uio man page on FreeBSD

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UIO(9)			 BSD 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;	     /* 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);

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 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
		 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.

RETURN VALUES
     On success uiomove() will return 0, on error it will return an appropri‐
     ate errno.

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

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

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

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.

BSD				March 21, 2010				   BSD
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