INT 21 - DOS 3+ - "FLOCK" - RECORD LOCKING
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
SeeAlso: AH=BCh,AH=BEh,INT 2F/AX=110Ah,INT 2F/AX=110Bh

See also: 21440B

215C - DOS 3+ - "FLOCK" - RECORD LOCKING