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

NAME
     uio,  uiomove,  uiomove_frombuf,  uiomove_nofault -- device driver I/O rou-
     tines

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 it-
     self 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 scat-
		 ter/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 trans-
		 fer.

     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.   The	supported  flags
		 are:

		 UIO_READ   Transfer data from the buffers into the I/O vectors.

		 UIO_WRITE  Transfer data from the I/O vectors into the buffers.

     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 ad-
     dresses 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,  effec-
     tively 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 Jorg Wunsch.

FreeBSD ports 15.quarterly	October 22, 2025			  UIO(9)

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