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.Dd August 29, 2008
.Dt SHM_OPEN 2
.Os Darwin
.Sh NAME
.Nm shm_open
.Nd open a shared memory object
.Sh SYNOPSIS
.Fd #include <sys/mman.h>
.Fd #include <fcntl.h>
.Ft int
.Fo shm_open
.Fa "const char *name"
.Fa "int oflag"
.Fa "..."
.Fc
.Pp
The parameter "mode_t mode" is optional.
.Sh DESCRIPTION
The shared memory object referenced by
.Fa name
is opened for reading and/or writing as specified by the argument
.Fa oflag
and the file descriptor returned to the calling process.
The returned file descriptor will be the lowest non-open file
descriptor for the calling process, and is not shared with any
other processes, as it is a new file descriptor. The new file
descriptor will have the
.Dv FD_CLOEXEC
flag set.
Repeated calls
to
.Nm shm_open
with the same string value for
.Fn name
will return a file descriptor referring to the same shared memory
object, provided that the object has not been unlinked by a call to
.Fn shm_unlink .
The
.Fa oflag
argument may indicate the file is to be
created if it does not exist (by specifying the
.Dv O_CREAT
flag), in which case the file is created with mode
.Fa mode
as described in
.Xr chmod 2
and modified by the process' umask value (see
.Xr umask 2 ) .
.Pp
The value of
.Fa oflag
is formed by
.Em or Ns 'ing
the following values:
.Pp
.Bd -literal -offset indent -compact
O_RDONLY	open for reading only
O_RDWR		open for reading and writing
O_CREAT		create object if it does not exist
O_EXCL		error if create and object exists
O_TRUNC		truncate size to 0
.Ed
.Pp
Exactly one of
.Dv O_RDONLY
or
.Dv O_RDWR
must be specified.
.Pp
If
.Dv O_TRUNC
is specified and the
file exists, the file is truncated to zero length.
If
.Dv O_EXCL
is set with
.Dv O_CREAT
and the file already
exists,
.Fn shm_open
returns an error.  This may be used to
implement a simple exclusive access locking mechanism.
.Pp
If successful,
.Fn shm_open
returns a non-negative integer, termed a file descriptor.
It returns -1 and sets
.Va errno
on failure.
The file pointer used to mark the current position within the
memory object is set to the beginning of the object.
.Pp
When a new shared memory object is created it is given the
owner and group corresponding to the effective user and
group of the calling process. There is no visible entry in the
file system for the created object in this implementation.
.Pp
When a shared memory object is created, it persists until it
it unlinked and all other references are gone. Objects do
not persist across a system reboot.
.Pp
The system imposes a limit on the number of file descriptors
open simultaneously by one process.
.Xr getdtablesize 2
returns the current system limit.
.Sh ERRORS
The named object 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.
.It Bq Er EACCES
.Dv O_CREAT
is specified, the object does not exist, and permission to
create the object is denied.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
were specified and the object exists.
.It Bq Er EINTR
The
.Fn shm_open
operation was interrupted by a signal.
.It Bq Er EINVAL
The
.Fn shm_open
operation is not supported.
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENAMETOOLONG
.Fa name
exceeded the name size limit.
This is currently
.Dv PSHMNAMLEN
characters (defined in
.In sys/posix_shm.h ) ,
but this may change in the future.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named object 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 object.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr close 2 ,
.Xr getdtablesize 2 ,
.Xr mmap 2 ,
.Xr shm_unlink 2 ,
.Xr umask 2
.Sh HISTORY
.Fn shm_open
is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).