File and device input/output

File and device input/output

This section discusses the UNIX System file and record locking facility. Mandatory and advisory file and record locking are both available on current releases of the UNIX System. The intent of this capability is to provide a synchronization mechanism for programs accessing the same stores of data simultaneously. Such processing is characteristic of many multiuser applications, and the need for a standard method of dealing with the problem has been recognized by standards advocates like /usr/group, an organization of UNIX System users from businesses and campuses across the country.

Advisory file and record locking can be used to coordinate self-synchronizing processes. In mandatory locking, the standard I/O subroutines and I/O system calls enforce the locking protocol. In this way, at the cost of a little efficiency, mandatory locking double checks the programs against accessing the data out of sequence.

Also included in this section is a description of how file and record locking capabilities can be used. Examples are given for the correct use of record locking. Misconceptions about the amount of protection that record locking affords are dispelled. Record locking should be viewed as a synchronization mechanism, not a security mechanism.

The remainder of this section describes the STREAMS mechanism as it relates to input/output operations.