DragonFly On-Line Manual Pages

UIO(9)                DragonFly Kernel Developer's Manual               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 Jorg Wunsch.

DragonFly 4.1                  January 16, 2015                  DragonFly 4.1