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MADVISE(2)		    BSD	System Calls Manual		    MADVISE(2)

NAME
     madvise --	give advice about use of memory

SYNOPSIS
     #include <sys/types.h>
     #include <sys/mman.h>

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

DESCRIPTION
     The madvise() system call allows a	process	that has knowledge of its mem-
     ory behavior to describe it to the	system.	 The known behaviors are given
     in	<sys/mman.h>:

     #define MADV_NORMAL     0 /* no further special treatment */
     #define MADV_RANDOM     1 /* expect random	page references	*/
     #define MADV_SEQUENTIAL 2 /* expect sequential references */
     #define MADV_WILLNEED   3 /* will need these pages	*/
     #define MADV_DONTNEED   4 /* don't	need these pages */
     #define MADV_FREE	     5 /* data is now unimportant */

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

     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 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
		      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 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.

RETURN VALUES
     Upon successful completion, madvise() returns 0.  Otherwise a value of -1
     is	returned and errno is set to indicate the error.

ERRORS
     The madvise() function will fail if:

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

SEE ALSO
     mincore(2), mprotect(2), msync(2),	munmap(2).

HISTORY
     The madvise() function first appeared in 4.4BSD.

BSD				 Jul 19, 1996				   BSD

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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