Provided by: freebsd-manpages_7.0-2_all
physio - initiate I/O on raw devices
physio(dev_t dev, struct uio *uio, int ioflag);
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
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
ioflag The ioflag argument from the read() or write() function calling
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.
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.