Name

msgget — get a message queue identifier

Synopsis

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgget( key_t   key,
  int   msgflg);

DESCRIPTION

The msgget() system call returns the message queue identifier associated with the value of the key argument. A new message queue is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message queue with the given key key exists, and IPC_CREAT is specified in msgflg.

If msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno set to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

Upon creation, the least significant bits of the argument msgflg define the permissions of the message queue. These permission bits have the same format and semantics as the permissions specified for the mode argument of open(2). (The execute permissions are not used.)

If a new message queue is created, then its associated data structure msqid_ds (see msgctl(2)) is initialized as follows:

  • msg_perm.cuid and msg_perm.uid are set to the effective user ID of the calling process.

  • msg_perm.cgid and msg_perm.gid are set to the effective group ID of the calling process.

  • The least significant 9 bits of msg_perm.mode are set to the least significant 9 bits of msgflg.

  • msg_qnum, msg_lspid, msg_lrpid, msg_stime and msg_rtime are set to 0.

  • msg_ctime is set to the current time.

  • msg_qbytes is set to the system limit MSGMNB.

If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction.

RETURN VALUE

If successful, the return value will be the message queue identifier (a non-negative integer), otherwise −1 with errno indicating the error.

ERRORS

On failure, errno is set to one of the following values:

EACCES

A message queue exists for key, but the calling process does not have permission to access the queue, and does not have the CAP_IPC_OWNER capability.

EEXIST

A message queue exists for key and msgflg specified both IPC_CREAT and IPC_EXCL.

ENOENT

No message queue exists for key and msgflg did not specify IPC_CREAT.

ENOMEM

A message queue has to be created but the system does not have enough memory for the new data structure.

ENOSPC

A message queue has to be created but the system limit for the maximum number of message queues (MSGMNI) would be exceeded.

CONFORMING TO

SVr4, POSIX.1-2001.

NOTES

IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the least significant 9 bits of msgflg and creates a new message queue (on success).

The following is a system limit on message queue resources affecting a msgget() call:

MSGMNI

System wide maximum number of message queues: policy dependent (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmni).

Linux Notes

Until version 2.3.20 Linux would return EIDRM for a msgget() on a message queue scheduled for deletion.

BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function.

SEE ALSO

msgctl(2), msgrcv(2), msgsnd(2), ftok(3), capabilities(7), mq_overview(7), svipc(7)

COLOPHON

This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


  Copyright 1993 Giorgio Ciucci <giorgiocrcc.it>

Permission is granted to make and distribute verbatim copies of this
manual provflags 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.

Added correction due to Nick Duffek <nsdbbc.com>, aeb, 960426
Modified Wed Nov  6 04:00:31 1996 by Eric S. Raymond <esrthyrsus.com>
Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpagesgmail.com>
Removed EIDRM from errors - that can't happen...
Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com>
    Added notes on capability requirements
Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com>
Language and formatting clean-ups
Added notes on /proc files