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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;
};

DESCRIPTION

As a result of any read(2), write(2), readv(2), or writev(2) system call that is being passed to a character-device driver, the appropriate driver d_read or d_write entry will be called with a pointer to a struct dev_read_args or struct dev_write_args being passed, a member of which is a pointer to a struct uio. The transfer request is encoded in this structure. The driver itself should use uiomove() to get at the data in this structure.

The fields in the uio structure are:

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.

uio_iovcnt

The number of I/O vectors present.

uio_offset

The offset into the device.

uio_resid

The number of bytes to process.

uio_segflg

One of the following flags:

UIO_USERSPACE

The I/O vector points into a process's address space.

UIO_SYSSPACE

The I/O vector points into the kernel address space.

UIO_NOCOPY

Don't copy, already in object.

uio_rw

The direction of the desired transfer, either UIO_READ, or UIO_WRITE.

uio_td

The pointer to a struct thread for the associated thread; used if uio_segflg indicates that the transfer is to be made from/to a process's address space.

/* 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;
}