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sem_overview.7: Clarify construction rules for semaphore object names 

See also http://groups.google.com/group/comp.os.linux.development.apps/browse_thread/thread/b4a67caa765cb65f

Reported-by: Jens Thoms Toerring <jt@toerring.de>
Reported-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2009-02-20 15:54:45 +13:00
parent db2fff8c01
commit e2cbd4754e
1 changed files with 16 additions and 3 deletions
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19
man7/sem_overview.7
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@ -23,7 +23,7 @@
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.TH SEM_OVERVIEW 7 2008-06-15 "Linux" "Linux Programmer's Manual"
.TH SEM_OVERVIEW 7 2009-02-20 "Linux" "Linux Programmer's Manual"
.SH NAME
sem_overview \- Overview of POSIX semaphores
.SH DESCRIPTION
@ -44,7 +44,20 @@ unnamed semaphores.
.TP
.B Named semaphores
A named semaphore is identified by a name of the form
.IR /somename .
.IR /somename ;
that is, a null-terminated string of up to
.BI NAME_MAX +1
(i.e., 256) characters consisting of an initial slash,
.\" glibc allows the initial slash to be omitted, and makes
.\" multiple initial slashes equivalent to a single slash.
.\" This differs from the implementation of POSIX message queues.
followed by one or more characters, none of which are slashes.
.\" glibc allows subdirectory components in the name, in which
.\" case the subdirectory tree must exist under /dev/shm, and
.\" the fist subdirectory component must exist as the name
.\" sem.name, and all of the subdirectory components must allow the
.\" required permissions if a user wants to create a semaphore
.\" object in a subdirectory.
Two processes can operate on the same named semaphore by passing
the same name to
.BR sem_open (3).
@ -113,7 +126,7 @@ On Linux, named semaphores are created in a virtual file system,
normally mounted under
.IR /dev/shm ,
with names of the form
.IR \fBsem.\fPname .
.IR \fBsem.\fPsomename .
Since Linux 2.6.19, ACLs can be placed on files under this directory,
to control object permissions on a per-user and per-group basis.