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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 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(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).

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 The value len is negative, addr is not page-aligned,  advice  is
              not  a  valid value, or the application is attempting to release
              locked or shared pages (with MADV_DONTNEED).

       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, and MADV_DOFORK 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 non-standard 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

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.