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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.
135 This flag creates a grow-down stack area with the specified maximum
137 It is no longer special-cased and will be converted to a normal anonymous
141 calls cannot sub-map ungrown areas returned by prior
145 The entire area is now applicable to the mapping.
147 Note that the kernel itself can still create auto-grow areas but will
148 do so for the user stack in order to maintain backwards compatibility
149 with older code that might otherwise assume it can map below the user
150 stack (in particular, older pthread libraries).
151 This compatibility is deprecated and will be removed in a future release.
152 .It Dv MAP_HASSEMAPHORE
153 Notify the kernel that the region may contain semaphores and that special
154 handling may be necessary.
156 Region is not included in a core file.
158 Causes data dirtied via this VM map to be flushed to physical media
159 only when necessary (usually by the pager) rather than gratuitously.
160 Typically this prevents the update daemons from flushing pages dirtied
161 through such maps and thus allows efficient sharing of memory across
162 unassociated processes using a file-backed shared memory map.
164 this option any VM pages you dirty may be flushed to disk every so often
165 (every 30-60 seconds usually) which can create performance problems if you
166 do not need that to occur (such as when you are using shared file-backed
167 mmap regions for IPC purposes).
168 Note that VM/filesystem coherency is maintained whether you use
171 This option is not portable across
173 platforms (yet), though some may implement the same behavior
177 Extending a file with
179 thus creating a big hole, and then filling the hole by modifying a shared
181 can lead to severe file fragmentation.
182 In order to avoid such fragmentation you should always pre-allocate the
183 file's backing store by
185 zero's into the newly extended area prior to modifying the area via your
187 The fragmentation problem is especially sensitive to
189 pages, because pages may be flushed to disk in a totally random order.
191 The same applies when using
193 to implement a file-based shared memory store.
194 It is recommended that you create the backing store by
196 zero's to the backing file rather than
199 You can test file fragmentation by observing the KB/t (kilobytes per
200 transfer) results from an
202 while reading a large file sequentially, e.g.,\& using
203 .Dq Li dd if=filename of=/dev/null bs=32k .
207 function will flush all dirty data and metadata associated with a file,
208 including dirty NOSYNC VM data, to physical media.
213 system call generally do not flush dirty NOSYNC VM data.
216 system call is obsolete since
218 implements a coherent filesystem buffer cache.
220 used to associate dirty VM pages with filesystem buffers and thus cause
221 them to be flushed to physical media sooner rather than later.
223 Modifications are private.
225 Modifications are shared.
227 Map the area as a stack.
235 should include at least
240 a memory region that grows to at most
242 bytes in size, starting from the stack top and growing down.
243 The stack top is the starting address returned by the call, plus
246 The bottom of the stack at maximum growth is the starting
247 address returned by the call.
249 The entire area is reserved from the point of view of other
251 calls, even if not faulted in yet.
257 is used, you cannot count on the returned address matching the hint
259 .It Dv MAP_VPAGETABLE
260 Memory accessed via this map is not linearly mapped and will be governed
261 by a virtual page table.
262 The base address of the virtual page table may be set using
266 Virtual page tables work with anonymous memory but there
267 is no way to populate the page table so for all intents and purposes
269 can only be used when mapping file descriptors.
270 Since the kernel will update the
272 bit in the virtual page table, the mapping must R+W
273 even though actual access to the memory will be properly governed by
274 the virtual page table.
276 Addressable backing store is limited by the range supported in the virtual
278 The kernel may implement a page table abstraction capable
279 of addressing a larger range within the backing store then could otherwise
280 be mapped into memory.
285 function does not unmap pages, see
287 for further information.
289 The current design does not allow a process to specify the location of
291 In the future we may define an additional mapping type,
294 the file descriptor argument specifies a file or device to which swapping
297 Upon successful completion,
299 returns a pointer to the mapped region.
300 Otherwise, a value of
304 is set to indicate the error.
312 was specified as part of the
316 was not open for reading.
321 were specified as part of the
327 was not open for writing.
330 is not a valid open file descriptor.
333 was specified and the
335 parameter was not page aligned, or part of the desired address space
336 resides out of the valid address space for a user process.
342 was specified and the
344 parameter was not -1.
347 has not been specified and
349 did not reference a regular or character special file.
352 was not page-aligned.
355 was specified and the
357 parameter wasn't available.
359 was specified and insufficient memory was available.
360 The system has reached the per-process mmap limit specified in the