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 application'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 behavior, 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.

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

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.

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 ufs file system type (see fstyp(1M)).

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

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

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.