NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
MADVISE(2) Linux Programmer's Manual MADVISE(2)
madvise - give advice about use of memory
#include <sys/mman.h>
int madvise(void *addr, size_t length, int advice);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
madvise(): _BSD_SOURCE
The madvise() system call advises the kernel about how to handle paging
input/output in the address range beginning at address addr 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 argument 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 file systems 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).
On success madvise() returns zero. On error, it returns -1 and errno is set
appropriately.
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, addr 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.
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.
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 addr 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).
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)
This page is part of release 3.23 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/.
Linux 2008-04-22 MADVISE(2)