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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 1.7 2008/10/06 21:01:37 swildner Exp $ 35.\" 36.Dd October 6, 2008 37.Dt MADVISE 2 38.Os 39.Sh NAME 40.Nm madvise , 41.Nm posix_madvise , 42.Nm mcontrol 43.Nd give advice about use of memory 44.Sh LIBRARY 45.Lb libc 46.Sh SYNOPSIS 47.In sys/types.h 48.In sys/mman.h 49.Ft int 50.Fn madvise "void *addr" "size_t len" "int behav" 51.Ft int 52.Fn posix_madvise "void *addr" "size_t len" "int behav" 53.Ft int 54.Fn mcontrol "void *addr" "size_t len" "int behav" "off_t value" 55.Sh DESCRIPTION 56The 57.Fn madvise 58system call 59allows a process that has knowledge of its memory behavior 60to describe it to the system. 61The 62.Fn posix_madvise 63interface is identical and is provided for standards conformance. 64The 65.Fn mcontrol 66system call is an extension of 67.Fn madvise 68that takes an additional 69.Fa value 70argument (see the description of the 71.Dv MADV_SETMAP 72behavior below). 73.Pp 74The known behaviors are: 75.Bl -tag -width MADV_SEQUENTIAL 76.It Dv MADV_NORMAL 77Tells the system to revert to the default paging 78behavior. 79.It Dv MADV_RANDOM 80Is a hint that pages will be accessed randomly, and prefetching 81is likely not advantageous. 82.It Dv MADV_SEQUENTIAL 83Causes the VM system to depress the priority of 84pages immediately preceding a given page when it is faulted in. 85.It Dv MADV_WILLNEED 86Causes pages that are in a given virtual address range 87to temporarily have higher priority, and if they are in 88memory, decrease the likelihood of them being freed. Additionally, 89the pages that are already in memory will be immediately mapped into 90the process, thereby eliminating unnecessary overhead of going through 91the entire process of faulting the pages in. This WILL NOT fault 92pages in from backing store, but quickly map the pages already in memory 93into the calling process. 94.It Dv MADV_DONTNEED 95Allows the VM system to decrease the in-memory priority 96of pages in the specified range. Additionally future references to 97this address range will incur a page fault. 98.It Dv MADV_FREE 99Gives the VM system the freedom to free pages, 100and tells the system that information in the specified page range 101is no longer important. This is an efficient way of allowing 102.Xr malloc 3 103to free pages anywhere in the address space, while keeping the address space 104valid. The next time that the page is referenced, the page might be demand 105zeroed, or might contain the data that was there before the 106.Dv MADV_FREE 107call. 108References made to that address space range will not make the VM system 109page the information back in from backing store until the page is 110modified again. 111.It Dv MADV_NOSYNC 112Request that the system not flush the data associated with this map to 113physical backing store unless it needs to. Typically this prevents the 114filesystem update daemon from gratuitously writing pages dirtied 115by the VM system to physical disk. Note that VM/filesystem coherency is 116always maintained, this feature simply ensures that the mapped data is 117only flush when it needs to be, usually by the system pager. 118.Pp 119This feature is typically used when you want to use a file-backed shared 120memory area to communicate between processes (IPC) and do not particularly 121need the data being stored in that area to be physically written to disk. 122With this feature you get the equivalent performance with mmap that you 123would expect to get with SysV shared memory calls, but in a more controllable 124and less restrictive manner. However, note that this feature is not portable 125across 126.Ux 127platforms (though some may do the right thing by default). 128For more information see the MAP_NOSYNC section of 129.Xr mmap 2 130.It Dv MADV_AUTOSYNC 131Undoes the effects of MADV_NOSYNC for any future pages dirtied within the 132address range. The effect on pages already dirtied is indeterminate - they 133may or may not be reverted. You can guarantee reversion by using the 134.Xr msync 2 135or 136.Xr fsync 2 137system calls. 138.It Dv MADV_NOCORE 139Region is not included in a core file. 140.It Dv MADV_CORE 141Include region in a core file. 142.It Dv MADV_INVAL 143Invalidate the hardware page table for a region of memory, forcing 144accesses to re-fault the pages. 145This command is primarily meant to be used in areas of memory 146governed by a virtual page table after modifications have been made 147to it. 148.It Dv MADV_SETMAP 149Set the offset of the page directory page to 150.Fa value 151for the virtual page table governing 152the specified area of memory. The entire memory area under virtual page table 153management should be specified. You may encounter unexpected effects 154if you only set the page directory page for part of the mapping. 155.El 156.Pp 157Portable programs that call the 158.Fn posix_madvise 159interface should use the aliases 160.Dv POSIX_MADV_NORMAL , POSIX_MADV_SEQUENTIAL , 161.Dv POSIX_MADV_RANDOM , POSIX_MADV_WILLNEED , 162and 163.Dv POSIX_MADV_DONTNEED 164rather than the flags described above. 165.Sh RETURN VALUES 166.Rv -std madvise posix_madvise mcontrol 167.Sh ERRORS 168The 169.Fn madvise , 170.Fn posix_madvise , 171and 172.Fn mcontrol 173functions will fail if: 174.Bl -tag -width Er 175.It Bq Er EINVAL 176The 177.Fa behav 178argument is not valid or the virtual address range specified by the 179.Fa addr 180and 181.Fa len 182arguments is not valid. 183.El 184.Sh SEE ALSO 185.Xr mincore 2 , 186.Xr mprotect 2 , 187.Xr msync 2 , 188.Xr munmap 2 189.Sh STANDARDS 190The 191.Fn posix_madvise 192interface conforms to 193.St -p1003.1-2001 . 194.Sh HISTORY 195The 196.Fn madvise 197function first appeared in 198.Bx 4.4 . 199The 200.Fn mcontrol 201function was added in 202.Dx 1.7 . 203