From QB64.org wiki
A LOCK access READ or WRITE can be specified in a file OPEN statement in a network environment(DOS 3.1 or higher).
- Default: When LOCK is not specified in an OPEN statement, all other processes are denied file access.
- SHARED: When specified, any file can be READ or WRITE accessed by other procedures.
- LOCK READ: When specified, read access is denied to other file procedures.
- LOCK WRITE: When specified, write access is denied to other file procedures.
- LOCK READ WRITE: When specified any access is denied to other file procedures.
- Requires that the DOS SHARED.EXE program be run for QBasic to use networking access modes.
- If another process already has access to a specified file, program access is denied for that file OPEN statement. A "Permission Denied" error 70 will be returned. A network program must be able to handle a denial of access error.