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