NAME

       uio, uiomove, uiomove_frombuf, uiomove_nofault — device driver I/O routines

SYNOPSIS

       #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 */
       };

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

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

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

DESCRIPTION

       The  functions  uiomove(),  uiomove_frombuf(),  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 transfer.

       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_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.

       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
       prohibited.

       The  uiomove_frombuf()  function  is  a  convenience wrapper around uiomove() for drivers that serve data
       which is wholly contained within an existing buffer in memory.  It validates the uio_offset and uio_resid
       values against the size of the existing buffer, handling  short  transfers  when  the  request  partially
       overlaps  the buffer.  When uio_offset is greater than or equal to the buffer size, the result is success
       with no bytes transferred, effectively signaling EOF.

RETURN VALUES

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

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

ERRORS

       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.

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 manual page was written by Jörg Wunsch.

Debian                                           March 11, 2017                                           UIO(9)