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MADVISE(2)		    BSD	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, 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 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 al-
		      lowing 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 de-
		      mand zeroed, or might contain the	data that was there
		      before the MADV_FREE call.  References made to that ad-
		      dress 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 co-
		      herency is always	maintained, this feature simply	en-
		      sures 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	de-
     scribed 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), posix_fadvise(2)

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

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

BSD			       January 30, 2014				   BSD


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