freebsd-nq/lib/libc/sys/madvise.2

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.\"	@(#)madvise.2	8.1 (Berkeley) 6/9/93
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.Dd Jul 19, 1996
.Dt MADVISE 2
.Os
.Sh NAME
.Nm madvise
.Nd give advice about use of memory
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/mman.h>
.Ft int
.Fn madvise "void *addr" "size_t len" "int behav"
.Sh DESCRIPTION
The
.Fn madvise
system call
allows a process that has knowledge of its memory behavior
to describe it to the system.
The known behaviors are given in
.Aq Pa sys/mman.h :
.Bd -literal
#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 */
#define MADV_NOSYNC	6 /* no explicit commit to physical backing store */
#define MADV_AUTOSYNC	7 /* default commit method to physical backing store */
#define MADV_NOCORE	8 /* do not include these pages in a core file */
#define MADV_CORE	9 /* revert to including pages in a core file */
.Ed
.Pp
.Bl -tag -width MADV_SEQUENTIAL 
.It Dv MADV_NORMAL
Tells the system to revert to the default paging
behavior.
.It Dv MADV_RANDOM
Is a hint that pages will be accessed randomly, and prefetching
is likely not advantageous.
.It Dv MADV_SEQUENTIAL
Causes the VM system to depress the priority of
pages immediately preceding a given page when it is faulted in.
.It Dv 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.
.It Dv 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.
.It Dv 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 
.Xr 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 
.Dv 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.
.It Dv 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
filesystem update daemon from gratuitously writing pages dirtied
by the VM system to physical disk.  Note that VM/filesystem 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.
.Pp
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
.Xr mmap 2
.It Dv 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 guarentee reversion by using the
.Xr msync 2
or
.Xr fsync 2
system calls.
.It Dv MADV_NOCORE
Region is not included in a core file.
.It Dv MADV_CORE
Include region in a core file.
.El
.Sh RETURN VALUES
Upon successful completion,
.Fn madvise
returns 0.  Otherwise a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn madvise
function will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The virtual address range specified by the
.Fa addr
and 
.Fa len
arguments is not valid.
.El
.Sh SEE ALSO
.Xr mincore 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 .
.Sh HISTORY
The
.Fn madvise
function first appeared in
.Bx 4.4 .