Cooperative Record Locking

The Amiga's cooperative record locking scheme allows an application to lock regions of a file rather than locking an entire file. These regions are known as records.

The Amiga thinks of a record in terms of size and offset. The size refers to the length of the record in bytes. The offset refers to the starting position of the record from the beginning of the file. The size and offset can vary from record to record. The only limit on a record's size and offset are the limits the file system has on the size of the file.

Record Lock Types

There are two basic types of record lock, exclusive and shared. An exclusive lock gives an application exclusive access to a record. While an application holds an exclusive record lock, no other process can obtain a record lock that overlaps the exclusively locked region. The file system can only grant an exclusive record lock on a region that is not part of an existing record lock.

The other type of record lock, the shared lock, gives an application shared access to a record. While an application holds a shared record lock, other applications can also obtain a shared record lock that overlaps the original shared record lock, but no other process can obtain an exclusive record lock that overlaps the shared record lock.