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Clarified semantics of relationship between flock() locks 

and open file entries and file descriptors.
See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=291121
This commit is contained in:
Michael Kerrisk 2005-06-21 14:43:56 +00:00
parent fb654466a6
commit d04e1109b9
1 changed files with 17 additions and 3 deletions
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20
man2/flock.2
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@ -43,7 +43,6 @@ The parameter
.I operation .I operation
is one of the following: is one of the following:
.RS .RS
.sp
.TP 1.0i .TP 1.0i
.B LOCK_SH .B LOCK_SH
Place a shared lock. Place a shared lock.
@ -59,7 +58,6 @@ file at a given time.
Remove an existing lock held by this process. Remove an existing lock held by this process.
.sp .sp
.RE .RE
A call to A call to
.BR flock () .BR flock ()
may block if an incompatible lock is held by another process. may block if an incompatible lock is held by another process.
@ -73,7 +71,7 @@ A single file may not simultaneously have both shared and exclusive locks.
Locks created by Locks created by
.BR flock () .BR flock ()
are associated with a file, or, more precisely, an open file table entry. are associated with an open file table entry.
This means that duplicate file descriptors (created by, for example, This means that duplicate file descriptors (created by, for example,
.BR fork "(2) or " dup (2)) .BR fork "(2) or " dup (2))
refer to the same lock, and this lock may be modified refer to the same lock, and this lock may be modified
@ -83,6 +81,15 @@ Furthermore, the lock is released either by an explicit
operation on any of these duplicate descriptors, or when all operation on any of these duplicate descriptors, or when all
such descriptors have been closed. such descriptors have been closed.
If a process uses
.BR open (2)
(or similar) to obtain more than one descriptor for the same file,
these descriptors are treated independently by
.BR flock ().
An attempt to lock the file using one of these file descriptors
may be denied by a lock that the calling process has
already placed via another descriptor.
A process may only hold one type of lock (shared or exclusive) A process may only hold one type of lock (shared or exclusive)
on a file. on a file.
Subsequent Subsequent
@ -164,6 +171,13 @@ and
.BR fcntl (2) .BR fcntl (2)
locks have different semantics with respect to forked processes and locks have different semantics with respect to forked processes and
.BR dup (2). .BR dup (2).
On systems that implement
.BR flock ()
using
.BR fcntl (),
the semantics of
.BR flock ()
will be different from those described in this manual page.
.PP .PP
Converting a lock Converting a lock
(shared to exclusive, or vice versa) is not guaranteed to be atomic: (shared to exclusive, or vice versa) is not guaranteed to be atomic: