Name
madvise — give advice about use of memory
Synopsis
#include <sys/mman.h>
int
madvise( |
void * | start, |
| size_t | length, | |
| int | advice); |
![[Note]](../stylesheet/note.png) |
Note | ||
|---|---|---|---|
|
DESCRIPTION
The madvise() system call
advises the kernel about how to handle paging input/output in
the address range beginning at address start and with size length bytes. It allows an
application to tell the kernel how it expects to use some
mapped or shared memory areas, so that the kernel can choose
appropriate read-ahead and caching techniques. This call does
not influence the semantics of the application (except in the
case of MADV_DONTNEED), but may
influence its performance. The kernel is free to ignore the
advice.
The advice is indicated in the advice parameter which can
be
MADV_NORMAL-
No special treatment. This is the default.
MADV_RANDOM-
Expect page references in random order. (Hence, read ahead may be less useful than normally.)
MADV_SEQUENTIAL-
Expect page references in sequential order. (Hence, pages in the given range can be aggressively read ahead, and may be freed soon after they are accessed.)
MADV_WILLNEED-
Expect access in the near future. (Hence, it might be a good idea to read some pages ahead.)
MADV_DONTNEED-
Do not expect access in the near future. (For the time being, the application is finished with the given range, so the kernel can free resources associated with it.) Subsequent accesses of pages in this range will succeed, but will result either in re-loading of the memory contents from the underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file.
MADV_REMOVE(Since Linux 2.6.16)-
Free up a given range of pages and its associated backing store. Currently, only shmfs/tmpfs supports this; other filesystems return with the error ENOSYS.
MADV_DONTFORK(Since Linux 2.6.16)-
Do not make the pages in this range available to the child after a fork(2). This is useful to prevent copy-on-write semantics from changing the physical location of a page(s) if the parent writes to it after a fork(2). (Such page relocations cause problems for hardware that DMAs into the page(s).)
MADV_DOFORK(Since Linux 2.6.16)-
Undo the effect of
MADV_DONTFORK, restoring the default behavior, whereby a mapping is inherited across fork(2).
RETURN VALUE
On success madvise() returns
zero. On error, it returns −1 and errno is set appropriately.
ERRORS
- EAGAIN
-
A kernel resource was temporarily unavailable.
- EBADF
-
The map exists, but the area maps something that isn't a file.
- EINVAL
-
The value
lenis negative,startis not page-aligned,adviceis not a valid value, or the application is attempting to release locked or shared pages (withMADV_DONTNEED). - EIO
-
(for
MADV_WILLNEED) Paging in this area would exceed the process's maximum resident set size. - ENOMEM
-
(for
MADV_WILLNEED) Not enough memory: paging in failed. - ENOMEM
-
Addresses in the specified range are not currently mapped, or are outside the address space of the process.
CONFORMING TO
POSIX.1b. POSIX.1-2001 describes posix_madvise(3) with constants
POSIX_MADV_NORMAL, etc., with a
behavior close to that described here. There is a similar
posix_fadvise(2) for file
access.
MADV_REMOVE, MADV_DONTFORK, and MADV_DOFORK are Linux-specific.
NOTES
Linux Notes
The current Linux implementation (2.4.0) views this system call more as a command than as advice and hence may return an error when it cannot do what it usually would do in response to this advice. (See the ERRORS description above.) This is non-standard behavior.
The Linux implementation requires that the address
start be
page-aligned, and allows length to be zero. If there
are some parts of the specified address range that are not
mapped, the Linux version of madvise() ignores them and applies the
call to the rest (but returns ENOMEM from the system call, as it
should).
COLOPHON
This page is part of release 2.79 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/.
|
Copyright (C) 2001 David Gómez <davidgejazzfree.com> 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. Based on comments from mm/filemap.c. Last modified on 10-06-2001 Modified, 25 Feb 2002, Michael Kerrisk, <mtk.manpagesgmail.com> Added notes on MADV_DONTNEED |