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;
size_t uio_resid;
enum uio_seg uio_segflg;
enum uio_rw uio_rw;
struct thread *uio_td;
};
int
uiomove(caddr_t 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
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.
RETURN VALUESuiomove() can return EFAULT from the invoked copyin(9) or copyout(9) in
case the transfer was to/from 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(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;
}
SEE ALSOread(2), readv(2), write(2), writev(2), copyin(9), copyout(9), physio(9),
sleep(9)HISTORY
The uio mechanism appeared in some early version of UNIX.
AUTHORS
This man page was written by Jörg Wunsch.
BSD November 10, 2008 BSD
