AT&T UNIX™PC Unix System V Users Manual - tenox

LOCKING ( 2 ) LOCKING ( 2 )
NAME locking - exclusive access to regions of a file
SYNOPSIS
int locking (filedes, mode, size);
int fildes, mode;
long size;
DESCRIPTION
Locking places or removes a lock on a region of a file. The calling
process has exclusive access to regions it has locked: another pro
cess that attempts to read or write any byte in the locked region
sleeps until the lock is removed. Filedes specifies the file to be
locked or unlocked; file des is a file descriptor returned by an
open, create , pipe, or dup system call. Mode specifies the action:
0 for lock removal; 1 for blocking lock; 2 for checking lock. The
region affected begins at the current file offset associated with
filedes and is size bytes longs. If size is zero, the region affected
ends at the end of the file.
A blocking lock differs from a checking lock only when a process
tries to lock bytes already locked by another process. A process
locked out of a blocking lock goes to sleep until the specified
region is free. A process locked out of a checking lock immediately
gets an error return from locking .
Locking imposes no structure on a UNIX file. A process can arbitrarily
lock any unlocked byte and unlock any locked byte. However,
creating a large number of noncontiguous locked regions can
fill up the lock table and make further locks impossible.
A process is said to be deadlocked if it is sleeping until an unlocking
which cannot occur until after the deadlocked process itself
does some unlocking. Locking , read, and write deliberately fail if
proceeding would cause the calling process to be deadlocked.
Special files and pipes can be locked, but no input/output is
blocked.
SEE ALSO
creat(2), close(2), dup(2), open(2), read(2), write(2).
RETURN VALUE
A return value of -1 indicates an error, with the error value in
errno. An unsuccessful checking lock sets errno to EA CCES . A
system call aborted by an incipient deadlock or by a lock table
overflow sets errno to EDEADLOCK.
WARNING
Do not apply any standard input/output library function to a
locked file.
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