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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 $
40 .Nd allocate memory, or map files or devices into memory
47 .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
51 function causes the pages starting at
53 and continuing for at most
55 bytes to be mapped from the object described by
57 starting at byte offset
61 is not a multiple of the pagesize, the mapped region may extend past the
63 Any such extension beyond the end of the mapped object will be zero-filled.
67 is non-zero, it is used as a hint to the system.
68 (As a convenience to the system, the actual address of the region may differ
69 from the address supplied.)
72 is zero, an address will be selected by the system.
73 The actual starting address of the region is returned.
76 deletes any previous mapping in the allocated address range.
78 The protections (region accessibility) are specified in the
84 .Bl -tag -width PROT_WRITE -compact
86 Pages may not be accessed.
92 Pages may be executed.
97 parameter specifies the type of the mapped object, mapping options and
98 whether modifications made to the mapped copy of the page are private
99 to the process or are to be shared with other references.
100 Sharing, mapping type and options are specified in the
104 the following values:
105 .Bl -tag -width MAP_HASSEMAPHORE
107 Map anonymous memory not associated with any specific file.
108 The file descriptor used for creating
113 parameter is ignored.
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 cannot be used,
126 must be a multiple of the pagesize.
127 Use of this option is discouraged.
128 .It Dv MAP_HASSEMAPHORE
129 Notify the kernel that the region may contain semaphores and that special
130 handling may be necessary.
132 Region is not included in a core file.
134 Causes data dirtied via this VM map to be flushed to physical media
135 only when necessary (usually by the pager) rather then gratuitously.
136 Typically this prevents the update daemons from flushing pages dirtied
137 through such maps and thus allows efficient sharing of memory across
138 unassociated processes using a file-backed shared memory map. Without
139 this option any VM pages you dirty may be flushed to disk every so often
140 (every 30-60 seconds usually) which can create performance problems if you
141 do not need that to occur (such as when you are using shared file-backed
142 mmap regions for IPC purposes). Note that VM/filesystem coherency is
143 maintained whether you use
145 or not. This option is not portable
148 platforms (yet), though some may implement the same behavior
152 Extending a file with
154 thus creating a big hole, and then filling the hole by modifying a shared
156 can lead to severe file fragmentation.
157 In order to avoid such fragmentation you should always pre-allocate the
158 file's backing store by
160 zero's into the newly extended area prior to modifying the area via your
162 The fragmentation problem is especially sensitive to
164 pages, because pages may be flushed to disk in a totally random order.
166 The same applies when using
168 to implement a file-based shared memory store.
169 It is recommended that you create the backing store by
171 zero's to the backing file rather then
174 You can test file fragmentation by observing the KB/t (kilobytes per
175 transfer) results from an
177 while reading a large file sequentially, e.g. using
178 .Dq Li dd if=filename of=/dev/null bs=32k .
182 function will flush all dirty data and metadata associated with a file,
183 including dirty NOSYNC VM data, to physical media. The
187 system call generally do not flush dirty NOSYNC VM data.
190 system call is obsolete since
192 implements a coherent filesystem buffer cache. However, it may be
193 used to associate dirty VM pages with filesystem buffers and thus cause
194 them to be flushed to physical media sooner rather then later.
196 Modifications are private.
198 Modifications are shared.
200 This option is only available if your system has been compiled with
202 defined when compiling the kernel.
203 This is the default for
211 to enable this option for other architechures.
221 must include at least
226 a memory region that grows to at most
228 bytes in size, starting from the stack top and growing down. The
229 stack top is the starting address returned by the call, plus
231 bytes. The bottom of the stack at maximum growth is the starting
232 address returned by the call.
237 function does not unmap pages, see
239 for further information.
241 The current design does not allow a process to specify the location of
243 In the future we may define an additional mapping type,
246 the file descriptor argument specifies a file or device to which swapping
249 Upon successful completion,
251 returns a pointer to the mapped region.
252 Otherwise, a value of
256 is set to indicate the error.
264 was specified as part of the
268 was not open for reading.
273 were specified as part of the
279 was not open for writing.
282 is not a valid open file descriptor.
285 was specified and the
287 parameter was not page aligned, or part of the desired address space
288 resides out of the valid address space for a user process.
294 was specified and the
296 parameter was not -1.
299 has not been specified and
301 did not reference a regular or character special file.
304 was not page-aligned.
310 was specified and the
312 parameter wasn't available.
314 was specified and insufficient memory was available.
315 The system has reached the per-process mmap limit specified in the
330 is limited to 2GB. Mmapping slightly more than 2GB doesn't work, but
331 it is possible to map a window of size (filesize % 2GB) for file sizes
332 of slightly less than 2G, 4GB, 6GB and 8GB.
334 The limit is imposed for a variety of reasons.
335 Most of them have to do
338 not wanting to use 64 bit offsets in the VM system due to
339 the extreme performance penalty.
342 uses 32bit page indexes and
345 a maximum of 8TB filesizes.
346 It's actually bugs in
347 the filesystem code that causes the limit to be further restricted to
348 1TB (loss of precision when doing blockno calculations).
350 Another reason for the 2GB limit is that filesystem metadata can
351 reside at negative offsets.