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.