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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(struct cdev *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 determined by
dev->si_iosize_max. The physio() call converts the I/O request into a strategy() request and passes the
new request to the driver'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 condition 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() function 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 requested 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.