SYNOPSIS

In sys/types.h In 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 */
};

Ft int Fn uiomove void *buf int howmuch struct uio *uiop Ft int Fn uiomove_nofault void *buf int howmuch struct uio *uiop

DESCRIPTION

The functions Fn uiomove and Fn 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 Vt struct uio being passed. The transfer request is encoded in this structure. The driver itself should use Fn uiomove or Fn uiomove_nofault 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 remaining number of bytes to process, updated after transfer.

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.

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

RETURN VALUES

On success Fn uiomove and Fn uiomove_nofault will return 0; on error they will return an appropriate error code.

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 will not work (the buffer pointer is not being advanced in case of a partial read), it is just here to demonstrate the 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)
                                break;
                        data_available -= amnt;
                } else
                        tsleep(...);    /* wait for a better time */
        }
        if (rv != 0) {
                /* do error cleanup here */
        }
        return (rv);
}

HISTORY

The mechanism appeared in some early version of UNIX

AUTHORS

This manual page was written by An Jörg Wunsch .