You do not need to use traditional file I/O to lock file elements. Use the lighter weight synchronization mechanisms that are described in Multithreaded Programming Guide with mapped files.
Locking files prevents errors that can occur when several users try to update a file at the same time. You can lock a portion of a file.
File locking blocks access to an entire file. Record locking blocks access to a specified segment of the file. In SunOS, all files are a sequence of bytes of data: a record is a concept of the programs that use the file.