Ausam/doc/man/man2/locking.2

.th LOCKING II 11-6-78
.sh NAME
readlock, writelock, unlock, lock \*- perform locking on a file
.sh SYNOPSIS
.br
(lock = 56.; not in assembler)
.br
(file descriptor in r0)
.br
.ft B
sys	lock;	mode
.ft R
.br
.ft B
lock(mode,fildes)
.br
readlock(fildes)
.br
writelock(fildes)
.br
unlock()
.ft R
.sh DESCRIPTION
These system calls are used to protect multiple accesses to a file through
an "n reader, one writer" type of protocol.
Readers and writers have effectively equal priorities \*- neither can
indefinitely lock out the other.
.s3
.it Mode
determines the type of locking:
.ti +5
0: lock for reading
.ti +5
1: lock for writing
.ti +5
2: unlock
.br
In the case of unlock, the file descriptor argument is ignored and need not
be specified.
.s3
.it Readlock
locks the file in question from being modified by writing until at least
the next
.it unlock.
(Note \*- this does not include truncation of a file by "creat".)
.s3
.it Writelock
locks the file for exclusive access (read or write) until the next
.it unlock.
.s3
.it Unlock
unlocks a file that was previously locked.
.s3
Only one file can be locked at a time; this restriction is not severe but
guarantees absence of deadlocks. If a file is already locked, a call to
.it readlock
or
.it writelock
effectively does an
.it unlock
first.
Children cannot inherit a lock; if a process forks while a file is locked,
the child loses its lock.
If a locked file is closed, it is unlocked as well.
.s3
Part of the mode of a file specifies whether or not it is to obey locking
protocols. If the bit 02000 (previously set-group-id)
is set in the mode, locking before accessing
will be enforced. If the bit is not set, calls to
.it readlock
or
.it writelock
will return an error.
.s3
If a file mode has the 010 bit (execute by group) set,
opening or creating the file will lock it automatically.
It is
.it readlocked
iff it was opened for reading only.
.s3
Files with both of these bits set will be
accessible by unmodified programs as if there were no locking operative,
but expect trouble if a program attempts to do overlapped input or output
operations on two such files simultaneously.
.sh "SEE ALSO"
Open(II), Creat(II), Close(II), Chmod(I & II)
.sh DIAGNOSTICS
From assembler, c bit set. From C, -1 is returned.
An error is returned from the locking routines if the file does not have the
"locking protocol" bit set.
An error is returned from
.it unlock
if the file was not locked.