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32 .\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
39 .Nd map files or devices into memory
41 .Fd #include <sys/types.h>
42 .Fd #include <sys/mman.h>
44 .Fn mmap "void * addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
48 function causes the pages starting at
50 and continuing for at most
52 bytes to be mapped from the object described by
54 starting at byte offset
58 is not a multiple of the pagesize, the mapped region may extend past the
60 Any such extension beyond the end of the mapped object will be zero-filled.
64 is non-zero, it is used as a hint to the system.
65 (As a convenience to the system, the actual address of the region may differ
66 from the address supplied.)
69 is zero, an address will be selected by the system.
70 The actual starting address of the region is returned.
73 deletes any previous mapping in the allocated address range.
75 The protections (region accessibility) are specified in the
81 .Bl -tag -width MAP_FIXEDX
83 Pages may be executed.
92 parameter 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
101 .Bl -tag -width MAP_FIXEDX
103 Map anonymous memory not associated with any specific file.
104 The file descriptor used for creating
109 parameter is ignored.
111 .\"Mapped from a regular file or character-special device memory.
113 Do not permit the system to select a different address than the one
115 If the specified address cannot be used,
118 If MAP_FIXED is specified,
120 must be a multiple of the pagesize.
121 Use of this option is discouraged.
122 .It Dv MAP_HASSEMAPHORE
123 Notify the kernel that the region may contain semaphores and that special
124 handling may be necessary.
126 Permit regions to be inherited across
130 Modifications are private.
132 Modifications are shared.
134 This option is only available if your system has been compiled with
135 VM_STACK defined when compiling the kernel. This is the default for
136 i386 only. Consider adding -DVM_STACK to COPTFLAGS in your /etc/make.conf
137 to enable this option for other architechures. MAP_STACK implies
144 must include at least PROT_READ and PROT_WRITE. This option creates
145 a memory region that grows to at most
147 bytes in size, starting from the stack top and growing down. The
148 stack top is the starting address returned by the call, plus
150 bytes. The bottom of the stack at maximum growth is the starting
151 address returned by the call.
156 function does not unmap pages, see
158 for further information.
160 The current design does not allow a process to specify the location of
162 In the future we may define an additional mapping type,
165 the file descriptor argument specifies a file or device to which swapping
168 Upon successful completion,
170 returns a pointer to the mapped region.
171 Otherwise, a value of MAP_FAILED is returned and
173 is set to indicate the error.
181 was specified as part of the
185 was not open for reading.
190 were specified as part of the
196 was not open for writing.
199 is not a valid open file descriptor.
202 was specified and the
204 parameter was not page aligned, or part of the desired address space
205 resides out of the valid address space for a user process.
211 was specified and the
213 parameter was not -1.
216 has not been specified and
218 did not reference a regular or character special file.
221 was not page-aligned. (See BUGS below.)
224 was specified and the
226 parameter wasn't available.
228 was specified and insufficient memory was available.
240 is limited to 2GB. Mmapping slightly more than 2GB doesn't work, but
241 it is possible to map a window of size (filesize % 2GB) for file sizes
242 of slightly less than 2G, 4GB, 6GB and 8GB.
244 The limit is imposed for a variety of reasons. Most of them have to do
247 not wanting to use 64 bit offsets in the VM system due to
248 the extreme performance penalty. So
250 uses 32bit page indexes and
253 a maximum of 8TB filesizes. It's actually bugs in
254 the filesystem code that causes the limit to be further restricted to
255 1TB (loss of precision when doing blockno calculations).
257 Another reason for the 2GB limit is that filesystem metadata can
258 reside at negative offsets.
260 We currently can only deal with page aligned file offsets.