posix_fadvise — predeclare an access pattern for file data
#define _XOPEN_SOURCE 600 #include <fcntl.h>
int posix_fadvise( |
int fd, |
| off_t offset, | |
| off_t len, | |
int advice); |
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_NORMALIndicates 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_SEQUENTIALThe application expects to access the specified data sequentially (with lower offsets read before higher ones).
POSIX_FADV_RANDOMThe specified data will be accessed in random order.
POSIX_FADV_NOREUSEThe specified data will be accessed only once.
POSIX_FADV_WILLNEEDThe specified data will be accessed in the near future.
POSIX_FADV_DONTNEEDThe specified data will not be accessed in the near future.
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. (Linux actually returns EINVAL in this case.)
posix_fadvise() appeared in
kernel 2.5.60. Glibc support has been provided since version
2.2.
POSIX.1-2001. Note that the type of the len argument was changed from
size_t to off_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,
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.
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.
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".
readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3), feature_test_macros(7)
This page is part of release 3.25 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Copyright 2003 Abhijit Menon-Sen <amswiw.org> 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. 2005-04-08 mtk, noted kernel version and added BUGS |