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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.

See also

Keyword Reference - Alphabetical
Keyword Reference - By Usage
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