man-pages/man2/madvise.2

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.\" Copyright (C) 2001 David G<>mez <davidge@jazzfree.com>
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.\" Based on comments from mm/filemap.c. Last modified on 10-06-2001
.\" Modified, 25 Feb 2002, Michael Kerrisk, <mtk-manpages@gmx.net>
.\"	Added notes on MADV_DONTNEED
.\"
.TH MADVISE 2 2001-06-10 "Linux 2.4.5" "Linux Programmer's Manual"
.SH NAME
madvise \- give advice about use of memory
.SH SYNOPSIS
.br
.B #include <sys/mman.h>
.sp
.BI "int madvise(void *" start ", size_t " length ", int " advice );
.SH DESCRIPTION
The
.B madvise
system call advises the kernel about how to handle paging input/output in
the address range beginning at address
.I start
and with size
.I 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
.BR MADV_DONTNEED ),
but
may influence its performance. The kernel is free to ignore the advice.
.LP
The advice is indicated in the 
.I advice
parameter which can be
.TP
.B MADV_NORMAL
No special treatment. This is the default.
.TP
.B MADV_RANDOM
Expect page references in random order.
(Hence, read ahead may be less useful than normally.)
.TP
.B 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.)
.TP
.B MADV_WILLNEED
Expect access in the near future.
(Hence, it might be a good idea to read some pages ahead.)
.TP
.B 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 \fBmmap\fP) or zero-fill-on-demand pages for mappings
without an underlying file.
.SH "RETURN VALUE"
On success 
.B madvise
returns zero. On error, it returns \-1 and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EAGAIN
A kernel resource was temporarily unavailable.
.TP
.B EBADF
The map exists, but the area maps something that isn't a file.
.TP
.B EINVAL
The value 
.IR len
is negative,
.\" .I len
.\" is zero,
.I start
is not page-aligned,
.I advice 
is not a valid value, or the application is attempting
to release locked or shared pages (with MADV_DONTNEED).
.TP
.B EIO
(for MADV_WILLNEED) Paging in this area would exceed the process's
maximum resident set size.
.TP
.B ENOMEM
(for MADV_WILLNEED) Not enough memory: paging in failed.
.TP
.B ENOMEM
Addresses in the specified range are not currently
mapped, or are outside the address space of the process.
.SH "LINUX NOTES"
.LP
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.
.LP
The Linux implementation requires that the address
.I start
be page-aligned, and allows
.I length
to be zero. If there are some parts of the specified address range
that are not mapped, the Linux version of
.B madvise
ignores them and applies the call to the rest (but returns 
.B ENOMEM
from the system call, as it should).
.SH HISTORY
The
.B madvise
function first appeared in 4.4BSD.
.SH "CONFORMING TO"
POSIX.1b (POSIX.4).
POSIX 1003.1-2001 describes
.B posix_madvise
with constants POSIX_MADV_NORMAL, etc.,
with a behaviour close to that described here. There is a similar
.I posix_fadvise
for file access.
.SH "SEE ALSO"
.BR getrlimit (2),
.BR mincore (2),
.BR mmap (2),
.BR mprotect (2),
.BR msync (2),
.BR munmap (2)