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.\" Copyright (c) 1997 Joerg Wunsch
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.\" $FreeBSD: src/share/man/man9/uio.9,v 1.5.2.4 2001/12/17 11:30:19 ru Exp $
.\"
.Dd January 16, 2015
.Dt UIO 9
.Os
.Sh NAME
.Nm uio ,
.Nm uiomove
.Nd device driver I/O routines
.Sh SYNOPSIS
.In sys/types.h
.In sys/uio.h
.Bd -literal
struct uio {
	struct	iovec *uio_iov;
	int	uio_iovcnt;
	off_t	uio_offset;
	size_t	uio_resid;
	enum	uio_seg uio_segflg;
	enum	uio_rw uio_rw;
	struct	thread *uio_td;
};
.Ed
.Ft int
.Fn uiomove "caddr_t buf" "size_t howmuch" "struct uio *uiop"
.Sh DESCRIPTION
The function
.Fn uiomove
is used to handle transfer of data between buffers and I/O vectors
that might possibly also cross the user/kernel space boundary.
.Pp
As a result of any
.Xr read 2 ,
.Xr write 2 ,
.Xr readv 2 ,
or
.Xr writev 2
system call that is being passed to a character-device driver, the
appropriate driver
.Nm d_read
or
.Nm d_write
entry will be called with a pointer to a
.Vt "struct dev_read_args"
or
.Vt "struct dev_write_args"
being passed,
a member of which is a pointer to a
.Vt "struct uio" .
The transfer request is encoded in this structure.
The driver itself should use
.Fn uiomove
to get at the data in this structure.
.Pp
The fields in the
.Vt uio
structure are:
.Bl -tag -width ".Va uio_iovcnt"
.It Va uio_iov
The array of I/O vectors to be processed.
In the case of scatter/gather
I/O, this will be more than one vector.
.It Va uio_iovcnt
The number of I/O vectors present.
.It Va uio_offset
The offset into the device.
.It Va uio_resid
The number of bytes to process.
.It Va uio_segflg
One of the following flags:
.Bl -tag -width ".Dv UIO_USERSPACE"
.It Dv UIO_USERSPACE
The I/O vector points into a process's address space.
.It Dv UIO_SYSSPACE
The I/O vector points into the kernel address space.
.It Dv UIO_NOCOPY
Don't copy, already in object.
.El
.It Va uio_rw
The direction of the desired transfer, either
.Dv UIO_READ ,
or
.Dv UIO_WRITE .
.It Va uio_td
The pointer to a
.Vt "struct thread"
for the associated thread; used if
.Va uio_segflg
indicates that the transfer is to be made from/to a process's address
space.
.El
.Sh RETURN VALUES
.Fn uiomove
can return
.Er EFAULT
from the invoked
.Xr copyin 9
or
.Xr copyout 9
in case the transfer was to/from a process's address space.
.Sh EXAMPLES
The idea is that the driver maintains a private buffer for its data,
and processes the request in chunks of maximal the size of this
buffer.
Note that the buffer handling below is very simplified and
won't work (the buffer pointer is not being advanced in case of a
partial read), it's just here to demonstrate the
.Nm
handling.
.Bd -literal
/* MIN() can be found there: */
#include <sys/param.h>

#define BUFSIZE 512
static char buffer[BUFSIZE];

static int data_available;	/* amount of data that can be read */

static int
fooread(struct dev_read_args *ap)
{
	cdev_t dev = ap->a_head.a_dev;
	int rv, amnt;

	while (ap->a_uio->uio_resid > 0) {
		if (data_available > 0) {
			amnt = MIN(ap->a_uio->uio_resid, data_available);
			if ((rv = uiomove((caddr_t)buffer, amnt, ap->a_uio))
			    != 0)
				goto error;
			data_available -= amnt;
		} else {
			tsleep(...);	/* wait for a better time */
		}
	}
	return 0;
error:
	/* do error cleanup here */
	return rv;
}
.Ed
.Sh SEE ALSO
.Xr read 2 ,
.Xr readv 2 ,
.Xr write 2 ,
.Xr writev 2 ,
.Xr copyin 9 ,
.Xr copyout 9 ,
.Xr physio 9 ,
.Xr sleep 9
.Sh HISTORY
The
.Nm
mechanism appeared in some early version of
.Ux .
.Sh AUTHORS
This man page was written by
.An J\(:org Wunsch .