetFont() method before calling this function.