1.\" 2.\" Copyright (c) 1995 David Hovemeyer <[email protected]> 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.\" $FreeBSD$ 27.\" 28.Dd March 4, 2018 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 sys/shm.h 38.Ft int 39.Fn shmget "key_t key" "size_t size" "int flag" 40.Sh DESCRIPTION 41Based on the values of 42.Fa key 43and 44.Fa flag , 45.Fn shmget 46returns the identifier of a newly created or previously existing shared 47memory segment. 48.\" 49.\" The following bit about keys and modes also applies to semaphores 50.\" and message queues. 51.\" 52The key 53is analogous to a filename: it provides a handle that names an 54IPC object. 55There are three ways to specify a key: 56.Bl -bullet 57.It 58IPC_PRIVATE may be specified, in which case a new IPC object 59will be created. 60.It 61An integer constant may be specified. 62If no IPC object corresponding 63to 64.Fa key 65is specified and the IPC_CREAT bit is set in 66.Fa flag , 67a new one will be created. 68.It 69The 70.Xr ftok 3 71may be used to generate a key from a pathname. 72.El 73.Pp 74The mode of a newly created IPC object is determined by 75which are set by ORing these constants into the 76.Fa flag 77argument: 78.Bl -tag -width 0000 79.It Dv 0400 80Read access for owner. 81.It Dv 0200 82Write access for owner. 83.It Dv 0040 84Read access for group. 85.It Dv 0020 86Write access for group. 87.It Dv 0004 88Read access for other. 89.It Dv 0002 90Write access for other. 91.El 92.\" 93.\" XXX - we should also mention how uid, euid, and gid affect ownership 94.\" and use 95.\" 96.\" end section about keys and modes 97.\" 98.Pp 99When creating a new shared memory segment, 100.Fa size 101indicates the desired size of the new segment in bytes. 102The size 103of the segment may be rounded up to a multiple convenient to the 104kernel (i.e., the page size). 105.Sh RETURN VALUES 106Upon successful completion, 107.Fn shmget 108returns the positive integer identifier of a shared memory segment. 109Otherwise, -1 is returned and 110.Va errno 111set to indicate the error. 112.Sh ERRORS 113The 114.Fn shmget 115system call 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