1.\" 2.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com> 3.\" 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 16.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 17.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 18.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 19.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 20.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 21.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 22.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 23.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 24.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 25.\" 26.\" $FreeBSD: src/lib/libc/sys/shmget.2,v 1.8.2.6 2001/12/14 18:34:01 ru Exp $ 27.\" 28.Dd January 4, 2014 29.Dt SHMGET 2 30.Os 31.Sh NAME 32.Nm shmget 33.Nd obtain a shared memory identifier 34.Sh LIBRARY 35.Lb libc 36.Sh SYNOPSIS 37.In machine/param.h 38.In sys/types.h 39.In sys/ipc.h 40.In sys/shm.h 41.Ft int 42.Fn shmget "key_t key" "size_t size" "int flag" 43.Sh DESCRIPTION 44Based on the values of 45.Fa key 46and 47.Fa flag , 48.Fn shmget 49returns the identifier of a newly created or previously existing shared 50memory segment. 51.\" 52.\" The following bit about keys and modes also applies to semaphores 53.\" and message queues. 54.\" 55The key 56is analogous to a filename: it provides a handle that names an 57IPC object. There are three ways to specify a key: 58.Bl -bullet 59.It 60IPC_PRIVATE may be specified, in which case a new IPC object 61will be created. 62.It 63An integer constant may be specified. If no IPC object corresponding 64to 65.Fa key 66is specified and the IPC_CREAT bit is set in 67.Fa flag , 68a new one will be created. 69.It 70.Fn ftok 71may be used to generate a key from a pathname. See 72.Xr ftok 3 . 73.El 74.Pp 75The mode of a newly created IPC object is determined by 76.Em OR Ns 'ing 77the following constants into the 78.Fa flag 79parameter: 80.Bl -tag -width XSHM_WXX6XXX 81.It Dv SHM_R 82Read access for user. 83.It Dv SHM_W 84Write access for user. 85.It Dv ( SHM_R>>3 ) 86Read access for group. 87.It Dv ( SHM_W>>3 ) 88Write access for group. 89.It Dv ( SHM_R>>6 ) 90Read access for other. 91.It Dv ( SHM_W>>6 ) 92Write access for other. 93.El 94.\" 95.\" XXX - we should also mention how uid, euid, and gid affect ownership 96.\" and use 97.\" 98.\" end section about keys and modes 99.\" 100.Pp 101When creating a new shared memory segment, 102.Fa size 103indicates the desired size of the new segment in bytes. The size 104of the segment may be rounded up to a multiple convenient to the 105kernel (i.e., the page size). 106.Sh RETURN VALUES 107Upon successful completion, 108.Fn shmget 109returns the positive integer identifier of a shared memory segment. 110Otherwise, -1 is returned and 111.Va errno 112set to indicate the error. 113.Sh ENVIRONMENT 114The XSI Interprocess Communication family of functions is also available 115as an implementation in userspace. 116To use it, the 117.Xr sysvipcd 8 118daemon has to be running. 119.Pp 120If the 121.Ev USR_SYSVIPC 122variable is set in a process' environment, the process and its children 123will use the userspace implementation. 124.Sh ERRORS 125.Fn Shmget 126will fail if: 127.Bl -tag -width Er 128.\" 129.\" XXX What about ipcperm failing? 130.\" 131.It Bq Er EINVAL 132Size specified is greater than the size of the previously existing segment. 133Size specified is less than the system imposed minimum, or greater than 134the system imposed maximum. 135.It Bq Er ENOENT 136No shared memory segment was found matching 137.Fa key , 138and IPC_CREAT was not specified. 139.It Bq Er ENOSPC 140The kernel was unable to allocate enough memory to 141satisfy the request. 142.It Bq Er EEXIST 143IPC_CREAT and IPC_EXCL were specified, and a shared memory segment 144corresponding to 145.Fa key 146already exists. 147.El 148.Sh "SEE ALSO" 149.Xr shmat 2 , 150.Xr shmctl 2 , 151.Xr shmdt 2 , 152.Xr ftok 3 153.Sh AUTHORS 154.An -nosplit 155The 156.Dx 157specific userspace implementation (see 158.Sx ENVIRONMENT ) 159was written by 160.An Larisa Grigore . 161