Int 21H, AH=5cH, AL=01H

3 and higher

unlock region of file

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

CF clear if successful
CF set on error
    AX = error code (01h,06h,21h,24h) (see #01680)

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. Under Novell DOS 7, this function is fully supported as in MS-DOS.

AX=440Bh,INT 2F/AX=110Ah,INT 2F/AX=110Bh

Text based on Ralf Brown Interrupt List Release 61