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.\" $DragonFly: src/lib/libc/sys/shmget.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 28.\" 29.Dd July 3, 1995 30.Dt SHMGET 2 31.Os 32.Sh NAME 33.Nm shmget 34.Nd obtain a shared memory identifier 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In machine/param.h 39.In sys/types.h 40.In sys/ipc.h 41.In sys/shm.h 42.Ft int 43.Fn shmget "key_t key" "int size" "int flag" 44.Sh DESCRIPTION 45Based on the values of 46.Fa key 47and 48.Fa flag , 49.Fn shmget 50returns the identifier of a newly created or previously existing shared 51memory segment. 52.\" 53.\" The following bit about keys and modes also applies to semaphores 54.\" and message queues. 55.\" 56The key 57is analogous to a filename: it provides a handle that names an 58IPC object. There are three ways to specify a key: 59.Bl -bullet 60.It 61IPC_PRIVATE may be specified, in which case a new IPC object 62will be created. 63.It 64An integer constant may be specified. If no IPC object corresponding 65to 66.Fa key 67is specified and the IPC_CREAT bit is set in 68.Fa flag , 69a new one will be created. 70.It 71.Fn ftok 72may be used to generate a key from a pathname. See 73.Xr ftok 3 . 74.El 75.Pp 76The mode of a newly created IPC object is determined by 77.Em OR Ns 'ing 78the following constants into the 79.Fa flag 80parameter: 81.Bl -tag -width XSHM_WXX6XXX 82.It Dv SHM_R 83Read access for user. 84.It Dv SHM_W 85Write access for user. 86.It Dv ( SHM_R>>3 ) 87Read access for group. 88.It Dv ( SHM_W>>3 ) 89Write access for group. 90.It Dv ( SHM_R>>6 ) 91Read access for other. 92.It Dv ( SHM_W>>6 ) 93Write access for other. 94.El 95.\" 96.\" XXX - we should also mention how uid, euid, and gid affect ownership 97.\" and use 98.\" 99.\" end section about keys and modes 100.\" 101.Pp 102When creating a new shared memory segment, 103.Fa size 104indicates the desired size of the new segment in bytes. The size 105of the segment may be rounded up to a multiple convenient to the 106kernel (i.e., the page size). 107.Sh RETURN VALUES 108Upon successful completion, 109.Fn shmget 110returns the positive integer identifier of a shared memory segment. 111Otherwise, -1 is returned and 112.Va errno 113set to indicate the error. 114.Sh ERRORS 115.Fn Shmget 116will fail if: 117.Bl -tag -width Er 118.\" 119.\" XXX What about ipcperm failing? 120.\" 121.It Bq Er EINVAL 122Size specified is greater than the size of the previously existing segment. 123Size specified is less than the system imposed minimum, or greater than 124the system imposed maximum. 125.It Bq Er ENOENT 126No shared memory segment was found matching 127.Fa key , 128and IPC_CREAT was not specified. 129.It Bq Er ENOSPC 130The kernel was unable to allocate enough memory to 131satisfy the request. 132.It Bq Er EEXIST 133IPC_CREAT and IPC_EXCL were specified, and a shared memory segment 134corresponding to 135.Fa key 136already exists. 137.El 138.Sh "SEE ALSO" 139.Xr shmat 2 , 140.Xr shmctl 2 , 141.Xr shmdt 2 , 142.Xr ftok 3 143