8e38aeff17
implement shm_open(2) and shm_unlink(2) in the kernel: - Each shared memory file descriptor is associated with a swap-backed vm object which provides the backing store. Each descriptor starts off with a size of zero, but the size can be altered via ftruncate(2). The shared memory file descriptors also support fstat(2). read(2), write(2), ioctl(2), select(2), poll(2), and kevent(2) are not supported on shared memory file descriptors. - shm_open(2) and shm_unlink(2) are now implemented as system calls that manage shared memory file descriptors. The virtual namespace that maps pathnames to shared memory file descriptors is implemented as a hash table where the hash key is generated via the 32-bit Fowler/Noll/Vo hash of the pathname. - As an extension, the constant 'SHM_ANON' may be specified in place of the path argument to shm_open(2). In this case, an unnamed shared memory file descriptor will be created similar to the IPC_PRIVATE key for shmget(2). Note that the shared memory object can still be shared among processes by sharing the file descriptor via fork(2) or sendmsg(2), but it is unnamed. This effectively serves to implement the getmemfd() idea bandied about the lists several times over the years. - The backing store for shared memory file descriptors are garbage collected when they are not referenced by any open file descriptors or the shm_open(2) virtual namespace. Submitted by: dillon, peter (previous versions) Submitted by: rwatson (I based this on his version) Reviewed by: alc (suggested converting getmemfd() to shm_open())
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 March 20, 2007
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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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.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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.Pp
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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 dies 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 ftruncate 2 ,
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.Xr fstat 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 7.0 .
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.Sh AUTHORS
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.An Garrett A. Wollman Aq 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 dillon@FreeBSD.org
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.Pq Dv MAP_NOSYNC
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