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 applica
tion, 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 aggres
sively 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 asso
ciated with it.)
RETURN VALUE
On success madvise returns zero. On error, it returns -1
and errno is set appropiately.
ERRORS
EINVAL the value len is negative, start is not page-
aligned, advice is not a valid value, or the appli
cation is attempting to release locked or shared
pages (with MADV_DONTNEED).
mapped, or are outside the address space of the
process.
ENOMEM (for MADV_WILLNEED) Not enough memory - paging in
failed.
EIO (for MADV_WILLNEED) Paging in this area would
exceed the process's maximum resident set size.
EBADF the map exists, but the area maps something that
isn't a file.
EAGAIN a kernel resource was temporarily unavailable.
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.4). The Austin draft describes posix_mad
vise with constants POSIX_MADV_NORMAL, etc., with a
behaviour close to that described here. There is a similar
posix_fadvise for file access.
SEE ALSO
getrlimit(2), mmap(2), mincore(2), mprotect(2), msync(2),
munmap(2)
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