binary data insert fails using dblib

who still uses the old dblib can help me?
thanks in advance:

1.create table & procedure in db:
test_table(uniqueidentifier a,varbinary50 b)
CREATE PROCEDURE insert_table
@.b_in varbinary
AS
insert into test_table(b) values (@.b_in)
GO

2.write program use dblib:
wchar_t str[120]=L"ABC";
dbrpcparam(dbproc[i], "@.b_in", (BYTE)NULL, SQLVARBINARY,
-1, 6, &str)

3.I can find the program executed successfully,but only 1 Byte is inserted:
a b
---------------
199D71BE-327A-4BC1-AEC8-ACB0C96076CA 0x41

how to insert the whole string into the database?See if you can spot the difference in the code below (tsk...the answer is there...):

declare @.i varbinary
set @.i = 0xffffffff
select @.i, datalength(@.i)
go
declare @.i varbinary(10)
set @.i = 0xffffffff
select @.i, datalength(@.i)
go|||Thanks! I have made a serious mistake *_*