bsd/man/man2/semget.2

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   2 .\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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  26 .\" $FreeBSD: src/lib/libc/sys/semget.2,v 1.14 2002/12/19 09:40:25 ru Exp $
  27 .\"
  28 .Dd September 12, 1995
  29 .Dt SEMGET 2
  30 .Os
  31 .Sh NAME
  32 .Nm semget
  33 .Nd obtain a semaphore id
  34 .Sh LIBRARY
  35 .Lb libc
  36 .Sh SYNOPSIS
  37 .In sys/types.h
  38 .In sys/ipc.h
  39 .In sys/sem.h
  40 .Ft int
  41 .Fn semget "key_t key" "int nsems" "int flag"
  42 .Sh DESCRIPTION
  43 Based on the values of
  44 .Fa key
  45 and
  46 .Fa flag ,
  47 .Fn semget
  48 returns the identifier of a newly created or previously existing
  49 set of semaphores.
  50 .\"
  51 .\" This is copied verbatim from the shmget manpage.  Perhaps
  52 .\" it should go in a common manpage, such as .Xr ipc 2
  53 .\"
  54 The key
  55 is analogous to a filename: it provides a handle that names an
  56 IPC object.  There are three ways to specify a key:
  57 .Bl -bullet
  58 .It
  59 IPC_PRIVATE may be specified, in which case a new IPC object
  60 will be created.
  61 .It
  62 An integer constant may be specified.  If no IPC object corresponding
  63 to
  64 .Fa key
  65 is specified and the IPC_CREAT bit is set in
  66 .Fa flag ,
  67 a new one will be created.
  68 .It
  69 The
  70 .Xr ftok 3
  71 function
  72 may be used to generate a key from a pathname.
  73 .El
  74 .\"
  75 .\" Likewise for this section, except SHM_* becomes SEM_*.
  76 .\"
  77 .Pp
  78 The mode of a newly created IPC object is determined by
  79 .Em OR Ns 'ing
  80 the following constants into the
  81 .Fa flag
  82 argument:
  83 .Bl -tag -width XSEM_WXX6XXX
  84 .It Dv SEM_R
  85 Read access for user.
  86 .It Dv SEM_A
  87 Alter access for user.
  88 .It Dv ( SEM_R>>3 )
  89 Read access for group.
  90 .It Dv ( SEM_A>>3 )
  91 Alter access for group.
  92 .It Dv ( SEM_R>>6 )
  93 Read access for other.
  94 .It Dv ( SEM_A>>6 )
  95 Alter access for other.
  96 .El
  97 .Pp
  98 If a new set of semaphores is being created,
  99 .Fa nsems
 100 is used to indicate the number of semaphores the set should contain.
 101 Otherwise,
 102 .Fa nsems
 103 may be specified as 0.
 104 .Sh RETURN VALUES
 105 The
 106 .Fn semget
 107 system call
 108 returns the id of a semaphore set if successful; otherwise, -1
 109 is returned and
 110 .Va errno
 111 is set to indicate the error.
 112 .Sh ERRORS
 113 The
 114 .Fn semget
 115 system call
 116 will fail if:
 117 .Bl -tag -width Er
 118 .\" ipcperm could fail (we're opening to read and write, as it were)
 119 .It Bq Er EACCES
 120 Access permission failure.
 121 .\"
 122 .\" sysv_sem.c is quite explicit about these, so I'm pretty sure
 123 .\" this is accurate
 124 .\"
 125 .It Bq Er EEXIST
 126 IPC_CREAT and IPC_EXCL were specified, and a semaphore set
 127 corresponding to
 128 .Fa key
 129 already exists.
 130 .It Bq Er EINVAL
 131 The number of semaphores requested exceeds the system imposed maximum
 132 per set.
 133 .It Bq Er ENOSPC
 134 Insufficiently many semaphores are available.
 135 .It Bq Er ENOSPC
 136 The kernel could not allocate a
 137 .Fa "struct semid_ds" .
 138 .It Bq Er ENOENT
 139 No semaphore set was found corresponding to
 140 .Fa key ,
 141 and IPC_CREAT was not specified.
 142 .El
 143 .Sh SEE ALSO
 144 .Xr semctl 2 ,
 145 .Xr semop 2 ,
 146 .Xr ftok 3