Int 21 Fn 5C  - DOS 3+ - "flock" - Record Locking                          [D]

   AH = 5Ch
   AL = subfunction
       00h lock region of file
       01h unlock region of file
   BX = file handle
   CX:DX = start offset of region within file
   SI:DI = length of region in bytes

Return: CF clear if successful
   CF set on error
       AX = error code (01h,06h,21h,24h) (see AH=59h)

Notes: error returned unless SHARE or network installed
   an unlock call must specify the same region as some prior lock call
   locked regions become entirely inaccessible to other processes
   duplicate handles created with AH=45h or AH=46h inherit locks, but
     handles inherited by child processes (see AH=4Bh) do not
   under DR-DOS 3.41 and 5.0, if a process opens a file without the no-
     inherit flag and then starts a child, any locks set by the parent
     are ignored, and the child will only get an error if it tries to
     lock an area previously locked by the parent process

See Also: AX=440Bh,AH=BCh,AH=BEh,INT 2F/AX=110Ah,INT 2F/AX=110Bh