UIO(9)

NAME
uio, uiomove - device driver IO routines

SYNOPSIS
#include <sys/types.h>
#include <sys/cdefs.h>
#include <sys/uio.h>

    struct uio {
    	struct	iovec *uio_iov;
    	int	uio_iovcnt;
    	off_t	uio_offset;
    	int	uio_resid;
    	enum	uio_seg uio_segflg;
    	enum	uio_rw uio_rw;
    	struct	proc *uio_procp;
    };

int
uiomove(caddr_t buf, int howmuch, struct uio *uiop);

DESCRIPTION
The function uiomove() is used to handle transfer of data between buffers and IO 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 read or 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 uiomove() to get at the data in this structure.

The fields in the uio structure are:
uio_iov
    The array of IO vectors to be processed. In the case of scatter/gather IO, this will be more than one vector.
uio_iovcnt
    The number of IO 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 IO vector points into a process's address space.
    UIO_SYSSPACE
        The IO vector points into the kernel address space.
    UIO_USERISPACE
        The IO vector points into the instruction area of a process's address space.
    UIO_NOCOPY
        Don't copy, already in object.
uio_rw uio_rw
    The direction of the desired transfer, either UIO_READ, or UIO_WRITE.
uio_procp
    The pointer to a struct proc for the associated process; used if uio_segflg indicates that the transfer is to be made from/to 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(dev_t dev, struct uio *uio, int flag)
    {
    	int rv, amnt;
    	while (uio->uio_resid > 0) {
    		if (data_available > 0) {
    			amnt = MIN(uio->uio_resid, data_available);
    			if ((rv = uiomove((caddr_t)buffer, amnt, uio))
    			    != 0)
    				goto error;
    			data_available -= amnt;
    		} else {
    			tsleep(...);	/* wait for a better time */
    		}
    	}
    	return 0;
    error:
    	/* do error cleanup here */
    	return rv;
    }

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.

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

HISTORY
The uio mechanism appeared in some early version of UNIX.

AUTHORS
This man page has been written by Joerg Wunsch.