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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 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  pro-
              cess.

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
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-04-22                        MADVISE(2)