UIO(9S) Data Structures for Drivers UIO(9S)

NAME

uio - scatter/gather I/O request structure

SYNOPSIS

#include <sys/uio.h>

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI)

DESCRIPTION

A uio structure describes an I/O request that can be broken up into
different data storage areas (scatter/gather I/O). A request is a list of
iovec structures (base-length pairs) indicating where in user space or
kernel space the I/O data is to be read or written.


The contents of uio structures passed to the driver through the entry
points should not be written by the driver. The uiomove(9F) function
takes care of all overhead related to maintaining the state of the uio
structure.


uio structures allocated by the driver should be initialized to zero
before use, by bzero(9F), kmem_zalloc(9F), or an equivalent.

STRUCTURE MEMBERS

iovec_t *uio_iov; /* pointer to start of iovec */
/* list for uio struc. */
int uio_iovcnt; /* number of iovecs in list */
off_t uio_offset; /* 32-bit offset into file where
/* data is xferred. See NOTES. */
offset_t uio_loffset; /* 64-bit offset into file where */
/* data is xferred. See NOTES. */
uio_seg_t uio_segflg; /* ID's type of I/O transfer: */
/* UIO_SYSSPACE: kernel <-> kernel */
/* UIO_USERSPACE: kernel <-> user */
uint16_t uio_fmode; /* file mode flags (not driver setable) */
daddr_t uio_limit; /* 32-bit ulimit for file (max. block */
/* offset). not driver setable. */
/* See NOTES. */
diskaddr_t uio_llimit; /* 64-bit ulimit for file (max. block */
/* offset). not driver setable. */
/* See NOTES */
ssize_t uio_resid; /* residual count */


The uio_iov member is a pointer to the beginning of the iovec(9S) list
for the uio. When the uio structure is passed to the driver through an
entry point, the driver should not set uio_iov. When the uio structure
is created by the driver, uio_iov should be initialized by the driver
and not written to afterward.

SEE ALSO

aread(9E), awrite(9E), read(9E), write(9E), bzero(9F), kmem_zalloc(9F),
uiomove(9F), cb_ops(9S), iovec(9S)


Writing Device Drivers

NOTES

Only one structure, uio_offset or uio_loffset, should be interpreted by
the driver. Which field the driver interprets is dependent upon the
settings in the cb_ops(9S) structure.


Only one structure, uio_limit or uio_llimit, should be interpreted by the
driver. Which field the driver interprets is dependent upon the settings
in the cb_ops(9S) structure.


When performing I/O on a seekable device, the driver should not modify
either the uio_offset or the uio_loffset field of the uio structure. I/O
to such a device is constrained by the maximum offset value. When
performing I/O on a device on which the concept of position has no
relevance, the driver may preserve the uio_offset or uio_loffset, perform
the I/O operation, then restore the uio_offset or uio_loffset to the
field's initial value. I/O performed to a device in this manner is not
constrained.

March 26, 2009 UIO(9S)