man/man9/uio.9

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.Dd March 11, 2017
.Dt UIO 9
.Os
.Sh NAME
.Nm uio ,
.Nm uiomove ,
.Nm uiomove_frombuf ,
.Nm uiomove_nofault
.Nd device driver I/O routines
.Sh SYNOPSIS
.In sys/types.h
.In sys/uio.h
.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"
.Ft int
.Fn uiomove_frombuf "void *buf" "int howmuch" "struct uio *uiop"
.Ft int
.Fn uiomove_nofault "void *buf" "int howmuch" "struct uio *uiop"
.Sh DESCRIPTION
The functions
.Fn uiomove ,
.Fn uiomove_frombuf ,
and
.Fn uiomove_nofault
are used to transfer data between buffers and I/O vectors that might
possibly 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
or
.Fn uiomove_nofault
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
.Pp
The function
.Fn uiomove_nofault
requires that the buffer and I/O vectors be accessible without
incurring a page fault.
The source and destination addresses must be physically mapped for
read and write access, respectively, and neither the source nor
destination addresses may be pageable.
Thus, the function
.Fn uiomove_nofault
can be called from contexts where acquiring virtual memory system
locks or sleeping are prohibited.
.Pp
The
.Fn uiomove_frombuf
function is a convenience wrapper around
.Fn uiomove
for drivers that serve data which is wholly contained within an
existing buffer in memory.
It validates the
.Va uio_offset
and
.Va uio_resid
values against the size of the existing buffer, handling short
transfers when the request partially overlaps the buffer.
When
.Va uio_offset
is greater than or equal to the buffer size, the result is success
with no bytes transferred, effectively signaling EOF.
.Sh RETURN VALUES
On success
.Fn uiomove ,
.Fn uiomove_frombuf ,
and
.Fn uiomove_nofault
will return 0; on error they will return an appropriate error code.
.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(struct cdev *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 ERRORS
.Fn uiomove
and
.Fn uiomove_nofault
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
.Pp
In addition,
.Fn uiomove_nofault
will fail and return the following error code if:
.Bl -tag -width Er
.It Bq Er EFAULT
A page fault occurs.
.El
.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 .