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 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.
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 application 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.
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 can-
not 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). POSIX 1003.1-2001 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.
SEE ALSO
getrlimit(2), mmap(2), mincore(2), mprotect(2), msync(2), munmap(2)
Linux 2.4.5 2001-06-10 MADVISE(2)