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.Dd June 8, 2000
.Dt SEM_OPEN 2
.Os Darwin
.Sh NAME
.Nm sem_open
.Nd initialize and open a named semaphore
.Sh SYNOPSIS
.Fd #include <semaphore.h>
.Ft sem_t *
.Fo sem_open
.Fa "const char *name"
.Fa "int oflag"
.Fa "..."
.Fc
.Pp
The parameters "mode_t mode" and "unsigned int value"
are optional.
.Sh DESCRIPTION
The named semaphore named
.Fa name
is initialized and opened as specified by the argument
.Fa oflag
and a semaphore descriptor is returned to the calling process.
.Pp
The value of
.Fa oflag
is formed by
.Em or Ns 'ing
the following values:
.Pp
.Bd -literal -offset indent -compact
O_CREAT		create the semaphore if it does not exist
O_EXCL		error if create and semaphore exists
.Ed
.Pp
If
.Dv O_CREAT
is specified,
.Fn sem_open
requires an additional two arguments.
.Fa mode
specifies the permissions for the semaphore as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
The semaphore is created with an initial
.Fa value ,
which must be less than or equal to
.Dv SEM_VALUE_MAX .
.Pp
If
.Dv O_EXCL
is specified and the semaphore exists,
.Fn sem_open
fails.  The check for the existence of the semaphore and the creation
of the semaphore are atomic with respect to all processes calling
.Fn sem_open
with
.Dv O_CREAT
and
.Dv O_EXCL
set.
.Pp
When a new semaphore is created, it is given the user ID and group ID
which correspond to the effective user and group IDs of the calling
process. There is no visible entry in the file system for the created
object in this implementation.
.Pp
The returned semaphore descriptor is available to the calling process
until it is closed with
.Fn sem_close ,
or until the caller exits or execs.
.Pp
If a process makes repeated calls to
.Fn sem_open ,
with the same
.Fa name
argument, the same descriptor is returned for each successful call,
unless
.Fn sem_unlink
has been called on the semaphore in the interim.
.Pp
If
.Fn sem_open
fails for any reason, it will return a value of
.Dv SEM_FAILED
and sets
.Va errno .
On success, it returns a semaphore descriptor.
.Sh ERRORS
The named semaphore is opened unless:
.Bl -tag -width Er
.It Bq Er EACCES
The required permissions (for reading and/or writing)
are denied for the given flags; or
.Dv O_CREAT
is specified, the object does not exist, and permission to
create the semaphore is denied.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
were specified and the semaphore exists.
.It Bq Er EINTR
The
.Fn sem_open
operation was interrupted by a signal.
.It Bq Er EINVAL
The
.Fn shm_open
operation is not supported; or
.Dv O_CREAT
is specified and
.Fa value
exceeds
.Dv SEM_VALUE_MAX .
.It Bq Er EMFILE
The process has already reached its limit for semaphores or file
descriptors in use.
.It Bq Er ENAMETOOLONG
.Fa name
exceeded
.Dv PSEMNAMLEN
characters.
.It Bq Er ENFILE
Too many semaphores or file descriptors are open on the system.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named semaphore does not exist.
.It Bq Er ENOSPC
.Dv O_CREAT
is specified, the file does not exist, and there is insufficient
space available to create the semaphore.
.El
.Sh SEE ALSO
.Xr sem_close 2 ,
.Xr sem_post 2 ,
.Xr sem_trywait 2 ,
.Xr sem_unlink 2 ,
.Xr sem_wait 2 ,
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 ,
.Xr umask 2
.Sh HISTORY
.Fn sem_open
is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).