On input, Data Length specifies
the maximum number of returned data bytes requested by the caller. The
Length value on input includes the length field's size. An input length
field value of less than 2 is returned to the caller with an INVALID_PARAMETER
error.
On output, Data Length always indicates the maximum number of bytes that
this function can return. For OS/2 2.1, this return length size is 4 bytes.