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, 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_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), posix_fadvise(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.

Debian                                          January 30, 2014                                      MADVISE(2)