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MADVISE(2)                FreeBSD System Calls Manual               MADVISE(2)

     madvise, posix_madvise -- give advice about use of memory

     Standard C Library (libc, -lc)

     #include <sys/mman.h>

     madvise(void *addr, size_t len, int behav);

     posix_madvise(void *addr, size_t len, int behav);

     The madvise() system call allows a process that has knowledge of its mem-
     ory behavior to describe it to the system.  The posix_madvise() interface
     is identical and is provided for standards conformance.

     The known behaviors are:

     MADV_NORMAL      Tells the system to revert to the default paging behav-

     MADV_RANDOM      Is a hint that pages will be accessed randomly, and
                      prefetching is likely not advantageous.

     MADV_SEQUENTIAL  Causes the VM system to depress the priority of pages
                      immediately preceding a given page when it is faulted

     MADV_WILLNEED    Causes pages that are in a given virtual address range
                      to temporarily have higher priority, and if they are in
                      memory, decrease the likelihood of them being freed.
                      Additionally, the pages that are already in memory will
                      be immediately mapped into the process, thereby elimi-
                      nating unnecessary overhead of going through the entire
                      process of faulting the pages in.  This WILL NOT fault
                      pages in from backing store, but quickly map the pages
                      already in memory into the calling process.

     MADV_DONTNEED    Allows the VM system to decrease the in-memory priority
                      of pages in the specified range.  Additionally future
                      references to this address range will incur a page

     MADV_FREE        Gives the VM system the freedom to free pages, and tells
                      the system that information in the specified page range
                      is no longer important.  This is an efficient way of
                      allowing malloc(3) to free pages anywhere in the address
                      space, while keeping the address space valid.  The next
                      time that the page is referenced, the page might be
                      demand zeroed, or might contain the data that was there
                      before the MADV_FREE call.  References made to that
                      address space range will not make the VM system page the
                      information back in from backing store until the page is
                      modified again.

     MADV_NOSYNC      Request that the system not flush the data associated
                      with this map to physical backing store unless it needs
                      to.  Typically this prevents the file system update dae-
                      mon from gratuitously writing pages dirtied by the VM
                      system to physical disk.  Note that VM/file system
                      coherency is always maintained, this feature simply
                      ensures that the mapped data is only flush when it needs
                      to be, usually by the system pager.

                      This feature is typically used when you want to use a
                      file-backed shared memory area to communicate between
                      processes (IPC) and do not particularly need the data
                      being stored in that area to be physically written to
                      disk.  With this feature you get the equivalent perfor-
                      mance with mmap that you would expect to get with SysV
                      shared memory calls, but in a more controllable and less
                      restrictive manner.  However, note that this feature is
                      not portable across UNIX platforms (though some may do
                      the right thing by default).  For more information see
                      the MAP_NOSYNC section of mmap(2)

     MADV_AUTOSYNC    Undoes the effects of MADV_NOSYNC for any future pages
                      dirtied within the address range.  The effect on pages
                      already dirtied is indeterminate - they may or may not
                      be reverted.  You can guarantee reversion by using the
                      msync(2) or fsync(2) system calls.

     MADV_NOCORE      Region is not included in a core file.

     MADV_CORE        Include region in a core file.

     MADV_PROTECT     Informs the VM system this process should not be killed
                      when the swap space is exhausted.  The process must have
                      superuser privileges.  This should be used judiciously
                      in processes that must remain running for the system to
                      properly function.

     Portable programs that call the posix_madvise() interface should use the
     POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the flags
     described above.

     The madvise() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     The madvise() system call will fail if:

     [EINVAL]           The behav argument is not valid.

     [ENOMEM]           The virtual address range specified by the addr and
                        len arguments is not valid.

     [EPERM]            MADV_PROTECT was specified and the process does not
                        have superuser privileges.

     mincore(2), mprotect(2), msync(2), munmap(2)

     The posix_madvise() interface conforms to IEEE Std 1003.1-2001

     The madvise() system call first appeared in 4.4BSD.

FreeBSD 6.2                      July 19, 1996                     FreeBSD 6.2


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