Section (7) sysvipc
Name
sysvipc — System V interprocess communication mechanisms
Synopsis
#include <sys/msg.h> #include <sys/sem.h> #include <sys/shm.h>
DESCRIPTION
This manual page refers to the Linux implementation of the
System V interprocess communication (IPC) mechanisms: message
queues, semaphore sets, and shared memory segments. In the
following, the word resource
means an
instantiation of one among such mechanisms.
Resource access permissions
For each resource, the system uses a common structure of
type struct ipc_perm
to store information needed in determining permissions to
perform an IPC operation. The ipc_perm
structure includes
the following members:
struct ipc_perm { uid_t cuid
; /* creator user ID */gid_t cgid
; /* creator group ID */uid_t uid
; /* owner user ID */gid_t gid
; /* owner group ID */unsigned short mode
; /* r/w permissions */};
The mode
member
of the ipc_perm
structure defines, with its lower 9 bits, the access
permissions to the resource for a process executing an IPC
system call. The permissions are interpreted as
follows:
0400 Read by user. 0200 Write by user. 0040 Read by group. 0020 Write by group. 0004 Read by others. 0002 Write by others.
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. Furthermore, write effectively means alter for a semaphore set.
The same system header file also defines the following symbolic constants:
IPC_CREAT
-
Create entry if key doesn_zsingle_quotesz_t exist.
IPC_EXCL
-
Fail if key exists.
IPC_NOWAIT
-
Error if request must wait.
IPC_PRIVATE
-
Private key.
IPC_RMID
-
Remove resource.
IPC_SET
-
Set resource options.
IPC_STAT
-
Get resource options.
Note that IPC_PRIVATE
is a
key_t
type, while
all the other symbolic constants are flag fields and can be
OR_zsingle_quotesz_ed into an int
type variable.
Message queues
A message queue is uniquely identified by a positive
integer (its msqid
) and has an
associated data structure of type struct msqid_ds, defined in
<
sys/msg.h
>
containing the following members:
struct msqid_ds { struct ipc_perm msg_perm
;msgqnum_t msg_qnum
; /* no of messages on queue */msglen_t msg_qbytes
; /* bytes max on a queue */pid_t msg_lspid
; /* PID of last msgsnd(2) call */pid_t msg_lrpid
; /* PID of last msgrcv(2) call */time_t msg_stime
; /* last msgsnd(2) time */time_t msg_rtime
; /* last msgrcv(2) time */time_t msg_ctime
; /* last change time */};
msg_perm
-
ipc_perm
structure that specifies the access permissions on the message queue. msg_qnum
-
Number of messages currently on the message queue.
msg_qbytes
-
Maximum number of bytes of message text allowed on the message queue.
msg_lspid
-
ID of the process that performed the last msgsnd(2) system call.
msg_lrpid
-
ID of the process that performed the last msgrcv(2) system call.
msg_stime
-
Time of the last msgsnd(2) system call.
msg_rtime
-
Time of the last msgrcv(2) system call.
msg_ctime
-
Time of the last system call that changed a member of the
msqid_ds
structure.
Semaphore sets
A semaphore set is uniquely identified by a positive
integer (its semid
) and has an
associated data structure of type struct semid_ds, defined in
<
sys/sem.h
>
containing the following members:
-
struct semid_ds { struct ipc_perm sem_perm
;time_t sem_otime
;/* last operation time */time_t sem_ctime
;/* last change time */unsigned long sem_nsems
;/* count of sems in set */}; sem_perm
-
ipc_perm
structure that specifies the access permissions on the semaphore set. sem_otime
-
Time of last semop(2) system call.
sem_ctime
-
Time of last semctl(2) system call that changed a member of the above structure or of one semaphore belonging to the set.
sem_nsems
-
Number of semaphores in the set. Each semaphore of the set is referenced by a nonnegative integer ranging from
0
tosem_nsems−1
.
A semaphore is a data structure of type struct sem containing the following members:
struct sem { int semval
; /* semaphore value */int sempid
; /* PID of process that last modified */};
semval
-
Semaphore value: a nonnegative integer.
sempid
-
PID of the last process that modified the value of this semaphore.
Shared memory segments
A shared memory segment is uniquely identified by a
positive integer (its shmid
) and has an
associated data structure of type struct shmid_ds, defined in
<
sys/shm.h
>
containing the following members:
struct shmid_ds { struct ipc_perm shm_perm
;size_t shm_segsz
; /* size of segment */pid_t shm_cpid
; /* PID of creator */pid_t shm_lpid
; /* PID, last operation */shmatt_t shm_nattch
; /* no. of current attaches */time_t shm_atime
; /* time of last attach */time_t shm_dtime
; /* time of last detach */time_t shm_ctime
; /* time of last change */};
shm_perm
-
ipc_perm
structure that specifies the access permissions on the shared memory segment. shm_segsz
-
Size in bytes of the shared memory segment.
shm_cpid
-
ID of the process that created the shared memory segment.
shm_lpid
-
ID of the last process that executed a shmat(2) or shmdt(2) system call.
shm_nattch
-
Number of current alive attaches for this shared memory segment.
shm_atime
-
Time of the last shmat(2) system call.
shm_dtime
-
Time of the last shmdt(2) system call.
shm_ctime
-
Time of the last shmctl(2) system call that changed
shmid_ds
.
IPC namespaces
For a discussion of the interaction of System V IPC objects and IPC namespaces, see ipc_namespaces(7).
SEE ALSO
ipcmk(1), ipcrm(1), ipcs(1), lsipc(1), ipc(2), msgctl(2), msgget(2), msgrcv(2), msgsnd(2), semctl(2), semget(2), semop(2), shmat(2), shmctl(2), shmdt(2), shmget(2), ftok(3), ipc_namespaces(7)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END FIXME . There is now duplication of some of the information below in semctl.2, msgctl.2, and shmctl.2 -- MTK, Nov 04 FIXME . Ultimately, there should probably be svmq_overview(7), svshm_overview(7), and svsem_overview(7) that provide an overview of each System V IPC mechanism. In that case: * Those files should add a discussion of the /proc/sysvipc interfaces. * Documentation of the various /proc interfaces should move into those files (from proc(5)), and references in the various *.2 pages that refer to the /proc files should be adjusted. * The only part that uniquely belongs in sysvipc(7) is perhaps the discussion of ipc_perm. |