madvise(2)

NAME

madvise, posix_madvise - give advice about use of memory

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/mman.h>
int
madvise(void *addr, size_t len, int behav);
int
posix_madvise(void *addr, size_t len, int behav);

DESCRIPTION

The madvise() system call allows a process that has knowl

edge of its memory behavior to describe it to the system. The

posix_madvise() interface
is identical and is provided for standards conformance.

The known behaviors are:

MADV_NORMAL Tells the system to revert to the default

paging behav

ior.

MADV_RANDOM Is a hint that pages will be accessed ran

domly, and

prefetching is likely not advantageous.

MADV_SEQUENTIAL Causes the VM system to depress the priori

ty of pages

immediately preceding a given page when it

is faulted
in.

MADV_WILLNEED Causes pages that are in a given virtual

address range

to temporarily have higher priority, and if

they are in
memory, decrease the likelihood of them be

ing freed.
Additionally, the pages that are already in

memory will
be immediately mapped into the process,

thereby eliminating unnecessary overhead of going

through the entire
process of faulting the pages in. This

WILL NOT fault
pages in from backing store, but quickly

map the pages
already in memory into the calling process.

MADV_DONTNEED Allows the VM system to decrease the in

memory priority

of pages in the specified range. Addition

ally future
references to this address range will incur

a page
fault.

MADV_FREE Gives the VM system the freedom to free

pages, and tells

the system that information in the speci

fied page range
is no longer important. This is an effi

cient way of
allowing malloc(3) to free pages anywhere

in the address
space, while keeping the address space

valid. The next
time that the page is referenced, the page

might be
demand zeroed, or might contain the data

that was there
before the MADV_FREE call. References made

to that
address space range will not make the VM

system page the
information back in from backing store un

til the page is
modified again.

MADV_NOSYNC Request that the system not flush the data

associated

with this map to physical backing store un

less it needs
to. Typically this prevents the file sys

tem update daemon from gratuitously writing pages dirtied

by the VM
system to physical disk. Note that VM/file

system
coherency is always maintained, this fea

ture simply
ensures that the mapped data is only flush

when it needs
to be, usually by the system pager.

This feature is typically used when you

want to use a
file-backed shared memory area to communi

cate between
processes (IPC) and do not particularly

need the data
being stored in that area to be physically

written to
disk. With this feature you get the equiv

alent performance with mmap that you would expect to

get with SysV
shared memory calls, but in a more control

lable and less
restrictive manner. However, note that

this feature is
not portable across UNIX platforms (though

some may do
the right thing by default). For more in

formation see
the MAP_NOSYNC section of mmap(2)

MADV_AUTOSYNC Undoes the effects of MADV_NOSYNC for any

future pages

dirtied within the address range. The ef

fect on pages
already dirtied is indeterminate - they may

or may not
be reverted. You can guarantee reversion

by using the
msync(2) or fsync(2) system calls.

MADV_NOCORE Region is not included in a core file.

MADV_CORE Include region in a core file.

MADV_PROTECT Informs the VM system this process should

not be killed

when the swap space is exhausted. The pro

cess must have
superuser privileges. This should be used

judiciously
in processes that must remain running for

the system to
properly function.

Portable programs that call the posix_madvise() interface

should use the
aliases POSIX_MADV_NORMAL, POSIX_MADV_SEQUENTIAL,

POSIX_MADV_RANDOM,
POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the

flags
described above.

RETURN VALUES

The madvise() function returns the value 0 if successful;

otherwise the
value -1 is returned and the global variable errno is set to

indicate the
error.

ERRORS

The madvise() system call will fail if:

[EINVAL] The behav argument is not valid.

[ENOMEM] The virtual address range specified by

the addr and

len arguments is not valid.

[EPERM] MADV_PROTECT was specified and the pro

cess does not

have superuser privileges.

SEE ALSO

mincore(2), mprotect(2), msync(2), munmap(2)

STANDARDS

The posix_madvise() interface conforms to .

HISTORY

The madvise() system call first appeared in 4.4BSD.

BSD July 19, 1996

BSD