From QB64.org wiki
The LOCK statement restricts access to parts of a file by other programs or processes.
- LOCK [#]fileNumber%
- LOCK [#]fileNumber%, record&
- LOCK [#]fileNumber%, [firstRecord&] TO lastRecord&
- fileNumber% is the file number of the file to lock.
- In the first syntax, the entire file is locked.
- In the second syntax, record& is the record number of the file to lock.
- In the third syntax, the records or bytes in the range [firstRecord&,lastRecord&] are locked. If firstRecord& is omitted, it is assumed to be one (the first record or byte).
- For files opened in BINARY mode, each record corresponds to a single byte.
- LOCK and UNLOCK statements are always used in pairs and each statement must match the other one.
- Files must be unlocked using UNLOCK before other programs can access them, and before the file is closed.
- Keyword not supported in Linux or macOS versions
- Required DOS SHARED.EXE to be run for QBasic to use networking access modes. No longer required.