Name

posix_fadvise — predeclare an access pattern for file data

Synopsis

        #include <fcntl.h>
int posix_fadvise( int fd,
  off_t offset,
  off_t len,
  int advice);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
posix_fadvise():
_POSIX_C_SOURCE >= 200112L

DESCRIPTION

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 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 the application.

Permissible values for advice include:

POSIX_FADV_NORMAL

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.

POSIX_FADV_SEQUENTIAL

The application expects to access the specified data sequentially (with lower offsets read before higher ones).

POSIX_FADV_RANDOM

The specified data will be accessed in random order.

POSIX_FADV_NOREUSE

The specified data will be accessed only once.

POSIX_FADV_WILLNEED

The specified data will be accessed in the near future.

POSIX_FADV_DONTNEED

The specified data will not be accessed in the near future.

RETURN VALUE

On success, zero is returned. On error, an error number is returned.

ERRORS

EBADF

The fd argument was not a valid file descriptor.

EINVAL

An invalid value was specified for advice.

ESPIPE

The specified file descriptor refers to a pipe or FIFO. (ESPIPE is the error specified by POSIX, but before kernel version 2.16, Linux returned EINVAL in this case.)

VERSIONS

Kernel support first appeared in Linux 2.5.60; the underlying system call is called fadvise64(). 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 configuration option.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008. Note that the type of the len argument was changed from size_t to off_t in POSIX.1-2003 TC1.

NOTES

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 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.

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 call fsync(2) or fdatasync(2) first.

C library/kernel differences

The name of the wrapper function in the C library is posix_fadvise(). The underlying system call is called fadvise64() (or, on some architectures, fadvise64_64()).

Architecture-specific variants

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 and offset arguments. Therefore, these architectures define a version of the system call that orders the arguments suitably, but is otherwise exactly the same as posix_fadvise().

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.

BUGS

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".

SEE ALSO

readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3)

COLOPHON

This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright 2003 Abhijit Menon-Sen <amswiw.org>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

2005-04-08 mtk, noted kernel version and added BUGS
2010-10-09, mtk, document arm_fadvise64_64()