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 .\" 4. 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
36 .Nd allocate memory, or map files or devices into memory
42 .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
46 system call causes the pages starting at
48 and continuing for at most
50 bytes to be mapped from the object described by
52 starting at byte offset
56 is not a multiple of the page size, the mapped region may extend past the
58 Any such extension beyond the end of the mapped object will be zero-filled.
62 references a regular file or a shared memory object, the range of
67 bytes must be legitimate for the possible (not necessarily
68 current) offsets in the object.
71 value cannot be negative.
72 If the object is truncated and the process later accesses a page that
73 is wholly within the truncated region, the access is aborted and a
75 signal is delivered to the process.
79 references a device file, the interpretation of the
81 value is device specific and defined by the device driver.
82 The virtual memory subsystem does not impose any restrictitions on the
84 value in this case, passing it unchanged to the driver.
88 is non-zero, it is used as a hint to the system.
89 (As a convenience to the system, the actual address of the region may differ
90 from the address supplied.)
93 is zero, an address will be selected by the system.
94 The actual starting address of the region is returned.
97 deletes any previous mapping in the allocated address range.
99 The protections (region accessibility) are specified in the
103 the following values:
105 .Bl -tag -width PROT_WRITE -compact
107 Pages may not be accessed.
111 Pages may be written.
113 Pages may be executed.
118 argument specifies the type of the mapped object, mapping options and
119 whether modifications made to the mapped copy of the page are private
120 to the process or are to be shared with other references.
121 Sharing, mapping type and options are specified in the
125 the following values:
126 .Bl -tag -width MAP_PREFAULT_READ
128 Request a region in the first 2GB of the current process's address space.
129 If a suitable region cannot be found,
132 This flag is only available on 64-bit platforms.
133 .It Dv MAP_ALIGNED Ns Pq Fa n
134 Align the region on a requested boundary.
135 If a suitable region cannot be found,
140 argument specifies the binary logarithm of the desired alignment.
141 .It Dv MAP_ALIGNED_SUPER
142 Align the region to maximize the potential use of large
145 If a suitable region cannot be found,
148 The system will choose a suitable page size based on the size of
150 The page size used as well as the alignment of the region may both be
151 affected by properties of the file being mapped.
153 the physical address of existing pages of a file may require a specific
155 The region is not guaranteed to be aligned on any specific boundary.
157 Map anonymous memory not associated with any specific file.
158 The file descriptor used for creating
165 .\"Mapped from a regular file or character-special device memory.
167 This flag is identical to
169 and is provided for compatibility.
171 This flag can only be used in combination with
173 Please see the definition of
175 for the description of its effect.
177 Do not permit the system to select a different address than the one
179 If the specified address cannot be used,
186 must be a multiple of the page size.
189 is not specified, a successful
191 request replaces any previous mappings for the process'
192 pages in the range from
200 is specified, the request will fail if a mapping
201 already exists within the range.
202 .It Dv MAP_HASSEMAPHORE
203 Notify the kernel that the region may contain semaphores and that special
204 handling may be necessary.
206 Region is not included in a core file.
208 Causes data dirtied via this VM map to be flushed to physical media
209 only when necessary (usually by the pager) rather than gratuitously.
210 Typically this prevents the update daemons from flushing pages dirtied
211 through such maps and thus allows efficient sharing of memory across
212 unassociated processes using a file-backed shared memory map.
214 this option any VM pages you dirty may be flushed to disk every so often
215 (every 30-60 seconds usually) which can create performance problems if you
216 do not need that to occur (such as when you are using shared file-backed
217 mmap regions for IPC purposes).
218 Dirty data will be flushed automatically when all mappings of an object are
219 removed and all descriptors referencing the object are closed.
220 Note that VM/file system coherency is
221 maintained whether you use
224 This option is not portable
227 platforms (yet), though some may implement the same behavior
231 Extending a file with
233 thus creating a big hole, and then filling the hole by modifying a shared
235 can lead to severe file fragmentation.
236 In order to avoid such fragmentation you should always pre-allocate the
237 file's backing store by
239 zero's into the newly extended area prior to modifying the area via your
241 The fragmentation problem is especially sensitive to
243 pages, because pages may be flushed to disk in a totally random order.
245 The same applies when using
247 to implement a file-based shared memory store.
248 It is recommended that you create the backing store by
250 zero's to the backing file rather than
253 You can test file fragmentation by observing the KB/t (kilobytes per
254 transfer) results from an
256 while reading a large file sequentially, e.g.,\& using
257 .Dq Li dd if=filename of=/dev/null bs=32k .
261 system call will flush all dirty data and metadata associated with a file,
262 including dirty NOSYNC VM data, to physical media.
267 system call generally do not flush dirty NOSYNC VM data.
270 system call is usually not needed since
272 implements a coherent file system buffer cache.
274 used to associate dirty VM pages with file system buffers and thus cause
275 them to be flushed to physical media sooner rather than later.
276 .It Dv MAP_PREFAULT_READ
277 Immediately update the calling process's lowest-level virtual address
278 translation structures, such as its page table, so that every memory
279 resident page within the region is mapped for read access.
280 Ordinarily these structures are updated lazily.
281 The effect of this option is to eliminate any soft faults that would
282 otherwise occur on the initial read accesses to the region.
283 Although this option does not preclude
287 it does not eliminate soft faults on the initial write accesses to the
290 Modifications are private.
292 Modifications are shared.
305 must include at least
310 a memory region that grows to at most
312 bytes in size, starting from the stack top and growing down.
314 stack top is the starting address returned by the call, plus
317 The bottom of the stack at maximum growth is the starting
318 address returned by the call.
323 system call does not unmap pages, see
325 for further information.
327 Although this implementation does not impose any alignment restrictions on
330 argument, a portable program must only use page-aligned values.
332 Large page mappings require that the pages backing an object be
333 aligned in matching blocks in both the virtual address space and RAM.
334 The system will automatically attempt to use large page mappings when
335 mapping an object that is already backed by large pages in RAM by
336 aligning the mapping request in the virtual address space to match the
337 alignment of the large physical pages.
338 The system may also use large page mappings when mapping portions of an
339 object that are not yet backed by pages in RAM.
341 .Dv MAP_ALIGNED_SUPER
342 flag is an optimization that will align the mapping request to the
343 size of a large page similar to
345 except that the system will override this alignment if an object already
346 uses large pages so that the mapping will be consistent with the existing
348 This flag is mostly useful for maximizing the use of large pages on the
349 first mapping of objects that do not yet have pages present in RAM.
351 Upon successful completion,
353 returns a pointer to the mapped region.
354 Otherwise, a value of
358 is set to indicate the error.
368 was specified as part of the
372 was not open for reading.
377 were specified as part of the
383 was not open for writing.
388 is not a valid open file descriptor.
390 An invalid (negative) value was passed in the
394 referenced a regular file or shared memory.
396 An invalid value was passed in the
400 An undefined option was set in the
417 At least one of these flags must be included.
420 was specified and the
422 argument was not page aligned, or part of the desired address space
423 resides out of the valid address space for a user process.
429 were specified and part of the desired address space resides outside
430 of the first 2GB of user address space.
438 was specified and the desired alignment was either larger than the
439 virtual address size of the machine or smaller than a page.
442 was specified and the
447 was specified and the
455 were specified, but the requested region is already used by a mapping.
463 has not been specified and
465 did not reference a regular or character special file.
468 was specified and the
470 argument was not available.
472 was specified and insufficient memory was available.
projects / FreeBSD / FreeBSD.git / blob