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NAME

     madvise, posix_madvise -- give advice about use of memory

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/mman.h>

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

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

DESCRIPTION

     The madvise() system call allows a process that has knowledge of its
     memory 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
                      behavior.

     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
                      in.

     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
                      eliminating 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
                      fault.

     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
                      daemon 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
                      performance 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
     aliases POSIX_MADV_NORMAL, POSIX_MADV_SEQUENTIAL, POSIX_MADV_RANDOM,
     POSIX_MADV_WILLNEED, and POSIX_MADV_DONTNEED rather than the flags
     described above.

RETURN VALUES

     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
     error.

ERRORS

     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.

SEE ALSO

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

STANDARDS

     The posix_madvise() interface conforms to IEEE Std 1003.1-2001
     (``POSIX.1'').

HISTORY

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