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.Dd March 4, 2018 27.Dt SEMGET 2 28.Os 29.Sh NAME 30.Nm semget 31.Nd obtain a semaphore id 32.Sh LIBRARY 33.Lb libc 34.Sh SYNOPSIS 35.In sys/sem.h 36.Ft int 37.Fn semget "key_t key" "int nsems" "int flag" 38.Sh DESCRIPTION 39Based on the values of 40.Fa key 41and 42.Fa flag , 43.Fn semget 44returns the identifier of a newly created or previously existing 45set of semaphores. 46.\" 47.\" This is copied verbatim from the shmget manpage. Perhaps 48.\" it should go in a common manpage, such as .Xr ipc 2 49.\" 50The key 51is analogous to a filename: it provides a handle that names an 52IPC object. 53There are three ways to specify a key: 54.Bl -bullet 55.It 56IPC_PRIVATE may be specified, in which case a new IPC object 57will be created. 58.It 59An integer constant may be specified. 60If no IPC object corresponding 61to 62.Fa key 63is specified and the IPC_CREAT bit is set in 64.Fa flag , 65a new one will be created. 66.It 67The 68.Xr ftok 3 69function 70may be used to generate a key from a pathname. 71.El 72.\" 73.\" Likewise for this section, except SHM_* becomes SEM_*. 74.\" 75.Pp 76The mode of a newly created IPC object is determined by ORing these constants 77into the 78.Fa flag 79argument: 80.Bl -tag -width 0000 81.It Dv 0400 82Read access for user. 83.It Dv 0200 84Alter access for user. 85.It Dv 0040 86Read access for group. 87.It Dv 0020 88Alter access for group. 89.It Dv 0004 90Read access for other. 91.It Dv 0002 92Alter access for other. 93.El 94.Pp 95If a new set of semaphores is being created, 96.Fa nsems 97is used to indicate the number of semaphores the set should contain. 98Otherwise, 99.Fa nsems 100may be specified as 0. 101.Sh RETURN VALUES 102The 103.Fn semget 104system call 105returns the id of a semaphore set if successful; otherwise, -1 106is returned and 107.Va errno 108is set to indicate the error. 109.Sh ERRORS 110The 111.Fn semget 112system call 113will fail if: 114.Bl -tag -width Er 115.\" ipcperm could fail (we are opening to read and write, as it were) 116.It Bq Er EACCES 117Access permission failure. 118.\" 119.\" sysv_sem.c is quite explicit about these, so I'm pretty sure 120.\" this is accurate 121.\" 122.It Bq Er EEXIST 123IPC_CREAT and IPC_EXCL were specified, and a semaphore set 124corresponding to 125.Fa key 126already exists. 127.It Bq Er EINVAL 128The number of semaphores requested exceeds the system imposed maximum 129per set. 130.It Bq Er EINVAL 131A semaphore set corresponding to 132.Fa key 133already exists and contains fewer semaphores than 134.Fa nsems . 135.It Bq Er EINVAL 136A semaphore set corresponding to 137.Fa key 138does not exist and 139.Fa nsems 140is 0 or negative. 141.It Bq Er ENOSPC 142Insufficiently many semaphores are available. 143.It Bq Er ENOSPC 144The kernel could not allocate a 145.Fa "struct semid_ds" . 146.It Bq Er ENOENT 147No semaphore set was found corresponding to 148.Fa key , 149and IPC_CREAT was not specified. 150.El 151.Sh SEE ALSO 152.Xr semctl 2 , 153.Xr semop 2 , 154.Xr ftok 3 155