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 .Pp
 152 VM_FLAGS_SUPERPAGE_SIZE_*       to use superpages for the allocation.
 153 See <mach/vm_statistics.h> for supported architectures and sizes (or use
 154 VM_FLAGS_SUPERPAGE_SIZE_ANY to have the kernel choose a size).
 155 The specified size must be divisible by the superpage size (except for
 156 VM_FLAGS_SUPERPAGE_SIZE_ANY), and if you use MAP_FIXED, the specified address
 157 must be properly aligned. If the system cannot satisfy the request with superpages,
 158 the call will fail. Note that currently, superpages are always wired and not
 159 inherited by children of the process.
 160 .It Dv MAP_FILE
 161 Mapped from a regular file.  (This is
 162 the default mapping type, and need not be specified.)
 163 .It Dv MAP_FIXED
 164 Do not permit the system to select a different address than the one
 165 specified.
 166 If the specified address cannot be used,
 167 .Fn mmap
 168 will fail.
 169 If
 170 .Dv MAP_FIXED
 171 is specified,
 172 .Fa addr
 173 must be a multiple of the pagesize.
 174 If a
 175 .Dv MAP_FIXED
 176 request is successful, the mapping established by
 177 .Fn mmap
 178 replaces any previous mappings for the process' pages in the range from
 179 .Fa addr
 180 to
 181 .Fa addr
 182 +
 183 .Fa len .
 184 Use of this option is discouraged.
 185 .It Dv MAP_HASSEMAPHORE
 186 Notify the kernel that the region may contain semaphores and that special
 187 handling may be necessary.
 188 .It Dv MAP_PRIVATE
 189 Modifications are private (copy-on-write).
 190 .It Dv MAP_SHARED
 191 Modifications are shared.
 192 .It Dv MAP_NOCACHE
 193 Pages in this mapping are not retained in the kernel's memory cache.
 194 If the system runs low on memory, pages in MAP_NOCACHE mappings will be among
 195 the first to be reclaimed.
 196 This flag is intended for mappings that have little locality and
 197 provides a hint to the kernel that pages in this mapping are unlikely to be needed
 198 again in the near future.
 199 .El
 200 .Pp
 201 Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
 202 .Pp
 203 The
 204 .Xr close 2
 205 system call does not unmap pages, see
 206 .Xr munmap 2
 207 for further information.
 208 .Pp
 209 The current design does not allow a process to specify the location of
 210 swap space.
 211 In the future we may define an additional mapping type,
 212 .Dv MAP_SWAP ,
 213 in which
 214 the file descriptor argument specifies a file or device to which swapping
 215 should be done.
 216 .Sh RETURN VALUES
 217 Upon successful completion,
 218 .Fn mmap
 219 returns a pointer to the mapped region.
 220 Otherwise, a value of
 221 .Dv MAP_FAILED
 222 is returned and
 223 .Va errno
 224 is set to indicate the error.
 225 .Sh ERRORS
 226 The
 227 .Fn mmap
 228 system call
 229 will fail if:
 230 .Bl -tag -width Er
 231 .It Bq Er EACCES
 232 The flag
 233 .Dv PROT_READ
 234 was specified as part of the
 235 .Fa prot
 236 argument and
 237 .Fa fd
 238 was not open for reading.
 239 The flags
 240 .Dv MAP_SHARED
 241 and
 242 .Dv PROT_WRITE
 243 were specified as part of the
 244 .Fa flags
 245 and
 246 .Fa prot
 247 argument and
 248 .Fa fd
 249 was not open for writing.
 250 .It Bq Er EBADF
 251 The
 252 .Fa fd
 253 argument
 254 is not a valid open file descriptor.
 255 .It Bq Er EINVAL
 256 .Dv MAP_FIXED
 257 was specified and the
 258 .Fa addr
 259 argument was not page aligned, or part of the desired address space
 260 resides out of the valid address space for a user process.
 261 .It Bq Er EINVAL
 262 .Fa flags
 263 does not include either MAP_PRIVATE or MAP_SHARED.
 264 .It Bq Er EINVAL
 265 The
 266 .Fa len
 267 argument
 268 was negative.
 269 .It Bq Er EINVAL
 270 The
 271 .Fa offset
 272 argument
 273 was not page-aligned based on the page size as returned by getpagesize(3).
 274 .It Bq Er ENODEV
 275 .Dv MAP_ANON
 276 has not been specified and the file
 277 .Fa fd
 278 refers to does not support mapping.
 279 .It Bq Er ENOMEM
 280 .Dv MAP_FIXED
 281 was specified and the
 282 .Fa addr
 283 argument was not available.
 284 .Dv MAP_FIXED
 285 was specified and the address range specified exceeds the address space
 286 limit for the process.
 287 .Dv MAP_ANON
 288 was specified and insufficient memory was available.
 289 .It Bq Er ENXIO
 290 Addresses in the specified range are invalid for
 291 .Fa fd .
 292 .It Bq Er EOVERFLOW
 293 Addresses in the specified range exceed the maximum offset
 294 set for
 295 .Fa fd .
 296 .El
 297 .Sh LEGACY SYNOPSIS
 298 .Fd #include <sys/types.h>
 299 .Fd #include <sys/mman.h>
 300 .Pp
 301 The include file
 302 .In sys/types.h
 303 is necessary.
 304 .Sh COMPATIBILITY
 305 .Fn mmap
 306 now returns with
 307 .Va errno
 308 set to EINVAL in places that historically succeeded.
 309 The rules have changed as follows:
 310 .Bl -bullet
 311 .It
 312 The
 313 .Fa flags
 314 parameter must specify either MAP_PRIVATE or MAP_SHARED.
 315 .It
 316 The
 317 .Fa size
 318 parameter must not be 0.
 319 .It
 320 The
 321 .Fa off
 322 parameter must be a multiple of pagesize,
 323 as returned by
 324 .Fn sysconf .
 325 .El
 326 .Sh SEE ALSO
 327 .Xr madvise 2 ,
 328 .Xr mincore 2 ,
 329 .Xr minherit 2 ,
 330 .Xr mlock 2 ,
 331 .Xr mprotect 2 ,
 332 .Xr msync 2 ,
 333 .Xr munlock 2 ,
 334 .Xr munmap 2 ,
 335 .Xr shmat 2 ,
 336 .Xr getpagesize 3