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Subject[OT] madvise.2
Now that I asked in public for manpages for the not-yet-documented
system calls, I suppose I should report on received pages in order
to avoid duplication. Yesterday or so I got an madvise.2 page
and polished it a bit. Current version below. Comments are welcome.
Note that comments in mm/filemap.c and what this code does in reality
differ quite a lot.

Andries

------------------------------------------------------------------
MADVISE(2) Linux Programmer's Manual 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 applica­
tion, but may influence its performance. The kernel is
free to ignore the advice.

For Linux, the address start must be page-aligned. For
Linux, if there are some parts of the specified address
range that are not mapped, madvise ignores them and
applies the call to the rest, but returns ENOMEM from the
system call. For Linux, zero length is permitted.

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).

ENOMEM addresses in the specified range are not currently
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.


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 the
behaviour 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)

Linux 2.4.5 2001-06-10 1
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