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.\"	@(#)madvise.2	8.1 (Berkeley) 6/9/93
.\"
.Dd March 29, 2011
.Dt MADVISE 2
.Os
.Sh NAME
.Nm madvise ,
.Nm posix_madvise
.Nd give advice about use of memory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/mman.h
.Ft int
.Fn madvise "void *addr" "size_t len" "int behav"
.Ft int
.Fn posix_madvise "void *addr" "size_t len" "int advice"
.Sh DESCRIPTION
The
.Fn madvise
system call
allows a process that has knowledge of its memory behavior
to describe it to the system.
The
.Fn posix_madvise
interface is identical and is provided for standards conformance.
.Pp
The known behaviors are:
.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
Is a hint that pages will be accessed sequentially, from the lower address to
higher address.
It might cause the VM system to depress the priority of
pages immediately preceding a given page when it is faulted in.
.It Dv MADV_WILLNEED
Is a hint that pages will be accessed in the near future.
It might cause the VM system to make 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.
It might immediately map the pages that are already in memory into the
process, thereby eliminating unnecessary overhead of going through
the entire process of faulting the pages in.
It might or might not fault pages in from backing store.
.It Dv MADV_DONTNEED
Is a hint that pages will not be accessed in the near future.
It might allow the VM system to decrease the in-memory priority
of pages in the specified range.
.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.
.El
.Pp
Portable programs that call the
.Fn posix_madvise
interface should use the aliases
.Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL ,
.Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED ,
and
.Dv POSIX_MADV_DONTNEED
rather than the flags described above.
.Sh RETURN VALUES
Upon successful completion,
a value of 0 is returned.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn madvise
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
Invalid parameters were provided.
.El
.Sh SEE ALSO
.Xr mincore 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr posix_fadvise 2
.Sh STANDARDS
The
.Fn posix_madvise
system call is expected to conform to the
.St -p1003.1-2001
standard.
.Sh HISTORY
The
.Nm madvise
system call first appeared in
.Bx 4.4 ,
but until
.Nx 1.5
it did not perform any of the requests on, or change any behavior of the
address range given.
The
.Fn posix_madvise
call was added in
.Nx 5.0 .