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, size_t 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 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.

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 ALSO
read(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.