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]](../stylesheet/note.png) |
Note | ||
|---|---|---|---|
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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.
ERRORS
- EBADF
-
The
fdargument 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".
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() |