bsd/man/man2/sem_open.2

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  23 .Dd June 8, 2000
  24 .Dt SEM_OPEN 2
  25 .Os Darwin
  26 .Sh NAME
  27 .Nm sem_open
  28 .Nd initialize and open a named semaphore
  29 .Sh SYNOPSIS
  30 .Fd #include <semaphore.h>
  31 .Ft sem_t *
  32 .Fo sem_open
  33 .Fa "const char *name"
  34 .Fa "int oflag"
  35 .Fa "..."
  36 .Fc
  37 .Pp
  38 The parameters "mode_t mode" and "unsigned int value"
  39 are optional.
  40 .Sh DESCRIPTION
  41 The named semaphore named
  42 .Fa name
  43 is initialized and opened as specified by the argument
  44 .Fa oflag
  45 and a semaphore descriptor is returned to the calling process.
  46 .Pp
  47 The value of
  48 .Fa oflag
  49 is formed by
  50 .Em or Ns 'ing
  51 the following values:
  52 .Pp
  53 .Bd -literal -offset indent -compact
  54 O_CREAT         create the semaphore if it does not exist
  55 O_EXCL          error if create and semaphore exists
  56 .Ed
  57 .Pp
  58 If
  59 .Dv O_CREAT
  60 is specified,
  61 .Fn sem_open
  62 requires an additional two arguments.
  63 .Fa mode
  64 specifies the permissions for the semaphore as described in
  65 .Xr chmod 2
  66 and modified by the process' umask value (see
  67 .Xr umask 2 ) .
  68 The semaphore is created with an initial
  69 .Fa value ,
  70 which must be less than or equal to
  71 .Dv SEM_VALUE_MAX .
  72 .Pp
  73 If
  74 .Dv O_EXCL
  75 is specified and the semaphore exists,
  76 .Fn sem_open
  77 fails.  The check for the existence of the semaphore and the creation
  78 of the semaphore are atomic with respect to all processes calling
  79 .Fn sem_open
  80 with
  81 .Dv O_CREAT
  82 and
  83 .Dv O_EXCL
  84 set.
  85 .Pp
  86 When a new semaphore is created, it is given the user ID and group ID
  87 which correspond to the effective user and group IDs of the calling
  88 process. There is no visible entry in the file system for the created
  89 object in this implementation.
  90 .Pp
  91 The returned semaphore descriptor is available to the calling process
  92 until it is closed with
  93 .Fn sem_close ,
  94 or until the caller exits or execs.
  95 .Pp
  96 If a process makes repeated calls to
  97 .Fn sem_open ,
  98 with the same
  99 .Fa name
 100 argument, the same descriptor is returned for each successful call,
 101 unless
 102 .Fn sem_unlink
 103 has been called on the semaphore in the interim.
 104 .Pp
 105 If
 106 .Fn sem_open
 107 fails for any reason, it will return a value of
 108 .Dv SEM_FAILED
 109 and sets
 110 .Va errno .
 111 On success, it returns a semaphore descriptor.
 112 .Sh ERRORS
 113 The named semaphore is opened unless:
 114 .Bl -tag -width Er
 115 .It Bq Er EACCES
 116 The required permissions (for reading and/or writing)
 117 are denied for the given flags; or
 118 .Dv O_CREAT
 119 is specified, the object does not exist, and permission to
 120 create the semaphore is denied.
 121 .It Bq Er EEXIST
 122 .Dv O_CREAT
 123 and
 124 .Dv O_EXCL
 125 were specified and the semaphore exists.
 126 .It Bq Er EINTR
 127 The
 128 .Fn sem_open
 129 operation was interrupted by a signal.
 130 .It Bq Er EINVAL
 131 The
 132 .Fn shm_open
 133 operation is not supported; or
 134 .Dv O_CREAT
 135 is specified and
 136 .Fa value
 137 exceeds
 138 .Dv SEM_VALUE_MAX .
 139 .It Bq Er EMFILE
 140 The process has already reached its limit for semaphores or file
 141 descriptors in use.
 142 .It Bq Er ENAMETOOLONG
 143 .Fa name
 144 exceeded
 145 .Dv PSEMNAMLEN
 146 characters.
 147 .It Bq Er ENFILE
 148 Too many semaphores or file descriptors are open on the system.
 149 .It Bq Er ENOENT
 150 .Dv O_CREAT
 151 is not set and the named semaphore does not exist.
 152 .It Bq Er ENOSPC
 153 .Dv O_CREAT
 154 is specified, the file does not exist, and there is insufficient
 155 space available to create the semaphore.
 156 .El
 157 .Sh SEE ALSO
 158 .Xr sem_close 2 ,
 159 .Xr sem_post 2 ,
 160 .Xr sem_trywait 2 ,
 161 .Xr sem_unlink 2 ,
 162 .Xr sem_wait 2 ,
 163 .Xr semctl 2 ,
 164 .Xr semget 2 ,
 165 .Xr semop 2 ,
 166 .Xr umask 2
 167 .Sh HISTORY
 168 .Fn sem_open
 169 is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).