bsd/man/man2/mmap.2

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  28 .\"     @(#)mmap.2      8.4 (Berkeley) 5/11/95
  29 .\" $FreeBSD: src/lib/libc/sys/mmap.2,v 1.56 2007/01/09 00:28:15 imp Exp $
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  31 .Dd April 21, 2006
  32 .Dt MMAP 2
  33 .Os
  34 .Sh NAME
  35 .Nm mmap
  36 .Nd allocate memory, or map files or devices into memory
  37 .Sh LIBRARY
  38 .Lb libc
  39 .Sh SYNOPSIS
  40 .In sys/mman.h
  41 .Ft void *
  42 .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
  43 .Sh DESCRIPTION
  44 The
  45 .Fn mmap
  46 system call causes the pages starting at
  47 .Fa addr
  48 and continuing for at most
  49 .Fa len
  50 bytes to be mapped from the object described by
  51 .Fa fd ,
  52 starting at byte offset
  53 .Fa offset .
  54 If
  55 .Fa offset
  56 or
  57 .Fa len
  58 is not a multiple of the pagesize, the mapped region may extend past the
  59 specified range.
  60 Any extension beyond the end of the mapped object will be zero-filled.
  61 .Pp
  62 The
  63 .Fa addr
  64 argument is used by the system to determine the starting address of the mapping,
  65 and its interpretation is dependent on the setting of the MAP_FIXED flag.
  66 If MAP_FIXED is specified in
  67 .Fa flags ,
  68 the system will try to place the mapping at the specified address,
  69 possibly removing a
  70 mapping that already exists at that location.
  71 If MAP_FIXED is not specified,
  72 then the system will attempt to use the range of addresses starting at
  73 .Fa addr
  74 if they do not overlap any existing mappings,
  75 including memory allocated by malloc(3) and other such allocators.
  76 Otherwise,
  77 the system will choose an alternate address for the mapping (using an implementation
  78 dependent algorithm)
  79 that does not overlap any existing
  80 mappings.
  81 In other words,
  82 without MAP_FIXED the system will attempt to find an empty location in the address space if the specified address
  83 range has already been mapped by something else.
  84 If
  85 .Fa addr
  86 is zero and MAP_FIXED is not specified,
  87 then an address will be selected by the system so as not to overlap
  88 any existing mappings in the address space.
  89 In all cases,
  90 the actual starting address of the region is returned.
  91 If MAP_FIXED is specified,
  92 a successful
  93 .Fa mmap
  94 deletes any previous mapping in the allocated address range.
  95 Previous mappings are never deleted if MAP_FIXED is not specified.
  96 .Pp
  97 The protections (region accessibility) are specified in the
  98 .Fa prot
  99 argument by
 100 .Em or Ns 'ing
 101 the following values:
 102 .Pp
 103 .Bl -tag -width PROT_WRITE -compact
 104 .It Dv PROT_NONE
 105 Pages may not be accessed.
 106 .It Dv PROT_READ
 107 Pages may be read.
 108 .It Dv PROT_WRITE
 109 Pages may be written.
 110 .It Dv PROT_EXEC
 111 Pages may be executed.
 112 .El
 113 .Pp
 114 Note that, due to hardware limitations, on some platforms PROT_WRITE may
 115 imply PROT_READ, and PROT_READ may imply PROT_EXEC.  Portable programs
 116 should not rely on these flags being separately enforcable.
 117 .Pp
 118 The
 119 .Fa flags
 120 argument specifies the type of the mapped object, mapping options and
 121 whether modifications made to the mapped copy of the page are private
 122 to the process (copy-on-write) or are to be shared with other references.
 123 Sharing, mapping type and options are specified in the
 124 .Fa flags
 125 argument by
 126 .Em or Ns 'ing
 127 the following values:
 128 .Bl -tag -width MAP_HASSEMAPHORE
 129 .It Dv MAP_ANON
 130 Map anonymous memory not associated with any specific file.
 131 The
 132 .Fa offset
 133 argument is ignored.
 134 Mac OS X specific: the file descriptor used for creating
 135 .Dv MAP_ANON
 136 regions can be used to pass some Mach VM flags, and can
 137 be specified as \-1 if no such flags are associated with
 138 the region.  Mach VM flags are defined in
 139 <mach/vm_statistics.h> and the ones that currently apply
 140 to
 141 .Nm mmap
 142 are:
 143 .Pp
 144 VM_FLAGS_PURGABLE       to create Mach purgable (i.e. volatile) memory
 145 .Pp
 146 VM_MAKE_TAG(tag)        to associate an 8-bit tag with the region
 147 .br
 148 <mach/vm_statistics.h> defines some preset tags (with a VM_MEMORY_ prefix).
 149 Users are encouraged to use tags between 240 and 255.
 150 Tags are used by tools such as vmmap(1) to help identify specific memory regions.
 151 .It Dv MAP_FILE
 152 Mapped from a regular file.  (This is
 153 the default mapping type, and need not be specified.)
 154 .It Dv MAP_FIXED
 155 Do not permit the system to select a different address than the one
 156 specified.
 157 If the specified address cannot be used,
 158 .Fn mmap
 159 will fail.
 160 If
 161 .Dv MAP_FIXED
 162 is specified,
 163 .Fa addr
 164 must be a multiple of the pagesize.
 165 If a
 166 .Dv MAP_FIXED
 167 request is successful, the mapping established by
 168 .Fn mmap
 169 replaces any previous mappings for the process' pages in the range from
 170 .Fa addr
 171 to
 172 .Fa addr
 173 +
 174 .Fa len .
 175 Use of this option is discouraged.
 176 .It Dv MAP_HASSEMAPHORE
 177 Notify the kernel that the region may contain semaphores and that special
 178 handling may be necessary.
 179 .It Dv MAP_PRIVATE
 180 Modifications are private (copy-on-write).
 181 .It Dv MAP_SHARED
 182 Modifications are shared.
 183 .It Dv MAP_NOCACHE
 184 Pages in this mapping are not retained in the kernel's memory cache.
 185 If the system runs low on memory, pages in MAP_NOCACHE mappings will be among
 186 the first to be reclaimed.
 187 This flag is intended for mappings that have little locality and
 188 provides a hint to the kernel that pages in this mapping are unlikely to be needed
 189 again in the near future.
 190 .El
 191 .Pp
 192 Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
 193 .Pp
 194 The
 195 .Xr close 2
 196 system call does not unmap pages, see
 197 .Xr munmap 2
 198 for further information.
 199 .Pp
 200 The current design does not allow a process to specify the location of
 201 swap space.
 202 In the future we may define an additional mapping type,
 203 .Dv MAP_SWAP ,
 204 in which
 205 the file descriptor argument specifies a file or device to which swapping
 206 should be done.
 207 .Sh RETURN VALUES
 208 Upon successful completion,
 209 .Fn mmap
 210 returns a pointer to the mapped region.
 211 Otherwise, a value of
 212 .Dv MAP_FAILED
 213 is returned and
 214 .Va errno
 215 is set to indicate the error.
 216 .Sh ERRORS
 217 The
 218 .Fn mmap
 219 system call
 220 will fail if:
 221 .Bl -tag -width Er
 222 .It Bq Er EACCES
 223 The flag
 224 .Dv PROT_READ
 225 was specified as part of the
 226 .Fa prot
 227 argument and
 228 .Fa fd
 229 was not open for reading.
 230 The flags
 231 .Dv MAP_SHARED
 232 and
 233 .Dv PROT_WRITE
 234 were specified as part of the
 235 .Fa flags
 236 and
 237 .Fa prot
 238 argument and
 239 .Fa fd
 240 was not open for writing.
 241 .It Bq Er EBADF
 242 The
 243 .Fa fd
 244 argument
 245 is not a valid open file descriptor.
 246 .It Bq Er EINVAL
 247 .Dv MAP_FIXED
 248 was specified and the
 249 .Fa addr
 250 argument was not page aligned, or part of the desired address space
 251 resides out of the valid address space for a user process.
 252 .It Bq Er EINVAL
 253 .Fa flags
 254 does not include either MAP_PRIVATE or MAP_SHARED.
 255 .It Bq Er EINVAL
 256 The
 257 .Fa len
 258 argument
 259 was negative.
 260 .It Bq Er EINVAL
 261 The
 262 .Fa offset
 263 argument
 264 was not page-aligned based on the page size as returned by getpagesize(3).
 265 .It Bq Er ENODEV
 266 .Dv MAP_ANON
 267 has not been specified and the file
 268 .Fa fd
 269 refers to does not support mapping.
 270 .It Bq Er ENOMEM
 271 .Dv MAP_FIXED
 272 was specified and the
 273 .Fa addr
 274 argument was not available.
 275 .Dv MAP_FIXED
 276 was specified and the address range specified exceeds the address space
 277 limit for the process.
 278 .Dv MAP_ANON
 279 was specified and insufficient memory was available.
 280 .It Bq Er ENXIO
 281 Addresses in the specified range are invalid for
 282 .Fa fd .
 283 .It Bq Er EOVERFLOW
 284 Addresses in the specified range exceed the maximum offset
 285 set for
 286 .Fa fd .
 287 .El
 288 .Sh LEGACY SYNOPSIS
 289 .Fd #include <sys/types.h>
 290 .Fd #include <sys/mman.h>
 291 .Pp
 292 The include file
 293 .In sys/types.h
 294 is necessary.
 295 .Sh COMPATIBILITY
 296 .Fn mmap
 297 now returns with
 298 .Va errno
 299 set to EINVAL in places that historically succeeded.
 300 The rules have changed as follows:
 301 .Bl -bullet
 302 .It
 303 The
 304 .Fa flags
 305 parameter must specify either MAP_PRIVATE or MAP_SHARED.
 306 .It
 307 The
 308 .Fa size
 309 parameter must not be 0.
 310 .It
 311 The
 312 .Fa off
 313 parameter must be a multiple of pagesize,
 314 as returned by
 315 .Fn sysconf .
 316 .El
 317 .Sh SEE ALSO
 318 .Xr madvise 2 ,
 319 .Xr mincore 2 ,
 320 .Xr minherit 2 ,
 321 .Xr mlock 2 ,
 322 .Xr mprotect 2 ,
 323 .Xr msync 2 ,
 324 .Xr munlock 2 ,
 325 .Xr munmap 2 ,
 326 .Xr shmat 2 ,
 327 .Xr getpagesize 3