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 behavior, 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_FAILURE.

       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 copied to a new page, 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 normally invisible to (i.e., does not change the
              semantics of) the calling process.  This feature is intended for testing of memory error-handling code; it is only
              available if the kernel was configured 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 regu-
              larly scans those areas of user memory that have been marked as mergeable, looking for pages with  identical  con-
              tent.  These are replaced by a single 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  feature  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  Documenta-
              tion/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 madvise() 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 3.25 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)