25.61. LOCK # fn, mode
Lock a file or release a lock on a file. The mode parameter can be read, write or release.
When a file is locked to read no other program is allowed to write the file. This ensures that the program reading the file gets consistent data from the file. If a program locks a file to read using the lock value read other programs may also get the read lock, but no program can get the write lock. This means that any program trying to write the file and issuing the command LOCK with the parameter write will stop and wait until all read locks are released.
When a program write locks a file no other program can read the file or write the file.
Note that the different operating systems and therefore ScriptBasic running on different operating systems implement file lock in different ways. UNIX operating systems implement so called advisory locking, while Windows NT implements mandatory lock.
This means that a program under UNIX can write a file while another program has a read or write lock on the file if the other program is not good behaving and does not ask for a write lock. Therefore this command under UNIX does not guarantee that any other program is not accessing the file simultaneously.
Contrary Windows NT does lock the file in a hard way, and this means that no other process can access the file in prohibited way while the file is locked.
This different behavior usually does not make harm, but in some rare cases knowing it may help in debugging some problems. Generally you should not have a headache because of this.
You should use this command to synchronize the BASIC programs running parallel and accessing the same file.
You can also use the command LOCK REGION to lock a part of the file while leaving other parts of the file accessible to other programs.
If you heavily use record oriented files and file locks you may consider using some data base module to store the data in database instead of plain files.