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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 pagesize, the mapped region may extend past the
58 Any such extension beyond the end of the mapped object will be zero-filled.
62 is non-zero, it is used as a hint to the system.
63 (As a convenience to the system, the actual address of the region may differ
64 from the address supplied.)
67 is zero, an address will be selected by the system.
68 The actual starting address of the region is returned.
71 deletes any previous mapping in the allocated address range.
73 The protections (region accessibility) are specified in the
79 .Bl -tag -width PROT_WRITE -compact
81 Pages may not be accessed.
87 Pages may be executed.
92 argument specifies the type of the mapped object, mapping options and
93 whether modifications made to the mapped copy of the page are private
94 to the process or are to be shared with other references.
95 Sharing, mapping type and options are specified in the
100 .Bl -tag -width MAP_PREFAULT_READ
102 Request a region in the first 2GB of the current process's address space.
103 If a suitable region cannot be found,
106 This flag is only available on 64-bit platforms.
107 .It Dv MAP_ALIGNED Ns Pq Fa n
108 Align the region on a requested boundary.
109 If a suitable region cannot be found,
114 argument specifies the binary logarithm of the desired alignment.
115 .It Dv MAP_ALIGNED_SUPER
116 Align the region to maximize the potential use of large
119 If a suitable region cannot be found,
122 The system will choose a suitable page size based on the size of
124 The page size used as well as the alignment of the region may both be
125 affected by properties of the file being mapped.
127 the physical address of existing pages of a file may require a specific
129 The region is not guaranteed to be aligned on any specific boundary.
131 Map anonymous memory not associated with any specific file.
132 The file descriptor used for creating
139 .\"Mapped from a regular file or character-special device memory.
141 This flag is identical to
143 and is provided for compatibility.
145 This flag can only be used in combination with
147 Please see the definition of
149 for the description of its effect.
151 Do not permit the system to select a different address than the one
153 If the specified address cannot be used,
160 must be a multiple of the pagesize.
163 is not specified, a successful
165 request replaces any previous mappings for the process'
166 pages in the range from
174 is specified, the request will fail if a mapping
175 already exists within the range.
176 .It Dv MAP_HASSEMAPHORE
177 Notify the kernel that the region may contain semaphores and that special
178 handling may be necessary.
180 Region is not included in a core file.
182 Causes data dirtied via this VM map to be flushed to physical media
183 only when necessary (usually by the pager) rather than gratuitously.
184 Typically this prevents the update daemons from flushing pages dirtied
185 through such maps and thus allows efficient sharing of memory across
186 unassociated processes using a file-backed shared memory map.
188 this option any VM pages you dirty may be flushed to disk every so often
189 (every 30-60 seconds usually) which can create performance problems if you
190 do not need that to occur (such as when you are using shared file-backed
191 mmap regions for IPC purposes).
192 Dirty data will be flushed automatically when all mappings of an object are
193 removed and all descriptors referencing the object are closed.
194 Note that VM/file system coherency is
195 maintained whether you use
198 This option is not portable
201 platforms (yet), though some may implement the same behavior
205 Extending a file with
207 thus creating a big hole, and then filling the hole by modifying a shared
209 can lead to severe file fragmentation.
210 In order to avoid such fragmentation you should always pre-allocate the
211 file's backing store by
213 zero's into the newly extended area prior to modifying the area via your
215 The fragmentation problem is especially sensitive to
217 pages, because pages may be flushed to disk in a totally random order.
219 The same applies when using
221 to implement a file-based shared memory store.
222 It is recommended that you create the backing store by
224 zero's to the backing file rather than
227 You can test file fragmentation by observing the KB/t (kilobytes per
228 transfer) results from an
230 while reading a large file sequentially, e.g.,\& using
231 .Dq Li dd if=filename of=/dev/null bs=32k .
235 system call will flush all dirty data and metadata associated with a file,
236 including dirty NOSYNC VM data, to physical media.
241 system call generally do not flush dirty NOSYNC VM data.
244 system call is usually not needed since
246 implements a coherent file system buffer cache.
248 used to associate dirty VM pages with file system buffers and thus cause
249 them to be flushed to physical media sooner rather than later.
250 .It Dv MAP_PREFAULT_READ
251 Immediately update the calling process's lowest-level virtual address
252 translation structures, such as its page table, so that every memory
253 resident page within the region is mapped for read access.
254 Ordinarily these structures are updated lazily.
255 The effect of this option is to eliminate any soft faults that would
256 otherwise occur on the initial read accesses to the region.
257 Although this option does not preclude
261 it does not eliminate soft faults on the initial write accesses to the
264 Modifications are private.
266 Modifications are shared.
279 must include at least
284 a memory region that grows to at most
286 bytes in size, starting from the stack top and growing down.
288 stack top is the starting address returned by the call, plus
291 The bottom of the stack at maximum growth is the starting
292 address returned by the call.
297 system call does not unmap pages, see
299 for further information.
301 Although this implementation does not impose any alignment restrictions on
304 argument, a portable program must only use page-aligned values.
306 Large page mappings require that the pages backing an object be
307 aligned in matching blocks in both the virtual address space and RAM.
308 The system will automatically attempt to use large page mappings when
309 mapping an object that is already backed by large pages in RAM by
310 aligning the mapping request in the virtual address space to match the
311 alignment of the large physical pages.
312 The system may also use large page mappings when mapping portions of an
313 object that are not yet backed by pages in RAM.
315 .Dv MAP_ALIGNED_SUPER
316 flag is an optimization that will align the mapping request to the
317 size of a large page similar to
319 except that the system will override this alignment if an object already
320 uses large pages so that the mapping will be consistent with the existing
322 This flag is mostly useful for maximizing the use of large pages on the
323 first mapping of objects that do not yet have pages present in RAM.
325 Upon successful completion,
327 returns a pointer to the mapped region.
328 Otherwise, a value of
332 is set to indicate the error.
342 was specified as part of the
346 was not open for reading.
351 were specified as part of the
357 was not open for writing.
362 is not a valid open file descriptor.
364 An invalid value was passed in the
368 An undefined option was set in the
385 At least one of these flags must be included.
388 was specified and the
390 argument was not page aligned, or part of the desired address space
391 resides out of the valid address space for a user process.
397 were specified and part of the desired address space resides outside
398 of the first 2GB of user address space.
406 was specified and the desired alignment was either larger than the
407 virtual address size of the machine or smaller than a page.
410 was specified and the
415 was specified and the
423 were specified, but the requested region is already used by a mapping.
431 has not been specified and
433 did not reference a regular or character special file.
436 was specified and the
438 argument was not available.
440 was specified and insufficient memory was available.
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