uio(9s)uio(9s)NAME
uio - General: Describes I/O, either single vector or multiple vectors
SYNOPSIS -----------------------------
Member Name Data Type
-----------------------------
uio_iov struct iovec *
uio_iovcnt int
uio_offset off_t
uio_segflg enum uio_seg
uio_resid int
uio_rw enum uio_rw
-----------------------------
MEMBERS
Specifies a pointer to the first iovec structure. The iovec structure has two members: one that specifies the address of the segment and
another that specifies the size of the segment. The system allocates contiguous iovec structures for a given transfer. Specifies the num-
ber of iovec structures for this transfer. Specifies the offset within the file. Specifies the segment type. This member can be set to
one of the following values: UIO_USERSPACE (the segment is from the user data space), UIO_SYSSPACE (the segment is from the system space),
or UIO_USERISPACE (the segment is from the user I space). Specifies the number of bytes that still need to be transferred. Specifies
whether the transfer is a read or a write. This member is set by read and write system calls according to the corresponding field in the
file descriptor. This member can be set to one of the following values: UIO_READ (read transfer), UIO_WRITE (write transfer), or UIO_AIORW
(Alpha I/O read/write transfer).
DESCRIPTION
The uio data structure describes, either singler-vector or multiple-vector I/O. Typically, kernel modules do not manipulate the members of
this data structure. However, the data structure is presented here for the purpose of understanding the uiomove routine, which operates on
the members of the uio structure.
FILES SEE ALSO
Routines: uiomove(9r)uio(9s)
Check Out this Related Man Page
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 struc-
ture. 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 appropriate 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 Jorg Wunsch.
BSD March 21, 2010 BSD