directio(3C)						   Standard C Library Functions 					      directio(3C)

NAME

       directio - provide advice to file system

SYNOPSIS

       #include <sys/types.h>
       #include <sys/fcntl.h>

       int directio(int fildes, int advice);

DESCRIPTION

       The  directio()	function  provides advice to the system about the expected behavior of the application when accessing the data in the file
       associated with the open file descriptor fildes. The system uses this information to help optimize accesses to the file's data.
	The directio() function has no effect on the semantics of the other operations on the data, though it may affect the performance of  other
       operations.

       The  advice  argument  is  kept per file; the last caller of directio() sets the advice for all applications using the file associated with
       fildes.

       Values for advice are defined in <sys/fcntl.h>.

       DIRECTIO_OFF    Applications get the default system behavior when accessing file data.

		       When an application reads data from a file, the data is first cached in system memory and then  copied  into  the  applica-
		       tion's  buffer  (see  read(2)).	If the system detects that the application is reading sequentially from a file, the system
		       will asynchronously "read ahead" from the file into system memory so the data is immediately available for the next read(2)
		       operation.

		       When  an  application  writes data into a file, the data is first cached in system memory and is written to the device at a
		       later time (see write(2)). When possible, the system increases the performance of write(2) operations by cacheing the  data
		       in  memory  pages. The data is copied into system memory and the write(2) operation returns immediately to the application.
		       The data is later written asynchronously to the device. When possible, the cached data is "clustered" into large chunks and
		       written to the device in a single write operation.

		       The system behavior for	DIRECTIO_OFF can change without notice.

       DIRECTIO_ON     The  system  behaves  as though the application is not going to reuse the file data in the near future. In other words, the
		       file data is not cached in the system's memory pages.

		       When possible, data is read or written directly between the application's memory and the device when the data  is  accessed
		       with  read(2) and write(2) operations. When such transfers are not possible, the system switches back to the default behav-
		       ior, but just for that operation. In general, the transfer is possible when the application's buffer is aligned on  a  two-
		       byte (short) boundary, the offset into the file is on a device sector boundary, and the size of the operation is a multiple
		       of device sectors.

		       This advisory is ignored while the file associated with fildes is mapped (see mmap(2)).

		       The system behavior for	DIRECTIO_ON can change without notice.

RETURN VALUES

       Upon successful completion, directio() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.

ERRORS

       The directio() function will fail if:

       EBADF	       The fildes argument is not a valid open file descriptor.

       ENOTTY	       The fildes argument is not associated with a file system that accepts advisory functions.

       EINVAL	       The value in advice is invalid.

USAGE

       Small sequential I/O generally performs best with  DIRECTIO_OFF.

       Large sequential I/O generally performs best with  DIRECTIO_ON, except when a file is sparse or is being extended and is opened with O_SYNC
       or O_DSYNC (see	open(2)).

       The directio() function is supported for the NFS and UFS file system types (see fstyp(1M)).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       fstyp(1M), mmap(2), open(2), read(2), write(2), fcntl.h(3HEAD), attributes(5)

WARNINGS

       Switching  between   DIRECTIO_OFF  and  DIRECTIO_ON can slow the system because each switch to DIRECTIO_ON might entail flushing the file's
       data from the system's memory.

SunOS 5.10							    9 Apr 2003							      directio(3C)