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--- old/usr/src/man/man2/mmap.2.man.txt
+++ new/usr/src/man/man2/mmap.2.man.txt
1 1 MMAP(2) System Calls MMAP(2)
2 2
3 3
4 4
5 5 NAME
6 6 mmap - map pages of memory
7 7
8 8 SYNOPSIS
9 9 #include <sys/mman.h>
10 10
11 11 void *mmap(void *addr, size_t len, int prot, int flags,
12 12 int fildes, off_t off);
13 13
14 14
15 15 DESCRIPTION
16 16 The mmap() function establishes a mapping between a process's address
17 17 space and a file or shared memory object. The format of the call is as
18 18 follows:
19 19
20 20
21 21 pa = mmap(addr, len, prot, flags, fildes, off);
22 22
23 23
24 24 The mmap() function establishes a mapping between the address space of
25 25 the process at an address pa for len bytes to the memory object
26 26 represented by the file descriptor fildes at offset off for len bytes.
27 27 The value of pa is a function of the addr argument and values of
28 28 flags, further described below. A successful mmap() call returns pa as
29 29 its result. The address range starting at pa and continuing for len
30 30 bytes will be legitimate for the possible (not necessarily current)
31 31 address space of the process. The range of bytes starting at off and
32 32 continuing for len bytes will be legitimate for the possible (not
33 33 necessarily current) offsets in the file or shared memory object
34 34 represented by fildes.
35 35
36 36
37 37 The mmap() function allows [pa, pa + len) to extend beyond the end of
38 38 the object both at the time of the mmap() and while the mapping
39 39 persists, such as when the file is created prior to the mmap() call and
40 40 has no contents, or when the file is truncated. Any reference to
41 41 addresses beyond the end of the object, however, will result in the
42 42 delivery of a SIGBUS or SIGSEGV signal. The mmap() function cannot be
43 43 used to implicitly extend the length of files.
44 44
45 45
46 46 The mapping established by mmap() replaces any previous mappings for
47 47 those whole pages containing any part of the address space of the
48 48 process starting at pa and continuing for len bytes.
49 49
50 50
51 51 If the size of the mapped file changes after the call to mmap() as a
52 52 result of some other operation on the mapped file, the effect of
53 53 references to portions of the mapped region that correspond to added or
54 54 removed portions of the file is unspecified.
55 55
56 56
57 57 The mmap() function is supported for regular files and shared memory
58 58 objects. Support for any other type of file is unspecified.
59 59
60 60
61 61 The prot argument determines whether read, write, execute, or some
62 62 combination of accesses are permitted to the data being mapped. The
63 63 prot argument should be either PROT_NONE or the bitwise inclusive OR of
64 64 one or more of the other flags in the following table, defined in the
65 65 header <sys/mman.h>.
66 66
67 67 PROT_READ
68 68 Data can be read.
69 69
70 70
71 71 PROT_WRITE
72 72 Data can be written.
73 73
74 74
75 75 PROT_EXEC
76 76 Data can be executed.
77 77
78 78
79 79 PROT_NONE
80 80 Data cannot be accessed.
81 81
82 82
83 83
84 84 If an implementation of mmap() for a specific platform cannot support
85 85 the combination of access types specified by prot, the call to mmap()
86 86 fails. An implementation may permit accesses other than those specified
87 87 by prot; however, the implementation will not permit a write to succeed
88 88 where PROT_WRITE has not been set or permit any access where PROT_NONE
89 89 alone has been set. Each platform-specific implementation of mmap()
90 90 supports the following values of prot: PROT_NONE, PROT_READ,
91 91 PROT_WRITE, and the inclusive OR of PROT_READ and PROT_WRITE. On some
92 92 platforms, the PROT_WRITE protection option is implemented as
93 93 PROT_READ|PROT_WRITE and PROT_EXEC as PROT_READ|PROT_EXEC. The file
94 94 descriptor fildes is opened with read permission, regardless of the
95 95 protection options specified. If PROT_WRITE is specified, the
96 96 application must have opened the file descriptor fildes with write
97 97 permission unless MAP_PRIVATE is specified in the flags argument as
98 98 described below.
99 99
100 100
101 101 The flags argument provides other information about the handling of
102 102 the mapped data. The value of flags is the bitwise inclusive OR of
103 103 these options, defined in <sys/mman.h>:
104 104
105 105 MAP_SHARED
106 106 Changes are shared.
107 107
108 108
109 109 MAP_PRIVATE
110 110 Changes are private.
111 111
112 112
113 113 MAP_FIXED
114 114 Interpret addr exactly.
115 115
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116 116
117 117 MAP_NORESERVE
118 118 Do not reserve swap space.
119 119
120 120
121 121 MAP_ANON
122 122 Map anonymous memory.
123 123
124 124
125 125 MAP_ALIGN
126 - Interpret addr as required aligment.
126 + Interpret addr as required alignment.
127 127
128 128
129 129 MAP_TEXT
130 130 Map text.
131 131
132 132
133 133 MAP_INITDATA
134 134 Map initialized data segment.
135 135
136 136
137 137 MAP_32BIT
138 138 Map to the lower 32 bits of address space.
139 139
140 140
141 141 MAP_FILE
142 142 Map a regular file. This is the default behavior;
143 143 specifying this flag is not required. It is provided
144 144 for compatibility with other systems and should not be
145 145 included in new code.
146 146
147 147
148 148
149 149 The MAP_SHARED and MAP_PRIVATE options describe the disposition of
150 150 write references to the underlying object. If MAP_SHARED is specified,
151 151 write references will change the memory object. If MAP_PRIVATE is
152 152 specified, the initial write reference will create a private copy of
153 153 the memory object page and redirect the mapping to the copy. The
154 154 private copy is not created until the first write; until then, other
155 155 users who have the object mapped MAP_SHARED can change the object.
156 156 Either MAP_SHARED or MAP_PRIVATE must be specified, but not both. The
157 157 mapping type is retained across fork(2).
158 158
159 159
160 160 When MAP_FIXED is set in the flags argument, the system is informed
161 161 that the value of pa must be addr, exactly. If MAP_FIXED is set, mmap()
162 162 may return (void *)-1 and set errno to EINVAL. If a MAP_FIXED request
163 163 is successful, the mapping established by mmap() replaces any previous
164 164 mappings for the process's pages in the range [pa, pa + len). The use
165 165 of MAP_FIXED is discouraged, since it may prevent a system from making
166 166 the most effective use of its resources.
167 167
168 168
169 169 When MAP_FIXED is set and the requested address is the same as previous
170 170 mapping, the previous address is unmapped and the new mapping is
171 171 created on top of the old one.
172 172
173 173
174 174 When MAP_FIXED is not set, the system uses addr to arrive at pa. The pa
175 175 so chosen will be an area of the address space that the system deems
176 176 suitable for a mapping of len bytes to the file. The mmap() function
177 177 interprets an addr value of 0 as granting the system complete freedom
178 178 in selecting pa, subject to constraints described below. A non-zero
179 179 value of addr is taken to be a suggestion of a process address near
180 180 which the mapping should be placed. When the system selects a value for
181 181 pa, it will never place a mapping at address 0, nor will it replace any
182 182 extant mapping, nor map into areas considered part of the potential
183 183 data or stack "segments".
184 184
185 185
186 186 When MAP_ALIGN is set, the system is informed that the alignment of pa
187 187 must be the same as addr. The alignment value in addr must be 0 or some
188 188 power of two multiple of page size as returned by sysconf(3C). If addr
189 189 is 0, the system will choose a suitable alignment.
190 190
191 191
192 192 The MAP_NORESERVE option specifies that no swap space be reserved for a
193 193 mapping. Without this flag, the creation of a writable MAP_PRIVATE
194 194 mapping reserves swap space equal to the size of the mapping; when the
195 195 mapping is written into, the reserved space is employed to hold private
196 196 copies of the data. A write into a MAP_NORESERVE mapping produces
197 197 results which depend on the current availability of swap space in the
198 198 system. If space is available, the write succeeds and a private copy
199 199 of the written page is created; if space is not available, the write
200 200 fails and a SIGBUS or SIGSEGV signal is delivered to the writing
201 201 process. MAP_NORESERVE mappings are inherited across fork(); at the
202 202 time of the fork(), swap space is reserved in the child for all private
203 203 pages that currently exist in the parent; thereafter the child's
204 204 mapping behaves as described above.
205 205
206 206
207 207 When MAP_ANON is set in flags, and fildes is set to -1, mmap() provides
208 208 a direct path to return anonymous pages to the caller. This operation
209 209 is equivalent to passing mmap() an open file descriptor on /dev/zero
210 210 with MAP_ANON elided from the flags argument.
211 211
212 212
213 213 The MAP_TEXT option informs the system that the mapped region will be
214 214 used primarily for executing instructions. This information can help
215 215 the system better utilize MMU resources on some platforms. This flag is
216 216 always passed by the dynamic linker when it maps text segments of
217 217 shared objects. When the MAP_TEXT option is used for regular file
218 218 mappings on some platforms, the system can choose a mapping size larger
219 219 than the page size returned by sysconf(3C). The specific page sizes
220 220 that are used depend on the platform and the alignment of the addr and
221 221 len arguments. Several different mapping sizes can be used to map the
222 222 region with larger page sizes used in the parts of the region that meet
223 223 alignment and size requirements for those page sizes.
224 224
225 225
226 226 The MAP_INITDATA option informs the system that the mapped region is an
227 227 initialized data segment of an executable or shared object. When the
228 228 MAP_INITDATA option is used for regular file mappings on some
229 229 platforms, the system can choose a mapping size larger than the page
230 230 size returned by sysconf(). The MAP_INITDATA option should be used only
231 231 by the dynamic linker for mapping initialized data of shared objects.
232 232
233 233
234 234 The MAP_32BIT option informs the system that the search space for
235 235 mapping assignment should be limited to the first 32 bits (4 Gbytes) of
236 236 the caller's address space. This flag is accepted in both 32-bit and
237 237 64-bit process models, but does not alter the mapping strategy when
238 238 used in a 32-bit process model.
239 239
240 240
241 241 The off argument is constrained to be aligned and sized according to
242 242 the value returned by sysconf() when passed _SC_PAGESIZE or
243 243 _SC_PAGE_SIZE. When MAP_FIXED is specified, the addr argument must also
244 244 meet these constraints. The system performs mapping operations over
245 245 whole pages. Thus, while the len argument need not meet a size or
246 246 alignment constraint, the system will include, in any mapping
247 247 operation, any partial page specified by the range [pa, pa + len).
248 248
249 249
250 250 The system will always zero-fill any partial page at the end of an
251 251 object. Further, the system will never write out any modified portions
252 252 of the last page of an object which are beyond its end. References to
253 253 whole pages following the end of an object will result in the delivery
254 254 of a SIGBUS or SIGSEGV signal. SIGBUS signals may also be delivered on
255 255 various file system conditions, including quota exceeded errors.
256 256
257 257
258 258 The mmap() function adds an extra reference to the file associated with
259 259 the file descriptor fildes which is not removed by a subsequent
260 260 close(2) on that file descriptor. This reference is removed when there
261 261 are no more mappings to the file by a call to the munmap(2) function.
262 262
263 263
264 264 The st_atime field of the mapped file may be marked for update at any
265 265 time between the mmap() call and the corresponding munmap(2) call. The
266 266 initial read or write reference to a mapped region will cause the
267 267 file's st_atime field to be marked for update if it has not already
268 268 been marked for update.
269 269
270 270
271 271 The st_ctime and st_mtime fields of a file that is mapped with
272 272 MAP_SHARED and PROT_WRITE, will be marked for update at some point in
273 273 the interval between a write reference to the mapped region and the
274 274 next call to msync(3C) with MS_ASYNC or MS_SYNC for that portion of the
275 275 file by any process. If there is no such call, these fields may be
276 276 marked for update at any time after a write reference if the underlying
277 277 file is modified as a result.
278 278
279 279
280 280 If the process calls mlockall(3C) with the MCL_FUTURE flag, the pages
281 281 mapped by all future calls to mmap() will be locked in memory. In this
282 282 case, if not enough memory could be locked, mmap() fails and sets errno
283 283 to EAGAIN.
284 284
285 285
286 286 The mmap() function aligns based on the length of the mapping. When
287 287 determining the amount of space to add to the address space, mmap()
288 288 includes two 8-Kbyte pages, one at each end of the mapping that are not
289 289 mapped and are therefore used as "red-zone" pages. Attempts to
290 290 reference these pages result in access violations.
291 291
292 292
293 293 The size requested is incremented by the 16 Kbytes for these pages and
294 294 is then subject to rounding constraints. The constraints are:
295 295
296 296 o For 32-bit processes:
297 297
298 298 If length > 4 Mbytes
299 299 round to 4-Mbyte multiple
300 300 elseif length > 512 Kbytes
301 301 round to 512-Kbyte multiple
302 302 else
303 303 round to 64-Kbyte multiple
304 304
305 305
306 306 o For 64-bit processes:
307 307
308 308 If length > 4 Mbytes
309 309 round to 4-Mbyte multiple
310 310 else
311 311 round to 1-Mbyte multiple
312 312
313 313
314 314
315 315 The net result is that for a 32-bit process:
316 316
317 317 o If an mmap() request is made for 4 Mbytes, it results in 4
318 318 Mbytes + 16 Kbytes and is rounded up to 8 Mbytes.
319 319
320 320 o If an mmap() request is made for 512 Kbytes, it results in
321 321 512 Kbytes + 16 Kbytes and is rounded up to 1 Mbyte.
322 322
323 323 o If an mmap() request is made for 1 Mbyte, it results in 1
324 324 Mbyte + 16 Kbytes and is rounded up to 1.5 Mbytes.
325 325
326 326 o Each 8-Kbyte mmap request "consumes" 64 Kbytes of virtual
327 327 address space.
328 328
329 329
330 330 To obtain maximal address space usage for a 32-bit process:
331 331
332 332 o Combine 8-Kbyte requests up to a limit of 48 Kbytes.
333 333
334 334 o Combine amounts over 48 Kbytes into 496-Kbyte chunks.
335 335
336 336 o Combine amounts over 496 Kbytes into 4080-Kbyte chunks.
337 337
338 338
339 339 To obtain maximal address space usage for a 64-bit process:
340 340
341 341 o Combine amounts < 1008 Kbytes into chunks <= 1008 Kbytes.
342 342
343 343 o Combine amounts over 1008 Kbytes into 4080-Kbyte chunks.
344 344
345 345
346 346 The following is the output from a 32-bit program demonstrating this:
347 347
348 348 map 8192 bytes: 0xff390000
349 349 map 8192 bytes: 0xff380000
350 350
351 351 64-Kbyte delta between starting addresses.
352 352
353 353
354 354 map 512 Kbytes: 0xff180000
355 355 map 512 Kbytes: 0xff080000
356 356
357 357 1-Mbyte delta between starting addresses.
358 358
359 359
360 360 map 496 Kbytes: 0xff000000
361 361 map 496 Kbytes: 0xfef80000
362 362
363 363 512-Kbyte delta between starting addresses
364 364
365 365
366 366 map 1 Mbyte: 0xfee00000
367 367 map 1 Mbyte: 0xfec80000
368 368
369 369 1536-Kbyte delta between starting addresses
370 370
371 371
372 372 map 1008 Kbytes: 0xfeb80000
373 373 map 1008 Kbytes: 0xfea80000
374 374
375 375 1-Mbyte delta between starting addresses
376 376
377 377
378 378 map 4 Mbytes: 0xfe400000
379 379 map 4 Mbytes: 0xfdc00000
380 380
381 381 8-Mbyte delta between starting addresses
382 382
383 383
384 384 map 4080 Kbytes: 0xfd800000
385 385 map 4080 Kbytes: 0xfd400000
386 386
387 387 4-Mbyte delta between starting addresses
388 388
389 389
390 390
391 391 The following is the output of the same program compiled as a 64-bit
392 392 application:
393 393
394 394 map 8192 bytes: 0xffffffff7f000000
395 395 map 8192 bytes: 0xffffffff7ef00000
396 396
397 397 1-Mbyte delta between starting addresses
398 398
399 399
400 400 map 512 Kbytes: 0xffffffff7ee00000
401 401 map 512 Kbytes: 0xffffffff7ed00000
402 402
403 403 1-Mbyte delta between starting addresses
404 404
405 405
406 406 map 496 Kbytes: 0xffffffff7ec00000
407 407 map 496 Kbytes: 0xffffffff7eb00000
408 408
409 409 1-Mbyte delta between starting addresses
410 410
411 411
412 412 map 1 Mbyte: 0xffffffff7e900000
413 413 map 1 Mbyte: 0xffffffff7e700000
414 414
415 415 2-Mbyte delta between starting addresses
416 416
417 417
418 418 map 1008 Kbytes: 0xffffffff7e600000
419 419 map 1008 Kbytes: 0xffffffff7e500000
420 420
421 421 1-Mbyte delta between starting addresses
422 422
423 423
424 424 map 4 Mbytes: 0xffffffff7e000000
425 425 map 4 Mbytes: 0xffffffff7d800000
426 426
427 427 8-Mbyte delta between starting addresses
428 428
429 429
430 430 map 4080 Kbytes: 0xffffffff7d400000
431 431 map 4080 Kbytes: 0xffffffff7d000000
432 432
433 433 4-Mbyte delta between starting addresses
434 434
435 435
436 436 RETURN VALUES
437 437 Upon successful completion, the mmap() function returns the address at
438 438 which the mapping was placed (pa); otherwise, it returns a value of
439 439 MAP_FAILED and sets errno to indicate the error. The symbol MAP_FAILED
440 440 is defined in the header <sys/mman.h>. No successful return from mmap()
441 441 will return the value MAP_FAILED.
442 442
443 443
444 444 If mmap() fails for reasons other than EBADF, EINVAL or ENOTSUP, some
445 445 of the mappings in the address range starting at addr and continuing
446 446 for len bytes may have been unmapped.
447 447
448 448 ERRORS
449 449 The mmap() function will fail if:
450 450
451 451 EACCES
452 452 The fildes file descriptor is not open for read,
453 453 regardless of the protection specified; or fildes is not
454 454 open for write and PROT_WRITE was specified for a
455 455 MAP_SHARED type mapping.
456 456
457 457
458 458 EAGAIN
459 459 The mapping could not be locked in memory.
460 460
461 461 There was insufficient room to reserve swap space for the
462 462 mapping.
463 463
464 464
465 465 EBADF
466 466 The fildes file descriptor is not open (and MAP_ANON was
467 467 not specified).
468 468
469 469
470 470 EINVAL
471 471 The arguments addr (if MAP_FIXED was specified) or off are
472 472 not multiples of the page size as returned by sysconf().
473 473
474 474 The argument addr (if MAP_ALIGN was specified) is not 0 or
475 475 some power of two multiple of page size as returned by
476 476 sysconf(3C).
477 477
478 478 MAP_FIXED and MAP_ALIGN are both specified.
479 479
480 480 The field in flags is invalid (neither MAP_PRIVATE or
481 481 MAP_SHARED is set).
482 482
483 483 The argument len has a value equal to 0.
484 484
485 485 MAP_ANON was specified, but the file descriptor was not
486 486 -1.
487 487
488 488 MAP_TEXT was specified but PROT_EXEC was not.
489 489
490 490 MAP_TEXT and MAP_INITDATA were both specified.
491 491
492 492
493 493 EMFILE
494 494 The number of mapped regions would exceed an
495 495 implementation-dependent limit (per process or per
496 496 system).
497 497
498 498
499 499 ENODEV
500 500 The fildes argument refers to an object for which mmap()
501 501 is meaningless, such as a terminal.
502 502
503 503
504 504 ENOMEM
505 505 The MAP_FIXED option was specified and the range [addr,
506 506 addr + len) exceeds that allowed for the address space of
507 507 a process.
508 508
509 509 The MAP_FIXED option was not specified and there is
510 510 insufficient room in the address space to effect the
511 511 mapping.
512 512
513 513 The mapping could not be locked in memory, if required by
514 514 mlockall(3C), because it would require more space than the
515 515 system is able to supply.
516 516
517 517 The composite size of len plus the lengths obtained from
518 518 all previous calls to mmap() exceeds RLIMIT_VMEM (see
519 519 getrlimit(2)).
520 520
521 521
522 522 ENOTSUP
523 523 The system does not support the combination of accesses
524 524 requested in the prot argument.
525 525
526 526
527 527 ENXIO
528 528 Addresses in the range [off, off + len) are invalid for
529 529 the object specified by fildes.
530 530
531 531 The MAP_FIXED option was specified in flags and the
532 532 combination of addr, len and off is invalid for the object
533 533 specified by fildes.
534 534
535 535
536 536 EOVERFLOW
537 537 The file is a regular file and the value of off plus len
538 538 exceeds the offset maximum establish in the open file
539 539 description associated with fildes.
540 540
541 541
542 542
543 543 The mmap() function may fail if:
544 544
545 545 EAGAIN
546 546 The file to be mapped is already locked using advisory or
547 547 mandatory record locking. See fcntl(2).
548 548
549 549
550 550 USAGE
551 551 Use of mmap() may reduce the amount of memory available to other memory
552 552 allocation functions.
553 553
554 554
555 555 MAP_ALIGN is useful to assure a properly aligned value of pa for
556 556 subsequent use with memcntl(2) and the MC_HAT_ADVISE command. This is
557 557 best used for large, long-lived, and heavily referenced regions.
558 558 MAP_FIXED and MAP_ALIGN are always mutually-exclusive.
559 559
560 560
561 561 Use of MAP_FIXED may result in unspecified behavior in further use of
562 562 brk(2), sbrk(2), malloc(3C), and shmat(2). The use of MAP_FIXED is
563 563 discouraged, as it may prevent an implementation from making the most
564 564 effective use of resources.
565 565
566 566
567 567 The application must ensure correct synchronization when using mmap()
568 568 in conjunction with any other file access method, such as read(2) and
569 569 write(2), standard input/output, and shmat(2).
570 570
571 571
572 572 The mmap() function has a transitional interface for 64-bit file
573 573 offsets. See lf64(5).
574 574
575 575
576 576 The mmap() function allows access to resources using address space
577 577 manipulations instead of the read()/write() interface. Once a file is
578 578 mapped, all a process has to do to access it is use the data at the
579 579 address to which the object was mapped.
580 580
581 581
582 582 Consider the following pseudo-code:
583 583
584 584 fildes = open(...)
585 585 lseek(fildes, offset, whence)
586 586 read(fildes, buf, len)
587 587 /* use data in buf */
588 588
589 589
590 590
591 591 The following is a rewrite using mmap():
592 592
593 593 fildes = open(...)
594 594 address = mmap((caddr_t) 0, len, (PROT_READ | PROT_WRITE),
595 595 MAP_PRIVATE, fildes, offset)
596 596 /* use data at address */
597 597
598 598
599 599 ATTRIBUTES
600 600 See attributes(5) for descriptions of the following attributes:
601 601
602 602
603 603
604 604
605 605 +--------------------+-------------------+
606 606 | ATTRIBUTE TYPE | ATTRIBUTE VALUE |
607 607 +--------------------+-------------------+
608 608 |Interface Stability | Standard |
609 609 +--------------------+-------------------+
610 610 |MT-Level | Async-Signal-Safe |
611 611 +--------------------+-------------------+
612 612
613 613 SEE ALSO
614 614 close(2), exec(2), fcntl(2), fork(2), getrlimit(2), memcntl(2),
615 615 mmapobj(2), mprotect(2), munmap(2), shmat(2), lockf(3C), mlockall(3C),
616 616 msync(3C), plock(3C), sysconf(3C), attributes(5), lf64(5),
617 617 standards(5), null(7D), zero(7D)
618 618
619 619
620 620
621 621 August 29, 2016 MMAP(2)
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