1 .\" Copyright (c) 1991, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
7 .\" 1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\" 2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\" 3. Neither the name of the University nor the names of its contributors
13 .\" may be used to endorse or promote products derived from this software
14 .\" without specific prior written permission.
16 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
17 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
20 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 .\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
29 .\" $FreeBSD: src/lib/libc/sys/mmap.2,v 1.22.2.12 2002/02/27 03:40:13 dd Exp $
36 .Nd allocate memory, or map files or devices into memory
43 .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
47 function causes the pages starting at
49 and continuing for at most
51 bytes to be mapped from the object described by
53 starting at byte offset
57 is not a multiple of the pagesize, the mapped region may extend past the
59 Any such extension beyond the end of the mapped object will be zero-filled.
63 is non-zero, it is used as a hint to the system.
64 (As a convenience to the system, the actual address of the region may differ
65 from the address supplied.)
68 is zero, an address will be selected by the system.
69 The actual starting address of the region is returned.
72 deletes any previous mapping in the allocated address range.
74 The protections (region accessibility) are specified in the
80 .Bl -tag -width ".Dv PROT_WRITE" -compact
82 Pages may not be accessed.
88 Pages may be executed.
93 parameter specifies the type of the mapped object, mapping options and
94 whether modifications made to the mapped copy of the page are private
95 to the process or are to be shared with other references.
96 Sharing, mapping type and options are specified in the
100 the following values:
101 .Bl -tag -width ".Dv MAP_HASSEMAPHORE"
103 Map anonymous memory not associated with any specific file.
104 The file descriptor used for creating
109 parameter is ignored.
111 This flag is an alias for
113 and is provided for compatibility.
115 .\"Mapped from a regular file or character-special device memory.
117 Do not permit the system to select a different address than the one
119 If the specified address contains other mappings those mappings will
121 If the specified address cannot otherwise be used,
128 must be a multiple of the pagesize.
130 Try to do a fixed mapping but fail if another mapping already exists in
131 the space instead of overwriting the mapping.
138 made within another (typically the master user stack), as long as
139 no pages have been faulted in the area requested.
140 .It Dv MAP_HASSEMAPHORE
141 Notify the kernel that the region may contain semaphores and that special
142 handling may be necessary.
144 Region is not included in a core file.
146 Causes data dirtied via this VM map to be flushed to physical media
147 only when necessary (usually by the pager) rather than gratuitously.
148 Typically this prevents the update daemons from flushing pages dirtied
149 through such maps and thus allows efficient sharing of memory across
150 unassociated processes using a file-backed shared memory map.
152 this option any VM pages you dirty may be flushed to disk every so often
153 (every 30-60 seconds usually) which can create performance problems if you
154 do not need that to occur (such as when you are using shared file-backed
155 mmap regions for IPC purposes).
156 Note that VM/filesystem coherency is maintained whether you use
159 This option is not portable across
161 platforms (yet), though some may implement the same behavior
165 Extending a file with
167 thus creating a big hole, and then filling the hole by modifying a shared
169 can lead to severe file fragmentation.
170 In order to avoid such fragmentation you should always pre-allocate the
171 file's backing store by
173 zero's into the newly extended area prior to modifying the area via your
175 The fragmentation problem is especially sensitive to
177 pages, because pages may be flushed to disk in a totally random order.
179 The same applies when using
181 to implement a file-based shared memory store.
182 It is recommended that you create the backing store by
184 zero's to the backing file rather than
187 You can test file fragmentation by observing the KB/t (kilobytes per
188 transfer) results from an
190 while reading a large file sequentially, e.g.,\& using
191 .Dq Li dd if=filename of=/dev/null bs=32k .
195 function will flush all dirty data and metadata associated with a file,
196 including dirty NOSYNC VM data, to physical media.
201 system call generally do not flush dirty NOSYNC VM data.
204 system call is obsolete since
206 implements a coherent filesystem buffer cache.
208 used to associate dirty VM pages with filesystem buffers and thus cause
209 them to be flushed to physical media sooner rather than later.
211 Modifications are private.
213 Modifications are shared.
215 Map the area as a stack.
223 should include at least
228 a memory region that grows to at most
230 bytes in size, starting from the stack top and growing down.
231 The stack top is the starting address returned by the call, plus
234 The bottom of the stack at maximum growth is the starting
235 address returned by the call.
237 The entire area is reserved from the point of view of other
239 calls, even if not faulted in yet.
244 mappings to provide a hint that points within an existing
246 mapping's space, and this will succeed as long as no page have been
247 faulted in the area specified, but this behavior is no longer supported
248 unless you also specify the
256 is used, you cannot count on the returned address matching the hint
258 .It Dv MAP_VPAGETABLE
259 Memory accessed via this map is not linearly mapped and will be governed
260 by a virtual page table.
261 The base address of the virtual page table may be set using
265 Virtual page tables work with anonymous memory but there
266 is no way to populate the page table so for all intents and purposes
268 can only be used when mapping file descriptors.
269 Since the kernel will update the
271 bit in the virtual page table, the mapping must R+W
272 even though actual access to the memory will be properly governed by
273 the virtual page table.
275 Addressable backing store is limited by the range supported in the virtual
277 The kernel may implement a page table abstraction capable
278 of addressing a larger range within the backing store then could otherwise
279 be mapped into memory.
284 function does not unmap pages, see
286 for further information.
288 The current design does not allow a process to specify the location of
290 In the future we may define an additional mapping type,
293 the file descriptor argument specifies a file or device to which swapping
296 Upon successful completion,
298 returns a pointer to the mapped region.
299 Otherwise, a value of
303 is set to indicate the error.
311 was specified as part of the
315 was not open for reading.
320 were specified as part of the
326 was not open for writing.
329 is not a valid open file descriptor.
332 was specified and the
334 parameter was not page aligned, or part of the desired address space
335 resides out of the valid address space for a user process.
341 was specified and the
343 parameter was not -1.
346 has not been specified and
348 did not reference a regular or character special file.
351 was not page-aligned.
357 was specified and the
359 parameter wasn't available.
361 was specified and insufficient memory was available.
362 The system has reached the per-process mmap limit specified in the