MADVISE(2)

NAME
madvise - give advice about use of memory

SYNOPSIS
#include <sys/mman.h>

int madvise(void *start, size_t length, int advice);

DESCRIPTION
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()) 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).

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, 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).

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.

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 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).

HISTORY
The madvise() function first appeared in 4.4BSD.

CONFORMING TO
POSIX.1b. POSIX.1-2001 describes posix_madvise() with constants POSIX_MADV_NORMAL, etc., with a behaviour close to that described here. There is a similar posix_fadvise() for file access.

MADV_REMOVE, MADV_DONTFORK, and MADV_DOFORK are Linux specific.

SEE ALSO
getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)