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	 pro-
	      cess’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 mad-
       vise 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)