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32 .\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
33 .\" $FreeBSD: src/lib/libc/sys/mmap.2,v 1.22.2.12 2002/02/27 03:40:13 dd Exp $
34 .\" $DragonFly: src/lib/libc/sys/mmap.2,v 1.4 2005/08/01 01:49:16 swildner Exp $
41 .Nd allocate memory, or map files or devices into memory
48 .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
52 function causes the pages starting at
54 and continuing for at most
56 bytes to be mapped from the object described by
58 starting at byte offset
62 is not a multiple of the pagesize, the mapped region may extend past the
64 Any such extension beyond the end of the mapped object will be zero-filled.
68 is non-zero, it is used as a hint to the system.
69 (As a convenience to the system, the actual address of the region may differ
70 from the address supplied.)
73 is zero, an address will be selected by the system.
74 The actual starting address of the region is returned.
77 deletes any previous mapping in the allocated address range.
79 The protections (region accessibility) are specified in the
85 .Bl -tag -width PROT_WRITE -compact
87 Pages may not be accessed.
93 Pages may be executed.
98 parameter specifies the type of the mapped object, mapping options and
99 whether modifications made to the mapped copy of the page are private
100 to the process or are to be shared with other references.
101 Sharing, mapping type and options are specified in the
105 the following values:
106 .Bl -tag -width MAP_HASSEMAPHORE
108 Map anonymous memory not associated with any specific file.
109 The file descriptor used for creating
114 parameter is ignored.
116 .\"Mapped from a regular file or character-special device memory.
118 Do not permit the system to select a different address than the one
120 If the specified address cannot be used,
127 must be a multiple of the pagesize.
128 Use of this option is discouraged.
129 .It Dv MAP_HASSEMAPHORE
130 Notify the kernel that the region may contain semaphores and that special
131 handling may be necessary.
133 Region is not included in a core file.
135 Causes data dirtied via this VM map to be flushed to physical media
136 only when necessary (usually by the pager) rather then gratuitously.
137 Typically this prevents the update daemons from flushing pages dirtied
138 through such maps and thus allows efficient sharing of memory across
139 unassociated processes using a file-backed shared memory map. Without
140 this option any VM pages you dirty may be flushed to disk every so often
141 (every 30-60 seconds usually) which can create performance problems if you
142 do not need that to occur (such as when you are using shared file-backed
143 mmap regions for IPC purposes). Note that VM/filesystem coherency is
144 maintained whether you use
146 or not. This option is not portable
149 platforms (yet), though some may implement the same behavior
153 Extending a file with
155 thus creating a big hole, and then filling the hole by modifying a shared
157 can lead to severe file fragmentation.
158 In order to avoid such fragmentation you should always pre-allocate the
159 file's backing store by
161 zero's into the newly extended area prior to modifying the area via your
163 The fragmentation problem is especially sensitive to
165 pages, because pages may be flushed to disk in a totally random order.
167 The same applies when using
169 to implement a file-based shared memory store.
170 It is recommended that you create the backing store by
172 zero's to the backing file rather then
175 You can test file fragmentation by observing the KB/t (kilobytes per
176 transfer) results from an
178 while reading a large file sequentially, e.g. using
179 .Dq Li dd if=filename of=/dev/null bs=32k .
183 function will flush all dirty data and metadata associated with a file,
184 including dirty NOSYNC VM data, to physical media. The
188 system call generally do not flush dirty NOSYNC VM data.
191 system call is obsolete since
193 implements a coherent filesystem buffer cache. However, it may be
194 used to associate dirty VM pages with filesystem buffers and thus cause
195 them to be flushed to physical media sooner rather then later.
197 Modifications are private.
199 Modifications are shared.
201 This option is only available if your system has been compiled with
203 defined when compiling the kernel.
204 This is the default for
212 to enable this option for other architechures.
222 must include at least
227 a memory region that grows to at most
229 bytes in size, starting from the stack top and growing down. The
230 stack top is the starting address returned by the call, plus
232 bytes. The bottom of the stack at maximum growth is the starting
233 address returned by the call.
238 function does not unmap pages, see
240 for further information.
242 The current design does not allow a process to specify the location of
244 In the future we may define an additional mapping type,
247 the file descriptor argument specifies a file or device to which swapping
250 Upon successful completion,
252 returns a pointer to the mapped region.
253 Otherwise, a value of
257 is set to indicate the error.
265 was specified as part of the
269 was not open for reading.
274 were specified as part of the
280 was not open for writing.
283 is not a valid open file descriptor.
286 was specified and the
288 parameter was not page aligned, or part of the desired address space
289 resides out of the valid address space for a user process.
295 was specified and the
297 parameter was not -1.
300 has not been specified and
302 did not reference a regular or character special file.
305 was not page-aligned.
311 was specified and the
313 parameter wasn't available.
315 was specified and insufficient memory was available.
316 The system has reached the per-process mmap limit specified in the
331 is limited to 2GB. Mmapping slightly more than 2GB doesn't work, but
332 it is possible to map a window of size (filesize % 2GB) for file sizes
333 of slightly less than 2G, 4GB, 6GB and 8GB.
335 The limit is imposed for a variety of reasons.
336 Most of them have to do
339 not wanting to use 64 bit offsets in the VM system due to
340 the extreme performance penalty.
343 uses 32bit page indexes and
346 a maximum of 8TB filesizes.
347 It's actually bugs in
348 the filesystem code that causes the limit to be further restricted to
349 1TB (loss of precision when doing blockno calculations).
351 Another reason for the 2GB limit is that filesystem metadata can
352 reside at negative offsets.