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diff --git a/man2/msgget.2 b/man2/msgget.2 new file mode 100644 index 0000000..0774f49 --- /dev/null +++ b/man2/msgget.2 @@ -0,0 +1,217 @@ +.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" Added correction due to Nick Duffek <nsd@bbc.com>, aeb, 960426 +.\" Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond <esr@thyrsus.com> +.\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com> +.\" Removed EIDRM from errors - that can't happen... +.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> +.\" Added notes on capability requirements +.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com> +.\" Language and formatting clean-ups +.\" Added notes on /proc files +.\" +.TH msgget 2 2023-03-30 "Linux man-pages 6.05.01" +.SH NAME +msgget \- get a System V message queue identifier +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <sys/msg.h> +.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 \[bu] 3 +.I msg_perm.cuid +and +.I msg_perm.uid +are set to the effective user ID of the calling process. +.IP \[bu] +.I msg_perm.cgid +and +.I msg_perm.gid +are set to the effective group ID of the calling process. +.IP \[bu] +The least significant 9 bits of +.I msg_perm.mode +are set to the least significant 9 bits of +.IR msgflg . +.IP \[bu] +.IR msg_qnum , +.IR msg_lspid , +.IR msg_lrpid , +.IR msg_stime , +and +.I msg_rtime +are set to 0. +.IP \[bu] +.I msg_ctime +is set to the current time. +.IP \[bu] +.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 +On success, +.BR msgget () +returns the message queue identifier (a nonnegative integer). +On failure, \-1 is returned, and +.I errno +is set to indicate the error. +.SH ERRORS +.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 +.B 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 STANDARDS +POSIX.1-2008. +.SH HISTORY +POSIX.1-2001, SVr4. +.SS Linux +Until Linux 2.3.20, Linux would return +.B EIDRM +for a +.BR msgget () +on a message queue scheduled for deletion. +.SH NOTES +.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 . +.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 sysvipc (7) |