PHYSIO(9)	       FreeBSD 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/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.  It calls
     back on the provided strategy routine one or more times to complete the
     transfer described by uio.  The maximum amount of data to transfer with
     each call to strategy is determined by the minphys routine.  Since uio
     normally describes user space addresses, physio() needs to lock the pro-
     cess into memory.	This is done by setting the P_PHYSIO flag on the pro-
     cess.  physio() always awaits the completion of the entire requested
     transfer before returning, unless an error condition is detected earlier.
     In all cases, the buffer passed in bp is locked (marked as ``busy'') for
     the duration of the entire transfer.

     A break-down of the arguments follows:

     strategy
	     The device strategy routine to call for each chunk of data to
	     initiate device I/O.

     bp      The buffer to use with the strategy routine.  The buffer flags
	     will have B_BUSY, and B_PHYS set when passed to the strategy rou-
	     tine.  If NULL, a buffer is allocated from a system pool.

     dev     The device number identifying the device to interact with.

     flags   Direction of transfer; the only valid settings are B_READ or
	     B_WRITE.

     minphys
	     A device specific routine called to determine the maximum trans-
	     fer size that the device's strategy routine can handle.

     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.

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 strat-
     egy 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)

FreeBSD 4.8			 June 15, 1996			   FreeBSD 4.8