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236 lines
6.1 KiB
Groff
236 lines
6.1 KiB
Groff
.\"
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.\" Copyright (c) 1997 Joerg Wunsch
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.\"
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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 DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd March 11, 2017
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.Dt UIO 9
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.Os
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.Sh NAME
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.Nm uio ,
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.Nm uiomove ,
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.Nm uiomove_frombuf ,
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.Nm uiomove_nofault
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.Nd device driver I/O routines
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/uio.h
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.Bd -literal
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struct uio {
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struct iovec *uio_iov; /* scatter/gather list */
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int uio_iovcnt; /* length of scatter/gather list */
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off_t uio_offset; /* offset in target object */
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ssize_t uio_resid; /* remaining bytes to copy */
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enum uio_seg uio_segflg; /* address space */
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enum uio_rw uio_rw; /* operation */
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struct thread *uio_td; /* owner */
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};
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.Ed
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.Ft int
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.Fn uiomove "void *buf" "int howmuch" "struct uio *uiop"
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.Ft int
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.Fn uiomove_frombuf "void *buf" "int howmuch" "struct uio *uiop"
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.Ft int
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.Fn uiomove_nofault "void *buf" "int howmuch" "struct uio *uiop"
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.Sh DESCRIPTION
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The functions
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.Fn uiomove ,
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.Fn uiomove_frombuf ,
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and
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.Fn uiomove_nofault
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are used to transfer data between buffers and I/O vectors that might
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possibly cross the user/kernel space boundary.
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.Pp
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As a result of any
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.Xr read 2 ,
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.Xr write 2 ,
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.Xr readv 2 ,
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or
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.Xr writev 2
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system call that is being passed to a character-device driver, the
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appropriate driver
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.Va d_read
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or
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.Va d_write
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entry will be called with a pointer to a
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.Vt "struct uio"
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being passed.
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The transfer request is encoded in this structure.
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The driver itself should use
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.Fn uiomove
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or
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.Fn uiomove_nofault
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to get at the data in this structure.
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.Pp
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The fields in the
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.Vt uio
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structure are:
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.Bl -tag -width ".Va uio_iovcnt"
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.It Va uio_iov
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The array of I/O vectors to be processed.
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In the case of scatter/gather
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I/O, this will be more than one vector.
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.It Va uio_iovcnt
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The number of I/O vectors present.
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.It Va uio_offset
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The offset into the device.
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.It Va uio_resid
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The remaining number of bytes to process, updated after transfer.
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.It Va uio_segflg
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One of the following flags:
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.Bl -tag -width ".Dv UIO_USERSPACE"
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.It Dv UIO_USERSPACE
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The I/O vector points into a process's address space.
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.It Dv UIO_SYSSPACE
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The I/O vector points into the kernel address space.
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.It Dv UIO_NOCOPY
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Do not copy, already in object.
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.El
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.It Va uio_rw
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The direction of the desired transfer, either
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.Dv UIO_READ
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or
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.Dv UIO_WRITE .
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.It Va uio_td
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The pointer to a
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.Vt "struct thread"
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for the associated thread; used if
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.Va uio_segflg
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indicates that the transfer is to be made from/to a process's address
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space.
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.El
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.Pp
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The function
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.Fn uiomove_nofault
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requires that the buffer and I/O vectors be accessible without
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incurring a page fault.
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The source and destination addresses must be physically mapped for
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read and write access, respectively, and neither the source nor
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destination addresses may be pageable.
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Thus, the function
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.Fn uiomove_nofault
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can be called from contexts where acquiring virtual memory system
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locks or sleeping are prohibited.
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.Pp
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The
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.Fn uiomove_frombuf
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function is a convenience wrapper around
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.Fn uiomove
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for drivers that serve data which is wholly contained within an
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existing buffer in memory.
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It validates the
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.Va uio_offset
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and
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.Va uio_resid
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values against the size of the existing buffer, handling short
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transfers when the request partially overlaps the buffer.
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When
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.Va uio_offset
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is greater than or equal to the buffer size, the result is success
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with no bytes transferred, effectively signaling EOF.
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.Sh RETURN VALUES
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On success
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.Fn uiomove ,
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.Fn uiomove_frombuf ,
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and
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.Fn uiomove_nofault
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will return 0; on error they will return an appropriate error code.
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.Sh EXAMPLES
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The idea is that the driver maintains a private buffer for its data,
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and processes the request in chunks of maximal the size of this
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buffer.
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Note that the buffer handling below is very simplified and
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will not work (the buffer pointer is not being advanced in case of a
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partial read), it is just here to demonstrate the
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.Nm
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handling.
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.Bd -literal
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/* MIN() can be found there: */
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#include <sys/param.h>
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#define BUFSIZE 512
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static char buffer[BUFSIZE];
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static int data_available; /* amount of data that can be read */
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static int
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fooread(struct cdev *dev, struct uio *uio, int flag)
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{
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int rv, amnt;
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rv = 0;
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while (uio->uio_resid > 0) {
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if (data_available > 0) {
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amnt = MIN(uio->uio_resid, data_available);
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rv = uiomove(buffer, amnt, uio);
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if (rv != 0)
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break;
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data_available -= amnt;
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} else
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tsleep(...); /* wait for a better time */
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}
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if (rv != 0) {
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/* do error cleanup here */
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}
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return (rv);
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}
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.Ed
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.Sh ERRORS
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.Fn uiomove
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and
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.Fn uiomove_nofault
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will fail and return the following error code if:
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.Bl -tag -width Er
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.It Bq Er EFAULT
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The invoked
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.Xr copyin 9
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or
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.Xr copyout 9
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returned
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.Er EFAULT
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.El
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.Pp
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In addition,
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.Fn uiomove_nofault
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will fail and return the following error code if:
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.Bl -tag -width Er
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.It Bq Er EFAULT
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A page fault occurs.
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.El
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.Sh SEE ALSO
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.Xr read 2 ,
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.Xr readv 2 ,
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.Xr write 2 ,
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.Xr writev 2 ,
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.Xr copyin 9 ,
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.Xr copyout 9 ,
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.Xr sleep 9
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.Sh HISTORY
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The
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.Nm
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mechanism appeared in some early version of
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.Ux .
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.Sh AUTHORS
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This manual page was written by
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.An J\(:org Wunsch .
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