Linux manual pages Section 7  

Name

svipc — 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'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'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 to sem_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 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), namespaces(7)

COLOPHON

This page is part of release 4.07 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)

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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 sem_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.