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UIO(9)		       FreeBSD Kernel Developer's Manual		UIO(9)

     uio, uiomove, uiomove_frombuf, uiomove_nofault -- device driver I/O rou-

     #include <sys/types.h>
     #include <sys/uio.h>

     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 */

     uiomove(void *buf,	int howmuch, struct uio	*uiop);

     uiomove_frombuf(void *buf,	int howmuch, struct uio	*uiop);

     uiomove_nofault(void *buf,	int howmuch, struct uio	*uiop);

     The functions uiomove(), uiomove_frombuf(), and uiomove_nofault() are
     used to transfer data between buffers and I/O vectors that	might possibly
     cross the user/kernel space boundary.

     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 uio be-
     ing passed.  The transfer request is encoded in this structure.  The
     driver itself should use uiomove()	or uiomove_nofault() 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 remaining number of bytes to process, updated after

     uio_segflg	 One of	the following flags:

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

		 UIO_SYSSPACE	The I/O	vector points into the kernel address

		 UIO_NOCOPY	Do not copy, already in	object.

     uio_rw	 The direction of the desired transfer,	either UIO_READ	or

     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.

     The function 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, respec-
     tively, and neither the source nor	destination addresses may be pageable.
     Thus, the function	uiomove_nofault() can be called	from contexts where
     acquiring virtual memory system locks or sleeping are prohibited.

     The uiomove_frombuf() function is a convenience wrapper around uiomove()
     for drivers that serve data which is wholly contained within an existing
     buffer in memory.	It validates the uio_offset and	uio_resid values
     against the size of the existing buffer, handling short transfers when
     the request partially overlaps the	buffer.	 When uio_offset is greater
     than or equal to the buffer size, the result is success with no bytes
     transferred, effectively signaling	EOF.

     On	success	uiomove(), uiomove_frombuf(), and uiomove_nofault() will re-
     turn 0; on	error they will	return an appropriate error code.

     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 uio handling.

     /*	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)
			     data_available -= amnt;
		     } else
			     tsleep(...);    /*	wait for a better time */
	     if	(rv != 0) {
		     /*	do error cleanup here */
	     return (rv);

     uiomove() and uiomove_nofault() will fail and return the following	error
     code if:

     [EFAULT]		The invoked copyin(9) or copyout(9) returned EFAULT

     In	addition, uiomove_nofault() will fail and return the following error
     code if:

     [EFAULT]		A page fault occurs.

     read(2), readv(2),	write(2), writev(2), copyin(9),	copyout(9), sleep(9)

     The uio mechanism appeared	in some	early version of UNIX.

     This manual page was written by Jorg Wunsch.

FreeBSD	13.0			March 11, 2017			  FreeBSD 13.0


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