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fcntl.2: Describe semantics for compatible and conflicting OFD locks 

And describe how threads can use OFD locks to ensure
exclusive access to a file.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-04-30 11:56:15 +02:00
parent 1a93af040c
commit 97e03523e5
1 changed files with 22 additions and 3 deletions
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@ -430,7 +430,7 @@ and close the same file.
The threads in a process share locks.
In other words,
a multithreaded program can't use record locking to ensure
that threads don't simulataneously access the same region of a file.
that threads don't simultaneously access the same region of a file.
.PP
Open file description locks solve both of these problems.
.SS Open file description locks (non-POSIX)
@ -459,8 +459,27 @@ instead of being released on any close of the file.
.PP
Open file description locks always conflict with traditional record locks,
even when they are acquired by the same process on the same file descriptor.
They only conflict with each other when they are acquired on different
open file descriptions.
Open file description locks placed via the same open file description
(i.e., via the same file descriptor,
or via a duplicate of the file descriptor created by
.BR fork (2),
.BR dup (2),
.BR fcntl (2)
.BR F_DUPFD ,
and so on) are always compatible:
if a new lock is placed on an already locked region,
then the existing lock is converted to the new lock type.
(Such conversions may result in splitting, shrinking, or coalescing with
an existing lock as discussed above.)
On the other hand, open file description locks may conflict with
each other when they are acquired via different open file descriptions.
Thus, the threads in a multithreaded program can use
open file description locks to synchronize access to a file region
by having each thread perform its own
.BR open (2)
on the file and applying locks via the resulting file descriptor.
.PP
As with traditional advisory locks, the third argument to
.BR fcntl (),