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     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 memory behavior to
     describe it to the system.  The posix_madvise() interface is identical, except it returns an
     error number on error and does not modify errno, 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_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 error.


     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

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


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


     The posix_madvise() interface conforms to IEEE Std 1003.1-2001 (“POSIX.1”).


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