Programs can use
to announce an intention to access
file data in a specific pattern in the future, thus allowing the kernel
to perform appropriate optimizations.
The advice applies to a (not necessarily existent) region starting
at offset and extending for len bytes (or until the end of
the file if len is 0) within the file referred to by fd.
The advice is not binding; it merely constitutes an expectation on behalf of
Permissible values for advice include:
Indicates that the application has no advice to give about its access
pattern for the specified data.
If no advice is given for an open file,
this is the default assumption.
The application expects to access the specified data sequentially (with
lower offsets read before higher ones).
The specified data will be accessed in random order.
The specified data will be accessed only once.
The specified data will be accessed in the near future.
The specified data will not be accessed in the near future.
On success, zero is returned.
On error, an error number is returned.
The fd argument was not a valid file descriptor.
An invalid value was specified for advice.
The specified file descriptor refers to a pipe or FIFO.
in this case.)
appeared in kernel 2.5.60.
Glibc support has been provided since version 2.2.
Note that the type of the
argument was changed from
in POSIX.1-2003 TC1.
Under Linux, POSIX_FADV_NORMAL sets the readahead window to the
default size for the backing device; POSIX_FADV_SEQUENTIAL doubles
this size, and POSIX_FADV_RANDOM disables file readahead entirely.
These changes affect the entire file, not just the specified region
(but other open file handles to the same file are unaffected).
POSIX_FADV_WILLNEED initiates a
nonblocking read of the specified region into the page cache.
The amount of data read may be decreased by the kernel depending
on virtual memory load.
(A few megabytes will usually be fully satisfied,
and more is rarely useful.)
In kernels before 2.6.18, POSIX_FADV_NOREUSE had the
same semantics as POSIX_FADV_WILLNEED.
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
POSIX_FADV_DONTNEED attempts to free cached pages associated with
the specified region.
This is useful, for example, while streaming large
A program may periodically request the kernel to free cached data
that has already been used, so that more useful cached pages are not
Pages that have not yet been written out will be unaffected, so if the
application wishes to guarantee that pages will be released, it should
In kernels before 2.6.6, if
was specified as 0, then this was interpreted literally as "zero bytes",
rather than as meaning "all bytes through to the end of the file".