freebsd man page for physio

PHYSIO(9)						   BSD Kernel Developer's Manual						 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(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.

BSD
								 January 19, 2012							       BSD