Name
madvise — give advice about use of memory
Synopsis
#include <sys/mman.h>
int
madvise( |
void * |
start, |
| |
size_t |
length, |
| |
int |
advice); |
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 -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 pagei(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
behaviour, 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 len is negative,
start is not
page-aligned, advice is not a valid
value, or the application is attempting to release
locked or shared pages (with MADV_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 behaviour close to that
described here. There is a similar posix_fadvise(3) 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 nonstandard behaviour.
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).
SEE ALSO
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)
Copyright (C) 2001 David Gómez <davidge@jazzfree.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-manpages@gmx.net>
Added notes on MADV_DONTNEED
|
