283 lines
6.6 KiB
Groff
283 lines
6.6 KiB
Groff
.\"
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.\" Copyright 2000 Massachusetts Institute of Technology
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.\"
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.\" Permission to use, copy, modify, and distribute this software and
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.\" its documentation for any purpose and without fee is hereby
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.\" granted, provided that both the above copyright notice and this
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.\" permission notice appear in all copies, that both the above
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.\" copyright notice and this permission notice appear in all
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.\" supporting documentation, and that the name of M.I.T. not be used
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.\" in advertising or publicity pertaining to distribution of the
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.\" software without specific, written prior permission. M.I.T. makes
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.\" no representations about the suitability of this software for any
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.\" purpose. It is provided "as is" without express or implied
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.\" warranty.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY M.I.T. ``AS IS''. M.I.T. DISCLAIMS
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.\" ALL EXPRESS OR IMPLIED WARRANTIES WITH REGARD TO THIS SOFTWARE,
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.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
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.\" SHALL M.I.T. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
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.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 18, 2013
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.Dt SHM_OPEN 2
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.Os
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.Sh NAME
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.Nm shm_open , shm_unlink
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.Nd "shared memory object operations"
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/mman.h
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.In fcntl.h
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.Ft int
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.Fn shm_open "const char *path" "int flags" "mode_t mode"
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.Ft int
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.Fn shm_unlink "const char *path"
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.Sh DESCRIPTION
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The
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.Fn shm_open
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system call opens (or optionally creates) a
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.Tn POSIX
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shared memory object named
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.Fa path .
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The
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.Fa flags
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argument contains a subset of the flags used by
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.Xr open 2 .
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An access mode of either
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.Dv O_RDONLY
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or
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.Dv O_RDWR
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must be included in
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.Fa flags .
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The optional flags
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.Dv O_CREAT ,
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.Dv O_EXCL ,
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and
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.Dv O_TRUNC
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may also be specified.
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.Pp
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If
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.Dv O_CREAT
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is specified,
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then a new shared memory object named
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.Fa path
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will be created if it does not exist.
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In this case,
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the shared memory object is created with mode
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.Fa mode
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subject to the process' umask value.
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If both the
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.Dv O_CREAT
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and
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.Dv O_EXCL
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flags are specified and a shared memory object named
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.Fa path
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already exists,
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then
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.Fn shm_open
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will fail with
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.Er EEXIST .
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.Pp
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Newly created objects start off with a size of zero.
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If an existing shared memory object is opened with
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.Dv O_RDWR
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and the
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.Dv O_TRUNC
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flag is specified,
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then the shared memory object will be truncated to a size of zero.
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The size of the object can be adjusted via
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.Xr ftruncate 2
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and queried via
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.Xr fstat 2 .
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.Pp
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The new descriptor is set to close during
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.Xr execve 2
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system calls;
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see
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.Xr close 2
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and
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.Xr fcntl 2 .
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.Pp
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As a FreeBSD extension,
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the constant
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.Dv SHM_ANON
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may be used for the
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.Fa path
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argument to
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.Fn shm_open .
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In this case, an anonymous, unnamed shared memory object is created.
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Since the object has no name,
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it cannot be removed via a subsequent call to
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.Fn shm_unlink .
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Instead,
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the shared memory object will be garbage collected when the last reference to
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the shared memory object is removed.
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The shared memory object may be shared with other processes by sharing the
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file descriptor via
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.Xr fork 2
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or
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.Xr sendmsg 2 .
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Attempting to open an anonymous shared memory object with
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.Dv O_RDONLY
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will fail with
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.Er EINVAL .
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All other flags are ignored.
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.Pp
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The
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.Fn shm_unlink
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system call removes a shared memory object named
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.Fa path .
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.Sh RETURN VALUES
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If successful,
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.Fn shm_open
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returns a non-negative integer,
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and
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.Fn shm_unlink
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returns zero.
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Both functions return -1 on failure, and set
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.Va errno
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to indicate the error.
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.Sh COMPATIBILITY
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The
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.Fa path
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argument does not necessarily represent a pathname (although it does in
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most other implementations).
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Two processes opening the same
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.Fa path
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are guaranteed to access the same shared memory object if and only if
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.Fa path
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begins with a slash
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.Pq Ql \&/
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character.
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.Pp
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Only the
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.Dv O_RDONLY ,
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.Dv O_RDWR ,
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.Dv O_CREAT ,
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.Dv O_EXCL ,
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and
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.Dv O_TRUNC
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flags may be used in portable programs.
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.Pp
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The result of using
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.Xr open 2 ,
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.Xr read 2 ,
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or
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.Xr write 2
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on a shared memory object, or on the descriptor returned by
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.Fn shm_open ,
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is undefined.
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It is also undefined whether the shared memory object itself, or its
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contents, persist across reboots.
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.Pp
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In FreeBSD,
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.Xr read 2
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and
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.Xr write 2
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on a shared memory object will fail with
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.Er EOPNOTSUPP
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and neither shared memory objects nor their contents persist across reboots.
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.Sh ERRORS
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The following errors are defined for
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.Fn shm_open :
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.Bl -tag -width Er
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.It Bq Er EINVAL
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A flag other than
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.Dv O_RDONLY ,
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.Dv O_RDWR ,
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.Dv O_CREAT ,
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.Dv O_EXCL ,
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or
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.Dv O_TRUNC
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was included in
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.Fa flags .
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.It Bq Er EMFILE
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The process has already reached its limit for open file descriptors.
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.It Bq Er ENFILE
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The system file table is full.
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.It Bq Er EINVAL
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.Dv O_RDONLY
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was specified while creating an anonymous shared memory object via
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.Dv SHM_ANON .
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.It Bq Er EFAULT
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The
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.Fa path
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argument points outside the process' allocated address space.
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.It Bq Er ENAMETOOLONG
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The entire pathname exceeded 1023 characters.
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.It Bq Er EINVAL
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The
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.Fa path
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does not begin with a slash
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.Pq Ql \&/
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character.
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.It Bq Er ENOENT
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.Dv O_CREAT
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is specified and the named shared memory object does not exist.
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.It Bq Er EEXIST
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.Dv O_CREAT
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and
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.Dv O_EXCL
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are specified and the named shared memory object does exist.
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.It Bq Er EACCES
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The required permissions (for reading or reading and writing) are denied.
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.El
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.Pp
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The following errors are defined for
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.Fn shm_unlink :
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.Bl -tag -width Er
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.It Bq Er EFAULT
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The
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.Fa path
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argument points outside the process' allocated address space.
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.It Bq Er ENAMETOOLONG
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The entire pathname exceeded 1023 characters.
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.It Bq Er ENOENT
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The named shared memory object does not exist.
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.It Bq Er EACCES
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The required permissions are denied.
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.Fn shm_unlink
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requires write permission to the shared memory object.
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.El
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.Sh SEE ALSO
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.Xr close 2 ,
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.Xr fstat 2 ,
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.Xr ftruncate 2 ,
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.Xr mmap 2 ,
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.Xr munmap 2
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.Sh STANDARDS
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The
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.Fn shm_open
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and
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.Fn shm_unlink
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functions are believed to conform to
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.St -p1003.1b-93 .
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.Sh HISTORY
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The
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.Fn shm_open
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and
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.Fn shm_unlink
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functions first appeared in
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.Fx 4.3 .
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The functions were reimplemented as system calls using shared memory objects
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directly rather than files in
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.Fx 8.0 .
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.Sh AUTHORS
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.An Garrett A. Wollman Aq Mt wollman@FreeBSD.org
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(C library support and this manual page)
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.Pp
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.An Matthew Dillon Aq Mt dillon@FreeBSD.org
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.Pq Dv MAP_NOSYNC
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