From 3d08cd331c1adcf0d917392f7e527b3f00511748 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Fri, 24 May 2024 06:52:22 +0200 Subject: Merging upstream version 6.8. Signed-off-by: Daniel Baumann --- man7/sem_overview.7 | 139 ---------------------------------------------------- 1 file changed, 139 deletions(-) delete mode 100644 man7/sem_overview.7 (limited to 'man7/sem_overview.7') diff --git a/man7/sem_overview.7 b/man7/sem_overview.7 deleted file mode 100644 index 67f2e41..0000000 --- a/man7/sem_overview.7 +++ /dev/null @@ -1,139 +0,0 @@ -.\" Copyright (C) 2006 Michael Kerrisk -.\" -.\" SPDX-License-Identifier: Linux-man-pages-copyleft -.\" -.TH sem_overview 7 2023-10-31 "Linux man-pages 6.7" -.SH NAME -sem_overview \- overview of POSIX semaphores -.SH DESCRIPTION -POSIX semaphores allow processes and threads to synchronize their actions. -.P -A semaphore is an integer whose value is never allowed to fall below zero. -Two operations can be performed on semaphores: -increment the semaphore value by one -.RB ( sem_post (3)); -and decrement the semaphore value by one -.RB ( sem_wait (3)). -If the value of a semaphore is currently zero, then a -.BR sem_wait (3) -operation will block until the value becomes greater than zero. -.P -POSIX semaphores come in two forms: named semaphores and -unnamed semaphores. -.TP -.B Named semaphores -A named semaphore is identified by a name of the form -.IR /somename ; -that is, a null-terminated string of up to -.BI NAME_MAX \-4 -(i.e., 251) 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). -.IP -The -.BR sem_open (3) -function creates a new named semaphore or opens an existing -named semaphore. -After the semaphore has been opened, it can be operated on using -.BR sem_post (3) -and -.BR sem_wait (3). -When a process has finished using the semaphore, it can use -.BR sem_close (3) -to close the semaphore. -When all processes have finished using the semaphore, -it can be removed from the system using -.BR sem_unlink (3). -.TP -.B Unnamed semaphores (memory-based semaphores) -An unnamed semaphore does not have a name. -Instead the semaphore is placed in a region of memory that -is shared between multiple threads (a -.IR "thread-shared semaphore" ) -or processes (a -.IR "process-shared semaphore" ). -A thread-shared semaphore is placed in an area of memory shared -between the threads of a process, for example, a global variable. -A process-shared semaphore must be placed in a shared memory region -(e.g., a System V shared memory segment created using -.BR shmget (2), -or a POSIX shared memory object built created using -.BR shm_open (3)). -.IP -Before being used, an unnamed semaphore must be initialized using -.BR sem_init (3). -It can then be operated on using -.BR sem_post (3) -and -.BR sem_wait (3). -When the semaphore is no longer required, -and before the memory in which it is located is deallocated, -the semaphore should be destroyed using -.BR sem_destroy (3). -.P -The remainder of this section describes some specific details -of the Linux implementation of POSIX semaphores. -.SS Versions -Before Linux 2.6, Linux supported only unnamed, -thread-shared semaphores. -On a system with Linux 2.6 and a glibc that provides the NPTL -threading implementation, -a complete implementation of POSIX semaphores is provided. -.SS Persistence -POSIX named semaphores have kernel persistence: -if not removed by -.BR sem_unlink (3), -a semaphore will exist until the system is shut down. -.SS Linking -Programs using the POSIX semaphores API must be compiled with -.I cc \-pthread -to link against the real-time library, -.IR librt . -.SS Accessing named semaphores via the filesystem -On Linux, named semaphores are created in a virtual filesystem, -normally mounted under -.IR /dev/shm , -with names of the form -.IR \fBsem.\fPsomename . -(This is the reason that semaphore names are limited to -.BI NAME_MAX \-4 -rather than -.B NAME_MAX -characters.) -.P -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. -.SH NOTES -System V semaphores -.RB ( semget (2), -.BR semop (2), -etc.) are an older semaphore API. -POSIX semaphores provide a simpler, and better designed interface than -System V semaphores; -on the other hand POSIX semaphores are less widely available -(especially on older systems) than System V semaphores. -.SH EXAMPLES -An example of the use of various POSIX semaphore functions is shown in -.BR sem_wait (3). -.SH SEE ALSO -.BR sem_close (3), -.BR sem_destroy (3), -.BR sem_getvalue (3), -.BR sem_init (3), -.BR sem_open (3), -.BR sem_post (3), -.BR sem_unlink (3), -.BR sem_wait (3), -.BR pthreads (7), -.BR shm_overview (7) -- cgit v1.2.3