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

NAME
     uio, uiomove -- device driver I/O routines

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

     struct uio	{
	     struct  iovec *uio_iov;
	     int     uio_iovcnt;
	     off_t   uio_offset;
	     int     uio_resid;
	     enum    uio_seg uio_segflg;
	     enum    uio_rw uio_rw;
	     struct  thread *uio_td;
     };

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

DESCRIPTION
     The function uiomove() is used to handle transfer of data between buffers
     and I/O vectors that might	possibly also 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()	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.

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 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(dev_t dev,	struct uio *uio, int flag)
     {
	     int rv, amnt;

	     while (uio->uio_resid > 0)	{
		     if	(data_available	> 0) {
			     amnt = MIN(uio->uio_resid,	data_available);
			     if	((rv = uiomove(buffer, amnt, uio))
				 != 0)
				     goto error;
			     data_available -= amnt;
		     } else {
			     tsleep(...);    /*	wait for a better time */
		     }
	     }
	     return 0;
     error:
	     /*	do error cleanup here */
	     return rv;
     }

RETURN VALUES
     uiomove() can return EFAULT from the invoked copyin(9) or copyout(9) in
     case the transfer was to/from a process's address space.

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

HISTORY
     The uio mechanism appeared	in some	early version of UNIX.

AUTHORS
     This man page was written by Jorg Wunsch.

BSD			       February	2, 1997				   BSD

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | RETURN VALUES | SEE ALSO | HISTORY | AUTHORS

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