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MADVISE(2)		   Linux Programmer's Manual		    MADVISE(2)

NAME
       madvise - give advice about use of memory

SYNOPSIS
       #include <sys/mman.h>

       int madvise(void *addr, size_t length, int advice);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       madvise(): _BSD_SOURCE

DESCRIPTION
       The madvise() system call advises the kernel about how to handle paging
       input/output in the address range beginning at address  addr  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 argument 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
	      reloading of the memory contents from the underlying mapped file
	      (see  mmap(2)) or zero-fill-on-demand pages for mappings without
	      an underlying file.

       MADV_REMOVE (Since Linux 2.6.16)
	      Free up a given range of pages and its associated backing store.
	      Currently,  only	shmfs/tmpfs  supports this; other file systems
	      return with the error ENOSYS.

       MADV_DONTFORK (Since Linux 2.6.16)
	      Do not make the pages in this range available to the child after
	      a	 fork(2).   This  is useful to prevent copy-on-write semantics
	      from changing the physical location of a page(s) if  the	parent
	      writes  to  it  after  a	fork(2).  (Such page relocations cause
	      problems for hardware that DMAs into the page(s).)

       MADV_DOFORK (Since Linux 2.6.16)
	      Undo the effect of MADV_DONTFORK, restoring the  default	behav‐
	      ior, whereby a mapping is inherited across fork(2).

       MADV_HWPOISON (Since Linux 2.6.32)
	      Poison  a	 page and handle it like a hardware memory corruption.
	      This operation is only available for privileged  (CAP_SYS_ADMIN)
	      processes.   This	 operation  may	 result in the calling process
	      receiving a SIGBUS and the page being unmapped.  This feature is
	      intended	for  testing of memory error-handling code; it is only
	      available if the kernel was configured with  CONFIG_MEMORY_FAIL‐
	      URE.

       MADV_SOFT_OFFLINE (Since Linux 2.6.33)
	      Soft  offline  the  pages	 in  the  range	 specified by addr and
	      length.  The memory of each page in the specified range is  pre‐
	      served (i.e., when next accessed, the same content will be visi‐
	      ble, but in a new physical page frame), and the original page is
	      offlined	(i.e.,	no longer used, and taken out of normal memory
	      management).  The effect of the MADV_SOFT_OFFLINE	 operation  is
	      invisible	 to (i.e., does not change the semantics of) the call‐
	      ing process.  This feature is intended  for  testing  of	memory
	      error-handling code; it is only available if the kernel was con‐
	      figured with CONFIG_MEMORY_FAILURE.

       MADV_MERGEABLE (since Linux 2.6.32)
	      Enable Kernel Samepage Merging (KSM) for the pages in the	 range
	      specified	 by addr and length.  The kernel regularly scans those
	      areas of user memory that have been marked as mergeable, looking
	      for  pages with identical content.  These are replaced by a sin‐
	      gle write-protected page (which is  automatically	 copied	 if  a
	      process  later  wants  to	 update the content of the page).  KSM
	      only merges private anonymous pages (see mmap(2)).  The KSM fea‐
	      ture  is	intended for applications that generate many instances
	      of the same data (e.g., virtualization systems such as KVM).  It
	      can  consume  a lot of processing power; use with care.  See the
	      kernel source file Documentation/vm/ksm.txt  for	more  details.
	      The  MADV_MERGEABLE  and	MADV_UNMERGEABLE  operations  are only
	      available if the kernel was configured with CONFIG_KSM.

       MADV_UNMERGEABLE (since Linux 2.6.32)
	      Undo the effect of an earlier MADV_MERGEABLE  operation  on  the
	      specified	 address  range;  KSM  unmerges	 whatever pages it had
	      merged in the address range specified by addr and length.

RETURN VALUE
       On success madvise() returns zero.  On error, it returns -1  and	 errno
       is set appropriately.

ERRORS
       EAGAIN A kernel resource was temporarily unavailable.

       EBADF  The map exists, but the area maps something that isn't a file.

       EINVAL This error can occur for the following reasons:

	      *	 The value len is negative.

	      *	 addr is not page-aligned.

	      *	 advice is not a valid value

	      *	 The  application  is  attempting  to release locked or shared
		 pages (with MADV_DONTNEED).

	      *	 MADV_MERGEABLE or MADV_UNMERGEABLE was specified  in  advice,
		 but the kernel was not configured with CONFIG_KSM.

       EIO    (for  MADV_WILLNEED)  Paging  in	this  area  would  exceed  the
	      process's maximum resident set size.

       ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.

       ENOMEM Addresses in the specified range are not	currently  mapped,  or
	      are outside the address space of the process.

CONFORMING TO
       POSIX.1b.    POSIX.1-2001  describes  posix_madvise(3)  with  constants
       POSIX_MADV_NORMAL, etc., with a behavior close to that described	 here.
       There is a similar posix_fadvise(2) for file access.

       MADV_REMOVE, MADV_DONTFORK, MADV_DOFORK, MADV_HWPOISON, MADV_MERGEABLE,
       and MADV_UNMERGEABLE are Linux-specific.

NOTES
   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 cannot
       do what it usually would do in  response	 to  this  advice.   (See  the
       ERRORS description above.)  This is nonstandard behavior.

       The  Linux  implementation  requires  that  the	address	 addr 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).

SEE ALSO
       getrlimit(2), mincore(2), mmap(2), mprotect(2), msync(2), munmap(2)

COLOPHON
       This  page  is  part of release 3.27 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2010-06-20			    MADVISE(2)
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