redhat man page for madvise

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)