DIRECTIO(3C)DIRECTIO(3C)NAMEdirectio - provide advice to file system
int directio(int fildes, int advice);
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 opera‐
tions on the data, though it may affect the performance of other opera‐
The advice argument is kept per file; the last caller of directio()
sets the advice for all applications using the file associated with
Values for advice are defined in <sys/fcntl.h>.
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) opera‐
tions 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 possi‐
ble, 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 with‐
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 sys‐
tem'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 opera‐
tion. 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 sec‐
tor boundary, and the size of the operation is a multi‐
ple 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
Upon successful completion, directio() returns 0. Otherwise, it returns
−1 and sets errno to indicate the error.
The directio() function will fail if:
The fildes argument is not a valid open file descriptor.
The fildes argument is not associated with a file system that
accepts advisory functions.
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 NFS and UFS file system
types (see fstyp(1M)).
See attributes(5) for descriptions of the following attributes:
│ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│MT-Level │ MT-Safe │
SEE ALSOfstyp(1M), mmap(2), open(2), read(2), write(2), fcntl.h(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.
Apr 9, 2003 DIRECTIO(3C)