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     physio — initiate I/O on raw devices


     #include <sys/param.h>
     #include <sys/systm.h>
     #include <sys/bio.h>
     #include <sys/buf.h>

     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
     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

     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.


     read(2), write(2)


     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.