.\" Copyright 1993 Giorgio Ciucci .\" .\" %%%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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 .\" .\" Added correction due to Nick Duffek , aeb, 960426 .\" Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond .\" Modified, 8 Jan 2003, Michael Kerrisk, .\" Removed EIDRM from errors - that can't happen... .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified, 11 Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Added notes on /proc files .\" .TH MSGGET 2 2018-04-30 "Linux" "Linux Programmer's Manual" .SH NAME msgget \- get a System V message queue identifier .SH SYNOPSIS .nf .B #include .B #include .B #include .PP .BI "int msgget(key_t " key ", int " msgflg ); .fi .SH DESCRIPTION The .BR msgget () system call returns the System\ V message queue identifier associated with the value of the .I key argument. It may be used either to obtain the identifier of a previously created message queue (when .I msgflg is zero and .I key does not have the value .BR IPC_PRIVATE ), or to create a new set. .PP A new message queue is created if .I key has the value .B IPC_PRIVATE or .I key isn't .BR IPC_PRIVATE , no message queue with the given key .I key exists, and .B IPC_CREAT is specified in .IR msgflg . .PP If .I msgflg specifies both .B IPC_CREAT and .B IPC_EXCL and a message queue already exists for .IR key , then .BR msgget () fails with .I errno set to .BR EEXIST . (This is analogous to the effect of the combination .B O_CREAT | O_EXCL for .BR open (2).) .PP Upon creation, the least significant bits of the argument .I msgflg define the permissions of the message queue. These permission bits have the same format and semantics as the permissions specified for the .I mode argument of .BR open (2). (The execute permissions are not used.) .PP If a new message queue is created, then its associated data structure .I msqid_ds (see .BR msgctl (2)) is initialized as follows: .IP .I msg_perm.cuid and .I msg_perm.uid are set to the effective user ID of the calling process. .IP .I msg_perm.cgid and .I msg_perm.gid are set to the effective group ID of the calling process. .IP The least significant 9 bits of .I msg_perm.mode are set to the least significant 9 bits of .IR msgflg . .IP .IR msg_qnum , .IR msg_lspid , .IR msg_lrpid , .IR msg_stime , and .I msg_rtime are set to 0. .IP .I msg_ctime is set to the current time. .IP .I msg_qbytes is set to the system limit .BR MSGMNB . .PP If the message queue already exists the permissions are verified, and a check is made to see if it is marked for destruction. .SH RETURN VALUE If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise \-1 with .I errno indicating the error. .SH ERRORS On failure, .I errno is set to one of the following values: .TP .B EACCES A message queue exists for .IR key , but the calling process does not have permission to access the queue, and does not have the .B CAP_IPC_OWNER capability in the user namespace that governs its IPC namespace. .TP .B EEXIST .B IPC_CREAT and .BR IPC_EXCL were specified in .IR msgflg , but a message queue already exists for .IR key . .TP .B ENOENT No message queue exists for .I key and .I msgflg did not specify .BR IPC_CREAT . .TP .B ENOMEM A message queue has to be created but the system does not have enough memory for the new data structure. .TP .B ENOSPC A message queue has to be created but the system limit for the maximum number of message queues .RB ( MSGMNI ) would be exceeded. .SH CONFORMING TO POSIX.1-2001, POSIX.1-2008, SVr4. .SH NOTES The inclusion of .I and .I isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. .\" Like Linux, the FreeBSD man pages still document .\" the inclusion of these header files. .PP .B IPC_PRIVATE isn't a flag field but a .I key_t type. If this special value is used for .IR key , the system call ignores everything but the least significant 9 bits of .I msgflg and creates a new message queue (on success). .PP The following is a system limit on message queue resources affecting a .BR msgget () call: .TP .B MSGMNI System-wide limit on the number of message queues. Before Linux 3.19, .\" commit 0050ee059f7fc86b1df2527aaa14ed5dc72f9973 the default value for this limit was calculated using a formula based on available system memory. Since Linux 3.19, the default value is 32,000. On Linux, this limit can be read and modified via .IR /proc/sys/kernel/msgmni . .SS Linux notes Until version 2.3.20, Linux would return .B EIDRM for a .BR msgget () on a message queue scheduled for deletion. .SH BUGS The name choice .B IPC_PRIVATE was perhaps unfortunate, .B IPC_NEW would more clearly show its function. .SH SEE ALSO .BR msgctl (2), .BR msgrcv (2), .BR msgsnd (2), .BR ftok (3), .BR capabilities (7), .BR mq_overview (7), .BR svipc (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/.