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 @(#)madvise.2 8.1 (Berkeley) 6/9/93

.Dd April 19, 2008
.Dt MADVISE 2
.Os
.Sh NAME
.Nm madvise
.Nd give advice about use of memory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
n 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.

p
The known behaviors are:
l -tag -width MADV_NORMAL t Dv MADV_NORMAL Tells the system to revert to the default paging
behavior.
t Dv MADV_RANDOM Is a hint that pages will be accessed randomly, and prefetching
is likely not advantageous.
t Dv MADV_SEQUENTIAL Causes the VM system to depress the priority of
pages immediately preceding a given page when it is faulted in.
t 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.
t 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.
t 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

p
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:
l -tag -width Er t 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
x 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
was invented in
.Nx 5.0 .