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.\" Copyright (c) 1997 Joerg Wunsch
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.\" $FreeBSD: src/share/man/man9/uio.9,v 1.22.2.3.2.1 2010/12/21 17:09:25 kensmith Exp $
.\"
.Dd March 21, 2010
.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
.Pp
.Bd -literal
struct uio {
	struct	iovec *uio_iov;		/* scatter/gather list */
	int	uio_iovcnt;		/* length of scatter/gather list */
	off_t	uio_offset;		/* offset in target object */
	ssize_t	uio_resid;		/* remaining bytes to copy */
	enum	uio_seg uio_segflg;	/* address space */
	enum	uio_rw uio_rw;		/* operation */
	struct	thread *uio_td;		/* owner */
};
.Ed
.Ft int
.Fn uiomove "void *buf" "int 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
.Va d_read
or
.Va d_write
entry will be called with a pointer to a
.Vt "struct uio"
being passed.
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 remaining number of bytes to process, updated after transfer.
.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
Do not 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
On success
.Fn uiomove
will return 0, on error it will return an appropriate errno.
.Sh ERRORS
.Fn uiomove
will fail and return the following error code if:
.Bl -tag -width Er
.It Bq Er EFAULT
The invoked
.Xr copyin 9
or
.Xr copyout 9
returned
.Er EFAULT
.El
.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
will not work (the buffer pointer is not being advanced in case of a
partial read), it is 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(dev_t dev, struct uio *uio, int flag)
{
	int rv, amnt;

	rv = 0;
	while (uio->uio_resid > 0) {
		if (data_available > 0) {
			amnt = MIN(uio->uio_resid, data_available);
			rv = uiomove(buffer, amnt, uio);
			if (rv != 0)
				break;
			data_available -= amnt;
		} else
			tsleep(...);	/* wait for a better time */
	}
	if (rv != 0) {
		/* 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 sleep 9
.Sh HISTORY
The
.Nm
mechanism appeared in some early version of
.Ux .
.Sh AUTHORS
This manual page was written by
.An J\(:org Wunsch .