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 feature_test_macros
_POSIX_C_SOURCE >= 200112L
Programs can use posix_fadvise
() to announce an intention to access file
data in a specific pattern in the future, thus allowing the kernel to perform
applies to a (not necessarily existent) region starting at
and extending for len
bytes (or until the end of the file
is 0) within the file referred to by fd
is not binding; it merely constitutes an expectation on behalf
of the application.
Permissible values for advice
- 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
- 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
- POSIX_FADV_DONTNEED attempts to free cached pages
associated with the specified region. This is useful, for example, while
streaming large files. A program may periodically request the kernel to
free cached data that has already been used, so that more useful cached
pages are not discarded instead.
- 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
offset and len must be page-aligned.
- The implementation may 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 fsync(2) or
On success, zero is returned. On error, an error number is returned.
- The fd argument was not a valid file
- An invalid value was specified for advice.
- The specified file descriptor refers to a pipe or FIFO.
(ESPIPE is the error specified by POSIX, but before kernel version
2.6.16, Linux returned EINVAL in this case.)
Kernel support first appeared in Linux 2.5.60; the underlying system call is
(). Library support has been provided since glibc
version 2.2, via the wrapper function posix_fadvise
Since Linux 3.18, support for the underlying system call is optional, depending
on the setting of the CONFIG_ADVISE_SYSCALLS
POSIX.1-2001, POSIX.1-2008. Note that the type of the len
changed from size_t
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,
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 proc
One can obtain a snapshot of which pages of a file are resident in the buffer
cache by opening a file, mapping it with mmap
(2), and then applying
(2) to the mapping.
The name of the wrapper function in the C library is posix_fadvise
underlying system call is called fadvise64
() (or, on some
Some architectures require 64-bit arguments to be aligned in a suitable pair of
registers (see syscall
(2) for further detail). On such architectures,
the call signature of posix_fadvise
() shown in the SYNOPSIS would force
a register to be wasted as padding between the fd
arguments. 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 posix_fadvise
() wrapper function, which invokes the
appropriate architecture-specific system call.
In kernels before 2.6.6, if len
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".
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