physio(9)

NAME

physio - initiate I/O on raw devices

SYNOPSIS

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/bio.h>
#include <sys/buf.h>
int
physio(dev_t dev, struct uio *uio, int ioflag);

DESCRIPTION

The physio() is a helper function typically called from

character device
read() and write() routines to start I/O on a user process

buffer. The
maximum amount of data to transfer with each call is deter

mined by
dev->si_iosize_max. The physio() call converts the I/O re

quest into a
strategy() request and passes the new request to the driv

er's strategy()
routine for processing.

Since uio normally describes user space addresses, physio()

needs to lock
those pages into memory. This is done by calling vmapbuf()

for the
appropriate pages. physio() always awaits the completion of

the entire
requested transfer before returning, unless an error condi

tion is
detected earlier.

A break-down of the arguments follows:

dev The device number identifying the device to interact

with.

uio The description of the entire transfer as requested

by the user

process. Currently, the results of passing a uio

structure with
the uio_segflg set to anything other than

UIO_USERSPACE are undefined.

ioflag The ioflag argument from the read() or write() func

tion calling

physio().

RETURN VALUES

If successful physio() returns 0. EFAULT is returned if the

address
range described by uio is not accessible by the requesting

process.
physio() will return any error resulting from calls to the

device strategy routine, by examining the B_ERROR buffer flag and the

b_error field.
Note that the actual transfer size may be less than request

ed by uio if
the device signals an ``end of file'' condition.

SEE ALSO

read(2), write(2)

HISTORY

The physio manual page is originally from NetBSD with minor

changes for
applicability with FreeBSD.

The physio call has been completely re-written for providing

higher I/O
and paging performance.

BSD July 8, 2004

BSD