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     uio, uiomove — device driver I/O routines


     #include <sys/types.h>
     #include <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 */

     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

     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 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 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

     uio_segflg  One of the following flags:

                 UIO_USERSPACE  The I/O vector points into a process's address

                 UIO_SYSSPACE   The I/O vector points into the kernel address

                 UIO_NOCOPY     Do not copy, already in object.

     uio_rw      The direction of the desired transfer, either UIO_READ, or

     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.


     On success uiomove() will return 0, on error it will return an
     appropriate errno.


     uiomove() will fail and return the following error code if:

     [EFAULT]           The invoked copyin(9) or copyout(9) returned EFAULT


     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)


     The uio mechanism appeared in some early version of UNIX.


     This manual page was written by Jörg Wunsch.