187 lines
5.0 KiB
Groff
187 lines
5.0 KiB
Groff
.\"
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.\" Copyright (c) 2011 Advanced Computing Technologies LLC
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.\" Written by: John H. Baldwin <jhb@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 14, 2011
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.Dt SHM_MAP 9
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.Os
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.Sh NAME
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.Nm shm_map , shm_unmap
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.Nd "map shared memory objects into the kernel's address space"
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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_map "struct file *fp" "size_t size" "off_t offset" "void **memp"
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.Ft int
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.Fn shm_unmap "struct file *fp" "void *mem" "size_t size"
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.Sh DESCRIPTION
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The
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.Fn shm_map
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and
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.Fn shm_unmap
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functions provide an API for mapping shared memory objects into the kernel.
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Shared memory objects are created by
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.Xr shm_open 2 .
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These objects can then be passed into the kernel via file descriptors.
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.Pp
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A shared memory object cannot be shrunk while it is mapped into the kernel.
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This is to avoid invalidating any pages that may be wired into the kernel's
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address space.
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Shared memory objects can still be grown while mapped into the kernel.
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.Pp
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To simplify the accounting needed to enforce the above requirement,
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callers of this API are required to unmap the entire region mapped by
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.Fn shm_map
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when calling
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.Fn shm_unmap .
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Unmapping only a portion of the region is not permitted.
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.Pp
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The
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.Fn shm_map
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function locates the shared memory object associated with the open file
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.Fa fp .
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It maps the region of that object described by
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.Fa offset
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and
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.Fa size
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into the kernel's address space.
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If it succeeds,
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.Fa *memp
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will be set to the start of the mapping.
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All pages for the range will be wired into memory upon successful return.
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.Pp
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The
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.Fn shm_unmap
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function unmaps a region previously mapped by
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.Fn shm_map .
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The
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.Fa mem
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argument should match the value previously returned in
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.Fa *memp ,
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and the
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.Fa size
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argument should match the value passed to
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.Fn shm_map .
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.Pp
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Note that
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.Fn shm_map
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will not hold an extra reference on the open file
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.Fa fp
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for the lifetime of the mapping.
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Instead,
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the calling code is required to do this if it wishes to use
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.Fn shm_unmap
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on the region in the future.
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.Sh RETURN VALUES
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The
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.Fn shm_map
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and
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.Fn shm_unmap
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functions return zero on success or an error on failure.
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.Sh EXAMPLES
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The following function accepts a file descriptor for a shared memory
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object.
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It maps the first sixteen kilobytes of the object into the kernel,
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performs some work on that address,
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and then unmaps the address before returning.
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.Bd -literal -offset indent
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int
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shm_example(int fd)
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{
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struct file *fp;
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void *mem;
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int error;
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error = fget(curthread, fd, CAP_MMAP, &fp);
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if (error)
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return (error);
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error = shm_map(fp, 16384, 0, &mem);
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if (error) {
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fdrop(fp, curthread);
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return (error);
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}
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/* Do something with 'mem'. */
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error = shm_unmap(fp, mem, 16384);
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fdrop(fp, curthread);
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return (error);
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}
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.Ed
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.Sh ERRORS
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The
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.Fn shm_map
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function returns the following errors on failure:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The open file
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.Fa fp
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is not a shared memory object.
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.It Bq Er EINVAL
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The requested region described by
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.Fa offset
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and
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.Fa size
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extends beyond the end of the shared memory object.
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.It Bq Er ENOMEM
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Insufficient address space was available.
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.It Bq Er EACCES
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The shared memory object could not be mapped due to a protection error.
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.It Bq Er EINVAL
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The shared memory object could not be mapped due to some other VM error.
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.El
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.Pp
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The
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.Fn shm_unmap
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function returns the following errors on failure:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The open file
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.Fa fp
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is not a shared memory object.
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.It Bq Er EINVAL
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The address range described by
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.Fa mem
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and
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.Fa size
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is not a valid address range.
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.It Bq Er EINVAL
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The address range described by
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.Fa mem
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and
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.Fa size
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is not backed by the shared memory object associated with the open file
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.Fa fp ,
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or the address range does not cover the entire mapping of the object.
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.El
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.Sh SEE ALSO
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.Xr shm_open 2
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.Sh HISTORY
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This API was first introduced in
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.Fx 10.0 .
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