Section: Linux Programmer's Manual (2)
posix_fadvise - predeclare an access pattern for file data
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
Feature Test Macro Requirements for glibc (see
_POSIX_C_SOURCE >= 200112L
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.
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.
The specified data will be accessed in the near future.
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.)
The specified data will not be accessed in the near future.
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
Requests to discard partial pages are ignored.
It is preferable to preserve needed data than discard unneeded data.
If the application requires that data be considered for discarding, then
must be page-aligned.
attempt to write back dirty pages in the specified region,
but this is not guaranteed.
Any unwritten dirty pages will not be freed.
If the application wishes to ensure that dirty pages will be released,
it should call
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.
is the error specified by POSIX,
but before kernel version 2.6.16,
in this case.)
Kernel support first appeared in Linux 2.5.60;
the underlying system call is called
Library support has been provided since glibc version 2.2,
via the wrapper function
Since Linux 3.18,
support for the underlying system call is optional,
depending on the setting of the
Note that the type of the
argument was changed from
in POSIX.1-2001 TC1.
Under Linux, POSIX_FADV_NORMAL
sets the readahead window to the
default size for the backing device; POSIX_FADV_SEQUENTIAL
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).
The contents of the kernel buffer cache can be cleared via the
interface described in
One can obtain a snapshot of which pages of a file are resident
in the buffer cache by opening a file, mapping it with
and then applying
to the mapping.
C library/kernel differences
The name of the wrapper function in the C library is
The underlying system call is called
(or, on some architectures,
the difference between the two is that the former system call
assumes that the type of the len
argument is size_t
while the latter expects loff_t
Some architectures require
64-bit arguments to be aligned in a suitable pair of registers (see
for further detail).
On such architectures, the call signature of
shown in the SYNOPSIS would force
a register to be wasted as padding between the
Therefore, these architectures define a version of the
system call that orders the arguments suitably,
but is otherwise exactly the same as
For example, since Linux 2.6.14, ARM has the following system call:
long arm_fadvise64_64(int fd, int advice,
loff_t offset, loff_t len);
These architecture-specific details are generally
hidden from applications by the glibc
which invokes the appropriate architecture-specific system call.
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".
This page is part of release 5.10 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at