Scroll to navigation

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


uio, uiomove, uiomove_frombuf, uiomove_nofaultdevice driver I/O routines


#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_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 being passed. The transfer request is encoded in this structure. The driver itself should use () or uiomove_nofault() to get at the data in this structure.

The fields in the uio structure are:

The array of I/O vectors to be processed. In the case of scatter/gather I/O, this will be more than one vector.
The number of I/O vectors present.
The offset into the device.
The remaining number of bytes to process, updated after transfer.
One of the following flags:
The I/O vector points into a process's address space.
The I/O vector points into the kernel address space.
Do not copy, already in object.
The direction of the desired transfer, either UIO_READ or UIO_WRITE.
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 () 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 uiomove_nofault() can be called from contexts where acquiring virtual memory system locks or sleeping are prohibited.

The () 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 return 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:

The invoked copyin(9) or copyout(9) returned

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

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 Jörg Wunsch.

March 11, 2017 Debian