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UIO(9)                 FreeBSD Kernel Developer's Manual                UIO(9)

     uio, uiomove, uiomove_nofault - 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);

     uiomove_nofault(void *buf, int howmuch, struct uio *uiop);

     The functions uiomove() and 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 struct uio
     being passed.  The transfer request is encoded in this structure.  The
     driver itself should use uiomove() or uiomove_nofault() 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 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_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.

     The function 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 uiomove_nofault() can be called from
     contexts where acquiring virtual memory system locks or sleeping are

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

     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(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)
                             data_available -= amnt;
                     } else
                             tsleep(...);    /* wait for a better time */
             if (rv != 0) {
                     /* do error cleanup here */
             return (rv);

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

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

     In addition, uiomove_nofault() will fail and return the following error
     code if:

     [EFAULT]           A page fault occurs.

     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 Jorg Wunsch.

FreeBSD 11.0-PRERELEASE        January 19, 2012        FreeBSD 11.0-PRERELEASE


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