NAME
madvise - give advice about use of memory
SYNOPSIS
#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
DESCRIPTION
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
reloading 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).
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, 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.
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 nonstandard 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).
SEE ALSO
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)
COLOPHON
This page is part of release 3.24 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/.