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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)madvise.2 8.1 (Berkeley) 6/9/93 33.\" $FreeBSD: src/lib/libc/sys/madvise.2,v 1.17.2.8 2003/01/06 23:33:59 trhodes Exp $ 34.\" $DragonFly: src/lib/libc/sys/madvise.2,v 1.4 2006/10/09 09:48:08 swildner Exp $ 35.\" 36.Dd July 19, 1996 37.Dt MADVISE 2 38.Os 39.Sh NAME 40.Nm madvise , 41.Nm mcontrol 42.Nd give advice about use of memory 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In sys/types.h 47.In sys/mman.h 48.Ft int 49.Fn madvise "void *addr" "size_t len" "int behav" 50.Ft int 51.Fn mcontrol "void *addr" "size_t len" "int behav" "off_t value" 52.Sh DESCRIPTION 53The 54.Fn madvise 55system call 56allows a process that has knowledge of its memory behavior 57to describe it to the system. 58.Pp 59The known behaviors are: 60.Bl -tag -width MADV_SEQUENTIAL 61.It Dv MADV_NORMAL 62Tells the system to revert to the default paging 63behavior. 64.It Dv MADV_RANDOM 65Is a hint that pages will be accessed randomly, and prefetching 66is likely not advantageous. 67.It Dv MADV_SEQUENTIAL 68Causes the VM system to depress the priority of 69pages immediately preceding a given page when it is faulted in. 70.It Dv MADV_WILLNEED 71Causes pages that are in a given virtual address range 72to temporarily have higher priority, and if they are in 73memory, decrease the likelihood of them being freed. Additionally, 74the pages that are already in memory will be immediately mapped into 75the process, thereby eliminating unnecessary overhead of going through 76the entire process of faulting the pages in. This WILL NOT fault 77pages in from backing store, but quickly map the pages already in memory 78into the calling process. 79.It Dv MADV_DONTNEED 80Allows the VM system to decrease the in-memory priority 81of pages in the specified range. Additionally future references to 82this address range will incur a page fault. 83.It Dv MADV_FREE 84Gives the VM system the freedom to free pages, 85and tells the system that information in the specified page range 86is no longer important. This is an efficient way of allowing 87.Xr malloc 3 88to free pages anywhere in the address space, while keeping the address space 89valid. The next time that the page is referenced, the page might be demand 90zeroed, or might contain the data that was there before the 91.Dv MADV_FREE 92call. 93References made to that address space range will not make the VM system 94page the information back in from backing store until the page is 95modified again. 96.It Dv MADV_NOSYNC 97Request that the system not flush the data associated with this map to 98physical backing store unless it needs to. Typically this prevents the 99filesystem update daemon from gratuitously writing pages dirtied 100by the VM system to physical disk. Note that VM/filesystem coherency is 101always maintained, this feature simply ensures that the mapped data is 102only flush when it needs to be, usually by the system pager. 103.Pp 104This feature is typically used when you want to use a file-backed shared 105memory area to communicate between processes (IPC) and do not particularly 106need the data being stored in that area to be physically written to disk. 107With this feature you get the equivalent performance with mmap that you 108would expect to get with SysV shared memory calls, but in a more controllable 109and less restrictive manner. However, note that this feature is not portable 110across UNIX platforms (though some may do the right thing by default). 111For more information see the MAP_NOSYNC section of 112.Xr mmap 2 113.It Dv MADV_AUTOSYNC 114Undoes the effects of MADV_NOSYNC for any future pages dirtied within the 115address range. The effect on pages already dirtied is indeterminate - they 116may or may not be reverted. You can guarantee reversion by using the 117.Xr msync 2 118or 119.Xr fsync 2 120system calls. 121.It Dv MADV_NOCORE 122Region is not included in a core file. 123.It Dv MADV_CORE 124Include region in a core file. 125.It Dv MADV_INVAL 126Invalidate the hardware page table for a region of memory, forcing 127accesses to re-fault the pages. 128This command is primarily meant to be used in areas of memory 129governed by a virtual page table after modifications have been made 130to it. 131.It Dv MADV_SETMAP 132Set the offset of the page directory page to 133.Fa value 134for the virtual page table governing 135the specified area of memory. The entire memory area under virtual page table 136management should be specified. You may encounter unexpected effects 137if you only set the page directory page for part of the mapping. 138.El 139.Sh RETURN VALUES 140.Rv -std madvise 141.Sh ERRORS 142The 143.Fn madvise 144function will fail if: 145.Bl -tag -width Er 146.It Bq Er EINVAL 147The virtual address range specified by the 148.Fa addr 149and 150.Fa len 151arguments is not valid. 152.El 153.Sh SEE ALSO 154.Xr mincore 2 , 155.Xr mprotect 2 , 156.Xr msync 2 , 157.Xr munmap 2 158.Sh HISTORY 159The 160.Fn madvise 161function first appeared in 162.Bx 4.4 . 163