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fcntl.2: Detail the limitations of traditional (process-associated) locks 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-04-30 11:24:42 +02:00
parent bfb147160f
commit 1a93af040c
1 changed files with 28 additions and 14 deletions
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42
man2/fcntl.2
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@ -391,20 +391,7 @@ To place both types of lock, open a file read-write.
.P
As well as being removed by an explicit
.BR F_UNLCK ,
record locks are automatically released when the process
terminates or if it closes
.I any
file descriptor referring to a file on which locks are held.
.\" (Additional file descriptors referring to the same file
.\" may have been obtained by calls to
.\" .BR open "(2), " dup "(2), " dup2 "(2), or " fcntl ().)
This is bad: it means that a process can lose the locks on
a file like
.I /etc/passwd
or
.I /etc/mtab
when for some reason a library function decides to open, read
and close it.
record locks are automatically released when the process terminates.
.P
Record locks are not inherited by a child created via
.BR fork (2),
@ -419,6 +406,33 @@ should be avoided; use
and
.BR write (2)
instead.
The record locks described above are associated with the process
(unlike the open file description locks described below).
This has some unfortunate consequences:
.IP * 3
If a process holding a lock on a file closes
.I any
file descriptor referring to the file,
then all of the process's locks on the file are released,
no matter which file descriptor they were obtained via.
.\" (Additional file descriptors referring to the same file
.\" may have been obtained by calls to
.\" .BR open "(2), " dup "(2), " dup2 "(2), or " fcntl ().)
This is bad: it means that a process can lose its locks on
a file such as
.I /etc/passwd
or
.I /etc/mtab
when for some reason a library function decides to open, read,
and close the same file.
.IP *
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.
.PP
Open file description locks solve both of these problems.
.SS Open file description locks (non-POSIX)
Open file description locks are advisory byte-range locks whose operation is
in most respects identical to the traditional record locks described above.