9afb12bab4
Add an atomic shm rename operation, similar in spirit to a file rename. Atomically unlink an shm from a source path and link it to a destination path. If an existing shm is linked at the destination path, unlink it as part of the same atomic operation. The caller needs the same permissions as shm_unlink to the shm being renamed, and the same permissions for the shm at the destination which is being unlinked, if it exists. If those fail, EACCES is returned, as with the other shm_* syscalls. truss support is included; audit support will come later. This commit includes only the implementation; the sysent-generated bits will come in a follow-on commit. Submitted by: Matthew Bryan <matthew.bryan@isilon.com> Reviewed by: jilles (earlier revision) Reviewed by: brueffer (manpages, earlier revision) Relnotes: yes Sponsored by: Dell EMC Isilon Differential Revision: https://reviews.freebsd.org/D21423
473 lines
11 KiB
Groff
473 lines
11 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 September 26, 2019
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.Dt SHM_OPEN 2
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.Os
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.Sh NAME
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.Nm memfd_create , shm_open , shm_rename, 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 memfd_create "const char *name" "unsigned int flags"
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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_rename "const char *path_from" "const char *path_to" "int flags"
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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
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.Fx
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extension, 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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or moved with a call to
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.Fn shm_rename .
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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_rename
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system call atomically removes a shared memory object named
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.Fa path_from
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and relinks it at
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.Fa path_to .
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If another object is already linked at
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.Fa path_to ,
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that object will be unlinked, unless one of the following flags are provided:
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.Bl -tag -offset indent -width Er
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.It Er SHM_RENAME_EXCHANGE
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Atomically exchange the shms at
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.Fa path_from
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and
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.Fa path_to .
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.It Er SHM_RENAME_NOREPLACE
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Return an error if an shm exists at
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.Fa path_to ,
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rather than unlinking it.
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.El
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.Fn shm_rename
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is also a
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.Fx
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extension.
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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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The
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.Fn memfd_create
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function creates an anonymous shared memory object, identical to that created
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by
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.Fn shm_open
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when
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.Dv SHM_ANON
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is specified.
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Newly created objects start off with a size of zero.
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The size of the new object must be adjusted via
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.Xr ftruncate 2 .
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.Pp
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The
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.Fa name
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argument must not be
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.Dv NULL ,
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but it may be an empty string.
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The length of the
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.Fa name
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argument may not exceed
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.Dv NAME_MAX
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minus six characters for the prefix
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.Dq memfd: ,
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which will be prepended.
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The
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.Fa name
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argument is intended solely for debugging purposes and will never be used by the
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kernel to identify a memfd.
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Names are therefore not required to be unique.
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.Pp
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The following
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.Fa flags
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may be specified to
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.Fn memfd_create :
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.Bl -tag -width MFD_ALLOW_SEALING
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.It Dv MFD_CLOEXEC
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Set
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.Dv FD_CLOEXEC
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on the resulting file descriptor.
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.It Dv MFD_ALLOW_SEALING
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Allow adding seals to the resulting file descriptor using the
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.Dv F_ADD_SEALS
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.Xr fcntl 2
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command.
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.It Dv MFD_HUGETLB
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This flag is currently unsupported.
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.El
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.Sh RETURN VALUES
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If successful,
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.Fn memfd_create
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and
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.Fn shm_open
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both return a non-negative integer,
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and
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.Fn shm_rename
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and
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.Fn shm_unlink
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return zero.
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All 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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.Fa path_from ,
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and
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.Fa path_to
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arguments do not necessarily represent a pathname (although they do 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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.Tn POSIX
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specifications state that 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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However, the
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.Fx
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kernel implementation explicitly includes support for
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.Xr read 2
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and
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.Xr write 2 .
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.Pp
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.Fx
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also supports zero-copy transmission of data from shared memory
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objects with
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.Xr sendfile 2 .
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.Pp
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Neither shared memory objects nor their contents persist across reboots.
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.Pp
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Writes do not extend shared memory objects, so
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.Xr ftruncate 2
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must be called before any data can be written.
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See
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.Sx EXAMPLES .
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.Sh EXAMPLES
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This example fails without the call to
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.Xr ftruncate 2 :
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.Bd -literal -compact
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uint8_t buffer[getpagesize()];
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ssize_t len;
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int fd;
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fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
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if (fd < 0)
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err(EX_OSERR, "%s: shm_open", __func__);
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if (ftruncate(fd, getpagesize()) < 0)
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err(EX_IOERR, "%s: ftruncate", __func__);
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len = pwrite(fd, buffer, getpagesize(), 0);
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if (len < 0)
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err(EX_IOERR, "%s: pwrite", __func__);
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if (len != getpagesize())
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errx(EX_IOERR, "%s: pwrite length mismatch", __func__);
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.Ed
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.Sh ERRORS
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.Fn memfd_create
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fails with these error codes for these conditions:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa name
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argument was NULL.
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.It Bq Er EINVAL
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The
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.Fa name
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argument was too long.
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.Pp
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An invalid or unsupported flag 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 ENOSYS
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In
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.Fa memfd_create ,
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.Dv MFD_HUGETLB
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was specified in
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.Fa flags ,
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and this system does not support forced hugetlb mappings.
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.El
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.Pp
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.Fn shm_open
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fails with these error codes for these conditions:
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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 exceeds 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_rename :
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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_from
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or
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.Fa path_to
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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 exceeds 1023 characters.
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.It Bq Er ENOENT
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The shared memory object at
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.Fa path_from
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does not exist.
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.It Bq Er EACCES
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The required permissions are denied.
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.It Bq Er EEXIST
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An shm exists at
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.Fa path_to ,
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and the
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.Dv SHM_RENAME_NOREPLACE
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flag was provided.
|
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.El
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.Pp
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.Fn shm_unlink
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fails with these error codes for these conditions:
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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 exceeds 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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.Xr sendfile 2
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.Sh STANDARDS
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The
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.Fn memfd_create
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function is expected to be compatible with the Linux system call of the same
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name.
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.Pp
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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
|
|
The
|
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.Fn memfd_create
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function appeared in
|
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.Fx 13.0 .
|
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.Pp
|
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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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.Pp
|
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.Fn shm_rename
|
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first appeared in
|
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.Fx 13.0
|
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as a
|
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.Fx
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|
extension.
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.Sh AUTHORS
|
|
.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
|
|
.An Matthew Dillon Aq Mt dillon@FreeBSD.org
|
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.Pq Dv MAP_NOSYNC
|
|
.Pp
|
|
.An Matthew Bryan Aq Mt matthew.bryan@isilon.com
|
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.Pq Dv shm_rename implementation
|