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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_HASSEMAPHORE
102 Map anonymous memory not associated with any specific file.
103 The file descriptor used for creating
110 .\"Mapped from a regular file or character-special device memory.
112 This flag is identical to
114 and is provided for compatibility.
116 Do not permit the system to select a different address than the one
118 If the specified address cannot be used,
125 must be a multiple of the pagesize.
128 request is successful, the mapping established by
130 replaces any previous mappings for the process' pages in the range from
136 Use of this option is discouraged.
137 .It Dv MAP_HASSEMAPHORE
138 Notify the kernel that the region may contain semaphores and that special
139 handling may be necessary.
141 This flag never operated as advertised and is no longer supported.
144 for further information.
146 Region is not included in a core file.
148 Causes data dirtied via this VM map to be flushed to physical media
149 only when necessary (usually by the pager) rather than gratuitously.
150 Typically this prevents the update daemons from flushing pages dirtied
151 through such maps and thus allows efficient sharing of memory across
152 unassociated processes using a file-backed shared memory map.
154 this option any VM pages you dirty may be flushed to disk every so often
155 (every 30-60 seconds usually) which can create performance problems if you
156 do not need that to occur (such as when you are using shared file-backed
157 mmap regions for IPC purposes).
158 Note that VM/file system coherency is
159 maintained whether you use
162 This option is not portable
165 platforms (yet), though some may implement the same behavior
169 Extending a file with
171 thus creating a big hole, and then filling the hole by modifying a shared
173 can lead to severe file fragmentation.
174 In order to avoid such fragmentation you should always pre-allocate the
175 file's backing store by
177 zero's into the newly extended area prior to modifying the area via your
179 The fragmentation problem is especially sensitive to
181 pages, because pages may be flushed to disk in a totally random order.
183 The same applies when using
185 to implement a file-based shared memory store.
186 It is recommended that you create the backing store by
188 zero's to the backing file rather than
191 You can test file fragmentation by observing the KB/t (kilobytes per
192 transfer) results from an
194 while reading a large file sequentially, e.g.\& using
195 .Dq Li dd if=filename of=/dev/null bs=32k .
199 system call will flush all dirty data and metadata associated with a file,
200 including dirty NOSYNC VM data, to physical media.
205 system call generally do not flush dirty NOSYNC VM data.
208 system call is usually not needed since
210 implements a coherent file system buffer cache.
212 used to associate dirty VM pages with file system buffers and thus cause
213 them to be flushed to physical media sooner rather than later.
214 .It Dv MAP_PREFAULT_READ
215 Immediately update the calling process's lowest-level virtual address
216 translation structures, such as its page table, so that every memory
217 resident page within the region is mapped for read access.
218 Ordinarily these structures are updated lazily.
219 The effect of this option is to eliminate any soft faults that would
220 otherwise occur on the initial read accesses to the region.
221 Although this option does not preclude
225 it does not eliminate soft faults on the initial write accesses to the
228 Modifications are private.
230 Modifications are shared.
243 must include at least
248 a memory region that grows to at most
250 bytes in size, starting from the stack top and growing down.
252 stack top is the starting address returned by the call, plus
255 The bottom of the stack at maximum growth is the starting
256 address returned by the call.
261 system call does not unmap pages, see
263 for further information.
265 The current design does not allow a process to specify the location of
267 In the future we may define an additional mapping type,
270 the file descriptor argument specifies a file or device to which swapping
273 Although this implementation does not impose any alignment restrictions on
276 argument, a portable program must only use page-aligned values.
278 Upon successful completion,
280 returns a pointer to the mapped region.
281 Otherwise, a value of
285 is set to indicate the error.
295 was specified as part of the
299 was not open for reading.
304 were specified as part of the
310 was not open for writing.
315 is not a valid open file descriptor.
318 was specified and the
320 argument was not page aligned, or part of the desired address space
321 resides out of the valid address space for a user process.
329 was specified and the
334 was specified and the
339 has not been specified and
341 did not reference a regular or character special file.
344 was specified and the
346 argument was not available.
348 was specified and insufficient memory was available.
364 is limited to the maximum file size or available userland address
366 Files may not be able to be made more than 1TB large on 32 bit systems
367 due to file systems restrictions and bugs, but address space is far more
369 Larger files may be possible on 64 bit systems.
371 The previous documented limit of 2GB was a documentation bug.
372 That limit has not existed since