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).
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 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).
This page is part of release 3.14 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/.
