623 lines
15 KiB
Groff
623 lines
15 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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.Dd January 30, 2023
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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_create_largepage , 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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.Fo shm_create_largepage
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.Fa "const char *path"
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.Fa "int flags"
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.Fa "int psind"
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.Fa "int alloc_policy"
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.Fa "mode_t mode"
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.Fc
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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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function opens (or optionally creates) a
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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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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_create_largepage
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function behaves similarly to
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.Fn shm_open ,
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except that the
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.Dv O_CREAT
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flag is implicitly specified, and the returned
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.Dq largepage
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object is always backed by aligned, physically contiguous chunks of memory.
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This ensures that the object can be mapped using so-called
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.Dq superpages ,
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which can improve application performance in some workloads by reducing the
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number of translation lookaside buffer (TLB) entries required to access a
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mapping of the object,
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and by reducing the number of page faults performed when accessing a mapping.
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This happens automatically for all largepage objects.
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.Pp
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An existing largepage object can be opened using the
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.Fn shm_open
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function.
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Largepage shared memory objects behave slightly differently from non-largepage
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objects:
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.Bl -bullet -offset indent
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.It
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Memory for a largepage object is allocated when the object is
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extended using the
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.Xr ftruncate 2
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system call, whereas memory for regular shared memory objects is allocated
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lazily and may be paged out to a swap device when not in use.
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.It
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The size of a mapping of a largepage object must be a multiple of the
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underlying large page size.
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Most attributes of such a mapping can only be modified at the granularity
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of the large page size.
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For example, when using
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.Xr munmap 2
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to unmap a portion of a largepage object mapping, or when using
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.Xr mprotect 2
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to adjust protections of a mapping of a largepage object, the starting address
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must be large page size-aligned, and the length of the operation must be a
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multiple of the large page size.
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If not, the corresponding system call will fail and set
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.Va errno
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to
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.Er EINVAL .
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.El
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.Pp
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The
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.Fa psind
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argument to
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.Fn shm_create_largepage
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specifies the size of large pages used to back the object.
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This argument is an index into the page sizes array returned by
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.Xr getpagesizes 3 .
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In particular, all large pages backing a largepage object must be of the
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same size.
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For example, on a system with large page sizes of 2MB and 1GB, a 2GB largepage
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object will consist of either 1024 2MB pages, or 2 1GB pages, depending on
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the value specified for the
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.Fa psind
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argument.
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The
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.Fa alloc_policy
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parameter specifies what happens when an attempt to use
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.Xr ftruncate 2
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to allocate memory for the object fails.
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The following values are accepted:
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.Bl -tag -offset indent -width SHM_
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.It Dv SHM_LARGEPAGE_ALLOC_DEFAULT
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If the (non-blocking) memory allocation fails because there is insufficient free
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contiguous memory, the kernel will attempt to defragment physical memory and
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try another allocation.
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The subsequent allocation may or may not succeed.
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If this subsequent allocation also fails,
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.Xr ftruncate 2
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will fail and set
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.Va errno
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to
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.Er ENOMEM .
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.It Dv SHM_LARGEPAGE_ALLOC_NOWAIT
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|
If the memory allocation fails,
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.Xr ftruncate 2
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will fail and set
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.Va errno
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to
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.Er ENOMEM .
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.It Dv SHM_LARGEPAGE_ALLOC_HARD
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The kernel will attempt defragmentation until the allocation succeeds,
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or an unblocked signal is delivered to the thread.
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However, it is possible for physical memory to be fragmented such that the
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allocation will never succeed.
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.El
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.Pp
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The
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.Dv FIOSSHMLPGCNF
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and
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.Dv FIOGSHMLPGCNF
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.Xr ioctl 2
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commands can be used with a largepage shared memory object to get and set
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largepage object parameters.
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Both commands operate on the following structure:
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|
.Bd -literal
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struct shm_largepage_conf {
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int psind;
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int alloc_policy;
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};
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|
.Ed
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|
The
|
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.Dv FIOGSHMLPGCNF
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|
command populates this structure with the current values of these parameters,
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while the
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.Dv FIOSSHMLPGCNF
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|
command modifies the largepage object.
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|
Currently only the
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.Va alloc_policy
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parameter may be modified.
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|
Internally,
|
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.Fn shm_create_largepage
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works by creating a regular shared memory object using
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.Fn shm_open ,
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and then converting it into a largepage object using the
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.Dv FIOSSHMLPGCNF
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ioctl command.
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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
|
|
.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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.Pp
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The
|
|
.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
|
|
.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.
|
|
Newly created objects start off with a size of zero.
|
|
The size of the new object must be adjusted via
|
|
.Xr ftruncate 2 .
|
|
.Pp
|
|
The
|
|
.Fa name
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argument must not be
|
|
.Dv NULL ,
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|
but it may be an empty string.
|
|
The length of the
|
|
.Fa name
|
|
argument may not exceed
|
|
.Dv NAME_MAX
|
|
minus six characters for the prefix
|
|
.Dq memfd: ,
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|
which will be prepended.
|
|
The
|
|
.Fa name
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argument is intended solely for debugging purposes and will never be used by the
|
|
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
|
|
.Fa flags
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|
may be specified to
|
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.Fn memfd_create :
|
|
.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
|
|
on the resulting file descriptor.
|
|
.It Dv MFD_ALLOW_SEALING
|
|
Allow adding seals to the resulting file descriptor using the
|
|
.Dv F_ADD_SEALS
|
|
.Xr fcntl 2
|
|
command.
|
|
.It Dv MFD_HUGETLB
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This flag is currently unsupported.
|
|
.El
|
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.Sh RETURN VALUES
|
|
If successful,
|
|
.Fn memfd_create
|
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and
|
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.Fn shm_open
|
|
both return a non-negative integer,
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and
|
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.Fn shm_rename
|
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and
|
|
.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
|
|
The
|
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.Fn shm_create_largepage
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and
|
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.Fn shm_rename
|
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functions are
|
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.Fx
|
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extensions, as is support for the
|
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.Dv SHM_ANON
|
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value in
|
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.Fn shm_open .
|
|
.Pp
|
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The
|
|
.Fa path ,
|
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.Fa path_from ,
|
|
and
|
|
.Fa path_to
|
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arguments do not necessarily represent a pathname (although they do in
|
|
most other implementations).
|
|
Two processes opening the same
|
|
.Fa path
|
|
are guaranteed to access the same shared memory object if and only if
|
|
.Fa path
|
|
begins with a slash
|
|
.Pq Ql \&/
|
|
character.
|
|
.Pp
|
|
Only the
|
|
.Dv O_RDONLY ,
|
|
.Dv O_RDWR ,
|
|
.Dv O_CREAT ,
|
|
.Dv O_EXCL ,
|
|
and
|
|
.Dv O_TRUNC
|
|
flags may be used in portable programs.
|
|
.Pp
|
|
POSIX
|
|
specifications state that the result of using
|
|
.Xr open 2 ,
|
|
.Xr read 2 ,
|
|
or
|
|
.Xr write 2
|
|
on a shared memory object, or on the descriptor returned by
|
|
.Fn shm_open ,
|
|
is undefined.
|
|
However, the
|
|
.Fx
|
|
kernel implementation explicitly includes support for
|
|
.Xr read 2
|
|
and
|
|
.Xr write 2 .
|
|
.Pp
|
|
.Fx
|
|
also supports zero-copy transmission of data from shared memory
|
|
objects with
|
|
.Xr sendfile 2 .
|
|
.Pp
|
|
Neither shared memory objects nor their contents persist across reboots.
|
|
.Pp
|
|
Writes do not extend shared memory objects, so
|
|
.Xr ftruncate 2
|
|
must be called before any data can be written.
|
|
See
|
|
.Sx EXAMPLES .
|
|
.Sh EXAMPLES
|
|
This example fails without the call to
|
|
.Xr ftruncate 2 :
|
|
.Bd -literal -compact
|
|
|
|
uint8_t buffer[getpagesize()];
|
|
ssize_t len;
|
|
int fd;
|
|
|
|
fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
|
|
if (fd < 0)
|
|
err(EX_OSERR, "%s: shm_open", __func__);
|
|
if (ftruncate(fd, getpagesize()) < 0)
|
|
err(EX_IOERR, "%s: ftruncate", __func__);
|
|
len = pwrite(fd, buffer, getpagesize(), 0);
|
|
if (len < 0)
|
|
err(EX_IOERR, "%s: pwrite", __func__);
|
|
if (len != getpagesize())
|
|
errx(EX_IOERR, "%s: pwrite length mismatch", __func__);
|
|
.Ed
|
|
.Sh ERRORS
|
|
.Fn memfd_create
|
|
fails with these error codes for these conditions:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EBADF
|
|
The
|
|
.Fa name
|
|
argument was NULL.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa name
|
|
argument was too long.
|
|
.Pp
|
|
An invalid or unsupported flag was included in
|
|
.Fa flags .
|
|
.It Bq Er EMFILE
|
|
The process has already reached its limit for open file descriptors.
|
|
.It Bq Er ENFILE
|
|
The system file table is full.
|
|
.It Bq Er ENOSYS
|
|
In
|
|
.Fa memfd_create ,
|
|
.Dv MFD_HUGETLB
|
|
was specified in
|
|
.Fa flags ,
|
|
and this system does not support forced hugetlb mappings.
|
|
.El
|
|
.Pp
|
|
.Fn shm_open
|
|
fails with these error codes for these conditions:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
A flag other than
|
|
.Dv O_RDONLY ,
|
|
.Dv O_RDWR ,
|
|
.Dv O_CREAT ,
|
|
.Dv O_EXCL ,
|
|
or
|
|
.Dv O_TRUNC
|
|
was included in
|
|
.Fa flags .
|
|
.It Bq Er EMFILE
|
|
The process has already reached its limit for open file descriptors.
|
|
.It Bq Er ENFILE
|
|
The system file table is full.
|
|
.It Bq Er EINVAL
|
|
.Dv O_RDONLY
|
|
was specified while creating an anonymous shared memory object via
|
|
.Dv SHM_ANON .
|
|
.It Bq Er EFAULT
|
|
The
|
|
.Fa path
|
|
argument points outside the process' allocated address space.
|
|
.It Bq Er ENAMETOOLONG
|
|
The entire pathname exceeds 1023 characters.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa path
|
|
does not begin with a slash
|
|
.Pq Ql \&/
|
|
character.
|
|
.It Bq Er ENOENT
|
|
.Dv O_CREAT
|
|
is not specified and the named shared memory object does not exist.
|
|
.It Bq Er EEXIST
|
|
.Dv O_CREAT
|
|
and
|
|
.Dv O_EXCL
|
|
are specified and the named shared memory object does exist.
|
|
.It Bq Er EACCES
|
|
The required permissions (for reading or reading and writing) are denied.
|
|
.It Bq Er ECAPMODE
|
|
The process is running in capability mode (see
|
|
.Xr capsicum 4 )
|
|
and attempted to create a named shared memory object.
|
|
.El
|
|
.Pp
|
|
.Fn shm_create_largepage
|
|
can fail for the reasons listed above.
|
|
It also fails with these error codes for the following conditions:
|
|
.Bl -tag -width Er
|
|
.It Bq Er ENOTTY
|
|
The kernel does not support large pages on the current platform.
|
|
.El
|
|
.Pp
|
|
The following errors are defined for
|
|
.Fn shm_rename :
|
|
.Bl -tag -width Er
|
|
.It Bq Er EFAULT
|
|
The
|
|
.Fa path_from
|
|
or
|
|
.Fa path_to
|
|
argument points outside the process' allocated address space.
|
|
.It Bq Er ENAMETOOLONG
|
|
The entire pathname exceeds 1023 characters.
|
|
.It Bq Er ENOENT
|
|
The shared memory object at
|
|
.Fa path_from
|
|
does not exist.
|
|
.It Bq Er EACCES
|
|
The required permissions are denied.
|
|
.It Bq Er EEXIST
|
|
An shm exists at
|
|
.Fa path_to ,
|
|
and the
|
|
.Dv SHM_RENAME_NOREPLACE
|
|
flag was provided.
|
|
.El
|
|
.Pp
|
|
.Fn shm_unlink
|
|
fails with these error codes for these conditions:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EFAULT
|
|
The
|
|
.Fa path
|
|
argument points outside the process' allocated address space.
|
|
.It Bq Er ENAMETOOLONG
|
|
The entire pathname exceeds 1023 characters.
|
|
.It Bq Er ENOENT
|
|
The named shared memory object does not exist.
|
|
.It Bq Er EACCES
|
|
The required permissions are denied.
|
|
.Fn shm_unlink
|
|
requires write permission to the shared memory object.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr posixshmcontrol 1 ,
|
|
.Xr close 2 ,
|
|
.Xr fstat 2 ,
|
|
.Xr ftruncate 2 ,
|
|
.Xr ioctl 2 ,
|
|
.Xr mmap 2 ,
|
|
.Xr munmap 2 ,
|
|
.Xr sendfile 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn memfd_create
|
|
function is expected to be compatible with the Linux system call of the same
|
|
name.
|
|
.Pp
|
|
The
|
|
.Fn shm_open
|
|
and
|
|
.Fn shm_unlink
|
|
functions are believed to conform to
|
|
.St -p1003.1b-93 .
|
|
.Sh HISTORY
|
|
The
|
|
.Fn memfd_create
|
|
function appeared in
|
|
.Fx 13.0 .
|
|
.Pp
|
|
The
|
|
.Fn shm_open
|
|
and
|
|
.Fn shm_unlink
|
|
functions first appeared in
|
|
.Fx 4.3 .
|
|
The functions were reimplemented as system calls using shared memory objects
|
|
directly rather than files in
|
|
.Fx 8.0 .
|
|
.Pp
|
|
.Fn shm_rename
|
|
first appeared in
|
|
.Fx 13.0
|
|
as a
|
|
.Fx
|
|
extension.
|
|
.Sh AUTHORS
|
|
.An Garrett A. Wollman Aq Mt wollman@FreeBSD.org
|
|
(C library support and this manual page)
|
|
.Pp
|
|
.An Matthew Dillon Aq Mt dillon@FreeBSD.org
|
|
.Pq Dv MAP_NOSYNC
|
|
.Pp
|
|
.An Matthew Bryan Aq Mt matthew.bryan@isilon.com
|
|
.Pq Dv shm_rename implementation
|