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UIO (9) | Kernel routines | Unix Manual Pages | :man


uio, uiomove - device driver I/O routines


Return Values
See Also


.In sys/types.h
.In sys/uio.h

struct uio {
struct iovec *uio_iov;
off_t uio_offset;
enum uio_seg uio_segflg;
enum uio_rw uio_rw;
struct thread *uio_td;

int uiomove "void *buf" "int howmuch" "struct uio *uiop"


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
.Vt "struct uio" being 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
.Vt 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 Do not copy, already in object.
uio_rw The direction of the desired transfer, either UIO_READ, or UIO_WRITE.
uio_td The pointer to a
.Vt "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.


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


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(dev_t 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);


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



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