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_ANONYMOUS
 130 Synonym for
 131 .Dv MAP_ANON.
 132 .It Dv MAP_ANON
 133 Map anonymous memory not associated with any specific file.
 134 The
 135 .Fa offset
 136 argument is ignored.
 137 Mac OS X specific: the file descriptor used for creating
 138 .Dv MAP_ANON
 139 regions can be used to pass some Mach VM flags, and can
 140 be specified as \-1 if no such flags are associated with
 141 the region.  Mach VM flags are defined in
 142 <mach/vm_statistics.h> and the ones that currently apply
 143 to
 144 .Nm mmap
 145 are:
 146 .Pp
 147 VM_FLAGS_PURGABLE       to create Mach purgable (i.e. volatile) memory.
 148 .Pp
 149 VM_MAKE_TAG(tag)        to associate an 8-bit tag with the region.
 150 .br
 151 <mach/vm_statistics.h> defines some preset tags (with a VM_MEMORY_ prefix).
 152 Users are encouraged to use tags between 240 and 255.
 153 Tags are used by tools such as vmmap(1) to help identify specific memory regions.
 154 .Pp
 155 .It Dv MAP_FILE
 156 Mapped from a regular file.  (This is
 157 the default mapping type, and need not be specified.)
 158 .It Dv MAP_FIXED
 159 Do not permit the system to select a different address than the one
 160 specified.
 161 If the specified address cannot be used,
 162 .Fn mmap
 163 will fail.
 164 If
 165 .Dv MAP_FIXED
 166 is specified,
 167 .Fa addr
 168 must be a multiple of the pagesize.
 169 If a
 170 .Dv MAP_FIXED
 171 request is successful, the mapping established by
 172 .Fn mmap
 173 replaces any previous mappings for the process' pages in the range from
 174 .Fa addr
 175 to
 176 .Fa addr
 177 +
 178 .Fa len .
 179 Use of this option is discouraged.
 180 .It Dv MAP_HASSEMAPHORE
 181 Notify the kernel that the region may contain semaphores and that special
 182 handling may be necessary.
 183 .It Dv MAP_PRIVATE
 184 Modifications are private (copy-on-write).
 185 .It Dv MAP_SHARED
 186 Modifications are shared.
 187 .It Dv MAP_NOCACHE
 188 Pages in this mapping are not retained in the kernel's memory cache.
 189 If the system runs low on memory, pages in MAP_NOCACHE mappings will be among
 190 the first to be reclaimed.
 191 This flag is intended for mappings that have little locality and
 192 provides a hint to the kernel that pages in this mapping are unlikely to be needed
 193 again in the near future.
 194 .El
 195 .Pp
 196 Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
 197 .Pp
 198 The
 199 .Xr close 2
 200 system call does not unmap pages, see
 201 .Xr munmap 2
 202 for further information.
 203 .Pp
 204 The current design does not allow a process to specify the location of
 205 swap space.
 206 In the future we may define an additional mapping type,
 207 .Dv MAP_SWAP ,
 208 in which
 209 the file descriptor argument specifies a file or device to which swapping
 210 should be done.
 211 .Sh RETURN VALUES
 212 Upon successful completion,
 213 .Fn mmap
 214 returns a pointer to the mapped region.
 215 Otherwise, a value of
 216 .Dv MAP_FAILED
 217 is returned and
 218 .Va errno
 219 is set to indicate the error.
 220 .Sh ERRORS
 221 The
 222 .Fn mmap
 223 system call
 224 will fail if:
 225 .Bl -tag -width Er
 226 .It Bq Er EACCES
 227 The flag
 228 .Dv PROT_READ
 229 was specified as part of the
 230 .Fa prot
 231 argument and
 232 .Fa fd
 233 was not open for reading.
 234 The flags
 235 .Dv MAP_SHARED
 236 and
 237 .Dv PROT_WRITE
 238 were specified as part of the
 239 .Fa flags
 240 and
 241 .Fa prot
 242 argument and
 243 .Fa fd
 244 was not open for writing.
 245 .It Bq Er EBADF
 246 The
 247 .Fa fd
 248 argument
 249 is not a valid open file descriptor.
 250 .It Bq Er EINVAL
 251 .Dv MAP_FIXED
 252 was specified and the
 253 .Fa addr
 254 argument was not page aligned, or part of the desired address space
 255 resides out of the valid address space for a user process.
 256 .It Bq Er EINVAL
 257 .Fa flags
 258 does not include either MAP_PRIVATE or MAP_SHARED.
 259 .It Bq Er EINVAL
 260 The
 261 .Fa len
 262 argument
 263 was negative or zero. Historically, the system call would not return an error if the argument was zero.
 264 See other potential additional restrictions in the
 265 COMPATIBILITY section below.
 266 .It Bq Er EINVAL
 267 The
 268 .Fa offset
 269 argument
 270 was not page-aligned based on the page size as returned by getpagesize(3).
 271 .It Bq Er ENODEV
 272 .Dv MAP_ANON
 273 has not been specified and the file
 274 .Fa fd
 275 refers to does not support mapping.
 276 .It Bq Er ENOMEM
 277 .Dv MAP_FIXED
 278 was specified and the
 279 .Fa addr
 280 argument was not available.
 281 .Dv MAP_FIXED
 282 was specified and the address range specified exceeds the address space
 283 limit for the process.
 284 .Dv MAP_ANON
 285 was specified and insufficient memory was available.
 286 .It Bq Er ENXIO
 287 Addresses in the specified range are invalid for
 288 .Fa fd .
 289 .It Bq Er EOVERFLOW
 290 Addresses in the specified range exceed the maximum offset
 291 set for
 292 .Fa fd .
 293 .El
 294 .Sh LEGACY SYNOPSIS
 295 .Fd #include <sys/types.h>
 296 .Fd #include <sys/mman.h>
 297 .Pp
 298 The include file
 299 .In sys/types.h
 300 is necessary.
 301 .Sh COMPATIBILITY
 302 .Fn mmap
 303 now returns with
 304 .Va errno
 305 set to EINVAL in places that historically succeeded.
 306 The rules have changed as follows:
 307 .Bl -bullet
 308 .It
 309 The
 310 .Fa flags
 311 parameter must specify either MAP_PRIVATE or MAP_SHARED.
 312 .It
 313 The
 314 .Fa len
 315 parameter must not be 0.
 316 .It
 317 The
 318 .Fa off
 319 parameter must be a multiple of pagesize,
 320 as returned by
 321 .Fn sysconf .
 322 .El
 323 .Sh SEE ALSO
 324 .Xr madvise 2 ,
 325 .Xr mincore 2 ,
 326 .Xr minherit 2 ,
 327 .Xr mlock 2 ,
 328 .Xr mprotect 2 ,
 329 .Xr msync 2 ,
 330 .Xr munlock 2 ,
 331 .Xr munmap 2 ,
 332 .Xr shmat 2 ,
 333 .Xr getpagesize 3