madvise — give advice about use of memory
#include <sys/mman.h>
int
madvise( |
void * | start, |
size_t | length, | |
int | advice) ; |
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).
On success madvise
() returns
zero. On error, it returns −1 and errno
is set appropriately.
A kernel resource was temporarily unavailable.
The map exists, but the area maps something that isn't a file.
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).
(for MADV_WILLNEED) Paging in this area would exceed the process's maximum resident set size.
(for MADV_WILLNEED) Not enough memory: paging in failed.
Addresses in the specified range are not currently mapped, or are outside the address space of the process.
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).
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.
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)
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