.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\"
.\" 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 svipc(7) is perhaps
.\"     the discussion of ipc_perm.
.\"
.TH SVIPC 7 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
svipc \- System V interprocess communication mechanisms
.SH SYNOPSIS
.nf
.B #include <sys/msg.h>
.B #include <sys/sem.h>
.B #include <sys/shm.h>
.fi
.SH 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
.I resource
means an instantiation of one among such mechanisms.
.SS Resource access permissions
For each resource, the system uses a common structure of type
.I "struct ipc_perm"
to store information needed in determining permissions to perform an
IPC operation.
The
.I ipc_perm
structure includes the following members:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
The
.I mode
member of the
.I 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:
.PP
.nf
    0400    Read by user.
    0200    Write by user.
    0040    Read by group.
    0020    Write by group.
    0004    Read by others.
    0002    Write by others.
.fi
.PP
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
Furthermore,
"write"
effectively means
"alter"
for a semaphore set.
.PP
The same system header file also defines the following symbolic
constants:
.TP 14
.B IPC_CREAT
Create entry if key doesn't exist.
.TP
.B IPC_EXCL
Fail if key exists.
.TP
.B IPC_NOWAIT
Error if request must wait.
.TP
.B IPC_PRIVATE
Private key.
.TP
.B IPC_RMID
Remove resource.
.TP
.B IPC_SET
Set resource options.
.TP
.B IPC_STAT
Get resource options.
.PP
Note that
.B IPC_PRIVATE
is a
.I key_t
type, while all the other symbolic constants are flag fields and can
be OR'ed into an
.I int
type variable.
.SS Message queues
A message queue is uniquely identified by a positive integer
.RI "(its " msqid )
and has an associated data structure of type
.IR "struct msqid_ds" ,
defined in
.IR <sys/msg.h> ,
containing the following members:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.TP 11
.I msg_perm
.I ipc_perm
structure that specifies the access permissions on the message
queue.
.TP
.I msg_qnum
Number of messages currently on the message queue.
.TP
.I msg_qbytes
Maximum number of bytes of message text allowed on the message
queue.
.TP
.I msg_lspid
ID of the process that performed the last
.BR msgsnd (2)
system call.
.TP
.I msg_lrpid
ID of the process that performed the last
.BR msgrcv (2)
system call.
.TP
.I msg_stime
Time of the last
.BR msgsnd (2)
system call.
.TP
.I msg_rtime
Time of the last
.BR msgrcv (2)
system call.
.TP
.I msg_ctime
Time of the last
system call that changed a member of the
.I msqid_ds
structure.
.SS Semaphore sets
A semaphore set is uniquely identified by a positive integer
.RI "(its " semid )
and has an associated data structure of type
.IR "struct semid_ds" ,
defined in
.IR <sys/sem.h> ,
containing the following members:
.IP
.in +4n
.EX
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 */
};
.EE
.in
.TP 11
.I sem_perm
.I ipc_perm
structure that specifies the access permissions on the semaphore
set.
.TP
.I sem_otime
Time of last
.BR semop (2)
system call.
.TP
.I sem_ctime
Time of last
.BR semctl (2)
system call that changed a member of the above structure or of one
semaphore belonging to the set.
.TP
.I sem_nsems
Number of semaphores in the set.
Each semaphore of the set is referenced by a nonnegative integer
ranging from
.B 0
to
.IR sem_nsems\-1 .
.PP
A semaphore is a data structure of type
.I "struct sem"
containing the following members:
.PP
.in +4n
.EX
struct sem {
    int semval;  /* semaphore value */
    int sempid;  /* PID of process that last modified */
.\"    unsigned short semncnt; /* nr awaiting semval to increase */
.\"    unsigned short semzcnt; /* nr awaiting semval = 0 */
};
.EE
.in
.TP 11
.I semval
Semaphore value: a nonnegative integer.
.TP
.I sempid
PID of the last process that modified the value of
this semaphore.
.\".TP
.\".I semncnt
.\"Number of processes suspended awaiting for
.\".I semval
.\"to increase.
.\".TP
.\".I semznt
.\"Number of processes suspended awaiting for
.\".I semval
.\"to become zero.
.SS Shared memory segments
A shared memory segment is uniquely identified by a positive integer
.RI "(its " shmid )
and has an associated data structure of type
.IR "struct shmid_ds" ,
defined in
.IR <sys/shm.h> ,
containing the following members:
.PP
.in +4n
.EX
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 */
};
.EE
.in
.TP 11
.I shm_perm
.I ipc_perm
structure that specifies the access permissions on the shared memory
segment.
.TP
.I shm_segsz
Size in bytes of the shared memory segment.
.TP
.I shm_cpid
ID of the process that created the shared memory segment.
.TP
.I shm_lpid
ID of the last process that executed a
.BR shmat (2)
or
.BR shmdt (2)
system call.
.TP
.I shm_nattch
Number of current alive attaches for this shared memory segment.
.TP
.I shm_atime
Time of the last
.BR shmat (2)
system call.
.TP
.I shm_dtime
Time of the last
.BR shmdt (2)
system call.
.TP
.I shm_ctime
Time of the last
.BR shmctl (2)
system call that changed
.IR shmid_ds .
.SS IPC namespaces
For a discussion of the interaction of System V IPC objects and
IPC namespaces, see
.BR namespaces (7).
.SH SEE ALSO
.BR ipcmk (1),
.BR ipcrm (1),
.BR ipcs (1),
.BR lsipc (1),
.BR ipc (2),
.BR msgctl (2),
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR semctl (2),
.BR semget (2),
.BR semop (2),
.BR shmat (2),
.BR shmctl (2),
.BR shmdt (2),
.BR shmget (2),
.BR ftok (3),
.BR namespaces (7)
.SH COLOPHON
This page is part of release 4.16 of the Linux
.I 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/.