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a62f5121 | 1 | .\" Copyright (C) 1996 Andries Brouwer <aeb@cwi.nl> |
c11b1abf | 2 | .\" and Copyright (C) 2006, 2007 Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 3 | .\" |
4b72fb64 | 4 | .\" %%%LICENSE_START(verbatim) |
fea681da MK |
5 | .\" Permission is granted to make and distribute verbatim copies of this |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 24 | .\" %%%LICENSE_END |
fea681da MK |
25 | .\" |
26 | .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com> | |
27 | .\" Modified 2000-03-25 by Jim Van Zandt <jrv@vanzandt.mv.com> | |
28 | .\" Modified 2001-10-04 by John Levon <moz@compsoc.man.ac.uk> | |
29 | .\" Modified 2003-02-02 by Andi Kleen <ak@muc.de> | |
c11b1abf | 30 | .\" Modified 2003-05-21 by Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 31 | .\" MAP_LOCKED works from 2.5.37 |
c11b1abf | 32 | .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 33 | .\" Modified 2004-09-11 by aeb |
1a956089 MK |
34 | .\" Modified 2004-12-08, from Eric Estievenart <eric.estievenart@free.fr> |
35 | .\" Modified 2004-12-08, mtk, formatting tidy-ups | |
a62f5121 | 36 | .\" Modified 2006-12-04, mtk, various parts rewritten |
74fa61b7 | 37 | .\" 2007-07-10, mtk, Added an example program. |
e6205b0c | 38 | .\" 2008-11-18, mtk, document MAP_STACK |
fea681da | 39 | .\" |
42b437ca | 40 | .TH MMAP 2 2013-02-25 "Linux" "Linux Programmer's Manual" |
fea681da MK |
41 | .SH NAME |
42 | mmap, munmap \- map or unmap files or devices into memory | |
43 | .SH SYNOPSIS | |
e0037472 | 44 | .nf |
fea681da MK |
45 | .B #include <sys/mman.h> |
46 | .sp | |
14f5ae6d | 47 | .BI "void *mmap(void *" addr ", size_t " length \ |
a62f5121 | 48 | ", int " prot ", int " flags , |
e0037472 | 49 | .BI " int " fd ", off_t " offset ); |
14f5ae6d | 50 | .BI "int munmap(void *" addr ", size_t " length ); |
e0037472 | 51 | .fi |
45e97e2a MK |
52 | |
53 | See NOTES for information on feature test macro requirements. | |
fea681da | 54 | .SH DESCRIPTION |
1a956089 | 55 | .BR mmap () |
c13182ef | 56 | creates a new mapping in the virtual address space of |
5e8cde2f MK |
57 | the calling process. |
58 | The starting address for the new mapping is specified in | |
14f5ae6d | 59 | .IR addr . |
5e8cde2f MK |
60 | The |
61 | .I length | |
62 | argument specifies the length of the mapping. | |
63 | ||
64 | If | |
14f5ae6d | 65 | .I addr |
5e8cde2f MK |
66 | is NULL, |
67 | then the kernel chooses the address at which to create the mapping; | |
68 | this is the most portable method of creating a new mapping. | |
c13182ef | 69 | If |
14f5ae6d | 70 | .I addr |
5e8cde2f MK |
71 | is not NULL, |
72 | then the kernel takes it as a hint about where to place the mapping; | |
6aa7db0a MK |
73 | on Linux, the mapping will be created at a nearby page boundary. |
74 | .\" Before Linux 2.6.24, the address was rounded up to the next page | |
29328361 | 75 | .\" boundary; since 2.6.24, it is rounded down! |
5e8cde2f MK |
76 | The address of the new mapping is returned as the result of the call. |
77 | ||
78 | The contents of a file mapping (as opposed to an anonymous mapping; see | |
79 | .B MAP_ANONYMOUS | |
d9bfdb9c | 80 | below), are initialized using |
fea681da MK |
81 | .I length |
82 | bytes starting at offset | |
83 | .I offset | |
5e8cde2f MK |
84 | in the file (or other object) referred to by the file descriptor |
85 | .IR fd . | |
a62f5121 MK |
86 | .I offset |
87 | must be a multiple of the page size as returned by | |
88 | .IR sysconf(_SC_PAGE_SIZE) . | |
fea681da MK |
89 | .LP |
90 | The | |
91 | .I prot | |
c13182ef | 92 | argument describes the desired memory protection of the mapping |
5e8cde2f MK |
93 | (and must not conflict with the open mode of the file). |
94 | It is either | |
fea681da | 95 | .B PROT_NONE |
a62f5121 | 96 | or the bitwise OR of one or more of the following flags: |
fea681da MK |
97 | .TP 1.1i |
98 | .B PROT_EXEC | |
99 | Pages may be executed. | |
100 | .TP | |
101 | .B PROT_READ | |
102 | Pages may be read. | |
103 | .TP | |
104 | .B PROT_WRITE | |
105 | Pages may be written. | |
106 | .TP | |
107 | .B PROT_NONE | |
108 | Pages may not be accessed. | |
109 | .LP | |
110 | The | |
111 | .I flags | |
a62f5121 MK |
112 | argument determines whether updates to the mapping |
113 | are visible to other processes mapping the same region, | |
ba7cb080 | 114 | and whether updates are carried through to the underlying file. |
d9bfdb9c | 115 | This behavior is determined by including exactly one |
a62f5121 | 116 | of the following values in |
5e8cde2f | 117 | .IR flags : |
fea681da | 118 | .TP 1.1i |
fea681da | 119 | .B MAP_SHARED |
c13182ef | 120 | Share this mapping. |
a62f5121 MK |
121 | Updates to the mapping are visible to other processes that map this file, |
122 | and are carried through to the underlying file. | |
fea681da MK |
123 | The file may not actually be updated until |
124 | .BR msync (2) | |
125 | or | |
2777b1ca | 126 | .BR munmap () |
a62f5121 | 127 | is called. |
fea681da MK |
128 | .TP |
129 | .B MAP_PRIVATE | |
130 | Create a private copy-on-write mapping. | |
a62f5121 MK |
131 | Updates to the mapping are not visible to other processes |
132 | mapping the same file, and are not carried through to | |
133 | the underlying file. | |
fea681da | 134 | It is unspecified whether changes made to the file after the |
1a956089 | 135 | .BR mmap () |
fea681da MK |
136 | call are visible in the mapped region. |
137 | .LP | |
a62f5121 MK |
138 | Both of these flags are described in POSIX.1-2001. |
139 | ||
140 | In addition, zero or more of the following values can be ORed in | |
141 | .IR flags : | |
fea681da | 142 | .TP |
c368e7ca MK |
143 | .BR MAP_32BIT " (since Linux 2.4.20, 2.6)" |
144 | Put the mapping into the first 2 Gigabytes of the process address space. | |
145 | This flag is only supported on x86-64, for 64-bit programs. | |
146 | It was added to allow thread stacks to be allocated somewhere | |
147 | in the first 2GB of memory, | |
148 | so as to improve context-switch performance on some early | |
149 | 64-bit processors. | |
150 | .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 | |
151 | Modern x86-64 processors no longer have this performance problem, | |
152 | so use of this flag is not required on those systems. | |
153 | The | |
83314009 | 154 | .B MAP_32BIT |
c368e7ca | 155 | flag is ignored when |
a62f5121 | 156 | .B MAP_FIXED |
83314009 | 157 | is set. |
fea681da | 158 | .TP |
a62f5121 | 159 | .B MAP_ANON |
c13182ef MK |
160 | Synonym for |
161 | .BR MAP_ANONYMOUS . | |
a62f5121 MK |
162 | Deprecated. |
163 | .TP | |
fea681da | 164 | .B MAP_ANONYMOUS |
5e8cde2f | 165 | The mapping is not backed by any file; |
d9bfdb9c | 166 | its contents are initialized to zero. |
5e8cde2f | 167 | The |
fea681da MK |
168 | .I fd |
169 | and | |
170 | .I offset | |
a62f5121 | 171 | arguments are ignored; |
c13182ef | 172 | however, some implementations require |
a62f5121 | 173 | .I fd |
c13182ef MK |
174 | to be \-1 if |
175 | .B MAP_ANONYMOUS | |
176 | (or | |
177 | .BR MAP_ANON ) | |
a62f5121 MK |
178 | is specified, |
179 | and portable applications should ensure this. | |
c13182ef | 180 | The use of |
a62f5121 | 181 | .B MAP_ANONYMOUS |
c13182ef | 182 | in conjunction with |
51ffcca0 | 183 | .B MAP_SHARED |
74f3f90b | 184 | is only supported on Linux since kernel 2.4. |
fea681da | 185 | .TP |
83314009 MK |
186 | .B MAP_DENYWRITE |
187 | This flag is ignored. | |
188 | .\" Introduced in 1.1.36, removed in 1.3.24. | |
d9bfdb9c | 189 | (Long ago, it signaled that attempts to write to the underlying file |
83314009 MK |
190 | should fail with |
191 | .BR ETXTBUSY . | |
192 | But this was a source of denial-of-service attacks.) | |
193 | .TP | |
194 | .B MAP_EXECUTABLE | |
195 | This flag is ignored. | |
196 | .\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link. | |
d9bfdb9c | 197 | .\" (Long ago, it signaled that the underlying file is an executable. |
83314009 MK |
198 | .\" However, that information was not really used anywhere.) |
199 | .\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of | |
200 | .\" MAP_DENYWRITE? | |
201 | .TP | |
fea681da | 202 | .B MAP_FILE |
c13182ef MK |
203 | Compatibility flag. |
204 | Ignored. | |
988db661 | 205 | .\" On some systems, this was required as the opposite of |
83314009 | 206 | .\" MAP_ANONYMOUS -- mtk, 1 May 2007 |
fea681da | 207 | .TP |
51ffcca0 | 208 | .B MAP_FIXED |
83314009 | 209 | Don't interpret |
14f5ae6d | 210 | .I addr |
83314009 | 211 | as a hint: place the mapping at exactly that address. |
14f5ae6d | 212 | .I addr |
83314009 MK |
213 | must be a multiple of the page size. |
214 | If the memory region specified by | |
14f5ae6d | 215 | .I addr |
83314009 MK |
216 | and |
217 | .I len | |
218 | overlaps pages of any existing mapping(s), then the overlapped | |
219 | part of the existing mapping(s) will be discarded. | |
220 | If the specified address cannot be used, | |
221 | .BR mmap () | |
222 | will fail. | |
223 | Because requiring a fixed address for a mapping is less portable, | |
224 | the use of this option is discouraged. | |
fea681da | 225 | .TP |
83314009 MK |
226 | .B MAP_GROWSDOWN |
227 | Used for stacks. | |
228 | Indicates to the kernel virtual memory system that the mapping | |
5fab2e7c | 229 | should extend downward in memory. |
83314009 | 230 | .TP |
76a34baa MK |
231 | .BR MAP_HUGETLB " (since Linux 2.6.32)" |
232 | Allocate the mapping using "huge pages." | |
66a9882e | 233 | See the Linux kernel source file |
76a34baa MK |
234 | .I Documentation/vm/hugetlbpage.txt |
235 | for further information. | |
236 | .TP | |
83314009 MK |
237 | .BR MAP_LOCKED " (since Linux 2.5.37)" |
238 | Lock the pages of the mapped region into memory in the manner of | |
74d32233 | 239 | .BR mlock (2). |
83314009 MK |
240 | This flag is ignored in older kernels. |
241 | .\" If set, the mapped pages will not be swapped out. | |
fea681da MK |
242 | .TP |
243 | .BR MAP_NONBLOCK " (since Linux 2.5.46)" | |
51ffcca0 MK |
244 | Only meaningful in conjunction with |
245 | .BR MAP_POPULATE . | |
c13182ef MK |
246 | Don't perform read-ahead: |
247 | only create page tables entries for pages | |
51ffcca0 | 248 | that are already present in RAM. |
7c40de08 | 249 | Since Linux 2.6.23, this flag causes |
f38fa944 MK |
250 | .BR MAP_POPULATE |
251 | to do nothing. | |
252 | One day the combination of | |
253 | .BR MAP_POPULATE | |
254 | and | |
255 | .BR MAP_NONBLOCK | |
3b777aff | 256 | may be reimplemented. |
83314009 MK |
257 | .TP |
258 | .B MAP_NORESERVE | |
259 | Do not reserve swap space for this mapping. | |
260 | When swap space is reserved, one has the guarantee | |
261 | that it is possible to modify the mapping. | |
8bd58774 MK |
262 | When swap space is not reserved one might get |
263 | .B SIGSEGV | |
264 | upon a write | |
83314009 MK |
265 | if no physical memory is available. |
266 | See also the discussion of the file | |
267 | .I /proc/sys/vm/overcommit_memory | |
268 | in | |
269 | .BR proc (5). | |
270 | In kernels before 2.6, this flag only had effect for | |
271 | private writable mappings. | |
272 | .TP | |
273 | .BR MAP_POPULATE " (since Linux 2.5.46)" | |
f38fa944 MK |
274 | Populate (prefault) page tables for a mapping. |
275 | For a file mapping, this causes read-ahead on the file. | |
83314009 | 276 | Later accesses to the mapping will not be blocked by page faults. |
f38fa944 MK |
277 | .BR MAP_POPULATE |
278 | is only supported for private mappings since Linux 2.6.23. | |
e6205b0c MK |
279 | .TP |
280 | .BR MAP_STACK " (since Linux 2.6.27)" | |
281 | Allocate the mapping at an address suitable for a process | |
282 | or thread stack. | |
283 | This flag is currently a no-op, | |
284 | but is used in the glibc threading implementation so that | |
285 | if some architectures require special treatment for stack allocations, | |
286 | support can later be transparently implemented for glibc. | |
67b59ff5 | 287 | .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 |
e6205b0c MK |
288 | .\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7 |
289 | .\" http://thread.gmane.org/gmane.linux.kernel/720412 | |
290 | .\" "pthread_create() slow for many threads; also time to revisit 64b | |
291 | .\" context switch optimization?" | |
12062404 MK |
292 | .TP |
293 | .BR MAP_UNINITIALIZED " (since Linux 2.6.33)" | |
294 | Don't clear anonymous pages. | |
295 | This flag is intended to improve performance on embedded devices. | |
296 | This flag is only honored if the kernel was configured with the | |
297 | .B CONFIG_MMAP_ALLOW_UNINITIALIZED | |
298 | option. | |
299 | Because of the security implications, | |
300 | that option is normally enabled only on embedded devices | |
301 | (i.e., devices where one has complete control of the contents of user memory). | |
a62f5121 | 302 | .LP |
7c7adcbe MK |
303 | Of the above flags, only |
304 | .B MAP_FIXED | |
305 | is specified in POSIX.1-2001. | |
306 | However, most systems also support | |
307 | .B MAP_ANONYMOUS | |
308 | (or its synonym | |
309 | .BR MAP_ANON ). | |
310 | .LP | |
2d6cfc1a MK |
311 | Some systems document the additional flags |
312 | .BR MAP_AUTOGROW , | |
313 | .BR MAP_AUTORESRV , | |
314 | .BR MAP_COPY , | |
315 | and | |
316 | .BR MAP_LOCAL . | |
fea681da | 317 | .LP |
fea681da | 318 | Memory mapped by |
1a956089 | 319 | .BR mmap () |
fea681da MK |
320 | is preserved across |
321 | .BR fork (2), | |
322 | with the same attributes. | |
323 | .LP | |
c13182ef MK |
324 | A file is mapped in multiples of the page size. |
325 | For a file that is not | |
fea681da | 326 | a multiple of the page size, the remaining memory is zeroed when mapped, |
c13182ef MK |
327 | and writes to that region are not written out to the file. |
328 | The effect of | |
fea681da MK |
329 | changing the size of the underlying file of a mapping on the pages that |
330 | correspond to added or removed regions of the file is unspecified. | |
de5f7e28 | 331 | .SS munmap() |
fea681da | 332 | The |
1a956089 | 333 | .BR munmap () |
fea681da MK |
334 | system call deletes the mappings for the specified address range, and |
335 | causes further references to addresses within the range to generate | |
c13182ef MK |
336 | invalid memory references. |
337 | The region is also automatically unmapped | |
338 | when the process is terminated. | |
339 | On the other hand, closing the file | |
fea681da MK |
340 | descriptor does not unmap the region. |
341 | .LP | |
342 | The address | |
14f5ae6d | 343 | .I addr |
c13182ef MK |
344 | must be a multiple of the page size. |
345 | All pages containing a part | |
fea681da | 346 | of the indicated range are unmapped, and subsequent references |
8bd58774 MK |
347 | to these pages will generate |
348 | .BR SIGSEGV . | |
c13182ef | 349 | It is not an error if the |
fea681da | 350 | indicated range does not contain any mapped pages. |
de5f7e28 | 351 | .SS Timestamps changes for file-backed mappings |
fea681da | 352 | For file-backed mappings, the |
8478ee02 | 353 | .I st_atime |
fea681da | 354 | field for the mapped file may be updated at any time between the |
1a956089 | 355 | .BR mmap () |
fea681da MK |
356 | and the corresponding unmapping; the first reference to a mapped |
357 | page will update the field if it has not been already. | |
358 | .LP | |
359 | The | |
8478ee02 | 360 | .I st_ctime |
fea681da | 361 | and |
8478ee02 | 362 | .I st_mtime |
c13182ef MK |
363 | field for a file mapped with |
364 | .B PROT_WRITE | |
365 | and | |
366 | .B MAP_SHARED | |
51ffcca0 | 367 | will be updated after |
fea681da | 368 | a write to the mapped region, and before a subsequent |
0bfa087b | 369 | .BR msync (2) |
c13182ef MK |
370 | with the |
371 | .B MS_SYNC | |
372 | or | |
0daa9e92 | 373 | .B MS_ASYNC |
51ffcca0 | 374 | flag, if one occurs. |
47297adb | 375 | .SH RETURN VALUE |
fea681da | 376 | On success, |
1a956089 | 377 | .BR mmap () |
fea681da MK |
378 | returns a pointer to the mapped area. |
379 | On error, the value | |
380 | .B MAP_FAILED | |
c13182ef | 381 | (that is, |
009df872 | 382 | .IR "(void\ *)\ \-1" ) |
5e8cde2f | 383 | is returned, and |
fea681da MK |
384 | .I errno |
385 | is set appropriately. | |
386 | On success, | |
1a956089 | 387 | .BR munmap () |
fea681da MK |
388 | returns 0, on failure \-1, and |
389 | .I errno | |
c13182ef | 390 | is set (probably to |
51ffcca0 | 391 | .BR EINVAL ). |
fea681da MK |
392 | .SH ERRORS |
393 | .TP | |
394 | .B EACCES | |
395 | A file descriptor refers to a non-regular file. | |
c13182ef MK |
396 | Or |
397 | .B MAP_PRIVATE | |
51ffcca0 | 398 | was requested, but |
fea681da MK |
399 | .I fd |
400 | is not open for reading. | |
c13182ef MK |
401 | Or |
402 | .B MAP_SHARED | |
403 | was requested and | |
404 | .B PROT_WRITE | |
51ffcca0 | 405 | is set, but |
fea681da | 406 | .I fd |
682edefb MK |
407 | is not open in read/write |
408 | .RB ( O_RDWR ) | |
409 | mode. | |
c13182ef MK |
410 | Or |
411 | .B PROT_WRITE | |
51ffcca0 | 412 | is set, but the file is append-only. |
fea681da MK |
413 | .TP |
414 | .B EAGAIN | |
83cd3686 MK |
415 | The file has been locked, or too much memory has been locked (see |
416 | .BR setrlimit (2)). | |
fea681da MK |
417 | .TP |
418 | .B EBADF | |
419 | .I fd | |
c13182ef | 420 | is not a valid file descriptor (and |
51ffcca0 MK |
421 | .B MAP_ANONYMOUS |
422 | was not set). | |
fea681da MK |
423 | .TP |
424 | .B EINVAL | |
425 | We don't like | |
14f5ae6d | 426 | .IR addr , |
62a04c81 | 427 | .IR length , |
fea681da | 428 | or |
0daa9e92 | 429 | .I offset |
62a04c81 MK |
430 | (e.g., they are too large, or not aligned on a page boundary). |
431 | .TP | |
432 | .B EINVAL | |
f99fc197 | 433 | (since Linux 2.6.12) |
fea681da | 434 | .I length |
62a04c81 MK |
435 | was 0. |
436 | .TP | |
437 | .B EINVAL | |
438 | .I flags | |
439 | contained neither | |
440 | .B MAP_PRIVATE | |
fea681da | 441 | or |
62a04c81 MK |
442 | .BR MAP_SHARED , |
443 | or contained both of these values. | |
fea681da MK |
444 | .TP |
445 | .B ENFILE | |
446 | .\" This is for shared anonymous segments | |
447 | .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() | |
448 | The system limit on the total number of open files has been reached. | |
449 | .\" .TP | |
450 | .\" .B ENOEXEC | |
451 | .\" A file could not be mapped for reading. | |
452 | .TP | |
453 | .B ENODEV | |
24d01c53 | 454 | The underlying file system of the specified file does not support |
fea681da MK |
455 | memory mapping. |
456 | .TP | |
457 | .B ENOMEM | |
458 | No memory is available, or the process's maximum number of mappings would | |
459 | have been exceeded. | |
460 | .TP | |
461 | .B EPERM | |
462 | The | |
463 | .I prot | |
464 | argument asks for | |
465 | .B PROT_EXEC | |
24d01c53 | 466 | but the mapped area belongs to a file on a file system that |
fea681da MK |
467 | was mounted no-exec. |
468 | .\" (Since 2.4.25 / 2.6.0.) | |
469 | .TP | |
470 | .B ETXTBSY | |
c13182ef | 471 | .B MAP_DENYWRITE |
51ffcca0 | 472 | was set but the object specified by |
fea681da MK |
473 | .I fd |
474 | is open for writing. | |
da3ce098 CH |
475 | .TP |
476 | .B EOVERFLOW | |
42b437ca MK |
477 | On 32-bit architecture together with the large file extension |
478 | (i.e., using 64-bit | |
479 | .IR off_t ): | |
480 | the number of pages used for | |
481 | .I length | |
482 | plus number of pages used for | |
483 | .I offset | |
484 | would overflow | |
485 | .I "unsigned long" | |
486 | (32 bits). | |
fea681da MK |
487 | .LP |
488 | Use of a mapped region can result in these signals: | |
489 | .TP | |
490 | .B SIGSEGV | |
1e321034 | 491 | Attempted write into a region mapped as read-only. |
fea681da MK |
492 | .TP |
493 | .B SIGBUS | |
494 | Attempted access to a portion of the buffer that does not correspond | |
495 | to the file (for example, beyond the end of the file, including the | |
496 | case where another process has truncated the file). | |
47297adb | 497 | .SH CONFORMING TO |
2b2581ee MK |
498 | SVr4, 4.4BSD, POSIX.1-2001. |
499 | .\" SVr4 documents additional error codes ENXIO and ENODEV. | |
500 | .\" SUSv2 documents additional error codes EMFILE and EOVERFLOW. | |
fea681da MK |
501 | .SH AVAILABILITY |
502 | On POSIX systems on which | |
1a956089 | 503 | .BR mmap (), |
0bfa087b | 504 | .BR msync (2) |
fea681da | 505 | and |
1a956089 | 506 | .BR munmap () |
fea681da MK |
507 | are available, |
508 | .B _POSIX_MAPPED_FILES | |
6387216b MK |
509 | is defined in \fI<unistd.h>\fP to a value greater than 0. |
510 | (See also | |
fea681da | 511 | .BR sysconf (3).) |
97c1eac8 | 512 | .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L. |
fea681da MK |
513 | .\" -1: unavailable, 0: ask using sysconf(). |
514 | .\" glibc defines it to 1. | |
a1d5f77c | 515 | .SH NOTES |
3062824d MK |
516 | This page describes the interface provided by the glibc |
517 | .BR mmap () | |
518 | wrapper function. | |
519 | Originally, this function invoked a system call of the same name. | |
520 | Since kernel 2.4, that system call has been superseded by | |
521 | .BR mmap2 (2), | |
522 | and nowadays | |
5636a29a MK |
523 | .\" Since around glibc 2.1/2.2, depending on the platform. |
524 | the glibc | |
525 | .BR mmap () | |
526 | wrapper function invokes | |
527 | .BR mmap2 (2) | |
528 | with a suitably adjusted value for | |
529 | .IR offset . | |
530 | ||
34ccb744 | 531 | On some hardware architectures (e.g., i386), |
0daa9e92 | 532 | .B PROT_WRITE |
f3edaabb MK |
533 | implies |
534 | .BR PROT_READ . | |
a1d5f77c MK |
535 | It is architecture dependent whether |
536 | .B PROT_READ | |
537 | implies | |
538 | .B PROT_EXEC | |
539 | or not. | |
540 | Portable programs should always set | |
541 | .B PROT_EXEC | |
542 | if they intend to execute code in the new mapping. | |
80d17cfa MK |
543 | |
544 | The portable way to create a mapping is to specify | |
545 | .I addr | |
546 | as 0 (NULL), and omit | |
547 | .B MAP_FIXED | |
548 | from | |
549 | .IR flags . | |
550 | In this case, the system chooses the address for the mapping; | |
46cdb997 | 551 | the address is chosen so as not to conflict with any existing mapping, |
80d17cfa MK |
552 | and will not be 0. |
553 | If the | |
554 | .B MAP_FIXED | |
555 | flag is specified, and | |
556 | .I addr | |
491cd2f0 | 557 | is 0 (NULL), then the mapped address will be 0 (NULL). |
45e97e2a MK |
558 | |
559 | Certain | |
560 | .I flags | |
561 | constants are defined only if either | |
562 | .BR _BSD_SOURCE | |
563 | or | |
564 | .BR _SVID_SOURCE | |
565 | is defined. | |
566 | (Requiring | |
567 | .BR _GNU_SOURCE | |
568 | also suffices, | |
569 | and requiring that macro specifically would have been more logical, | |
570 | since these flags are all Linux specific.) | |
571 | The relevant flags are: | |
572 | .BR MAP_32BIT , | |
573 | .BR MAP_ANONYMOUS | |
574 | (and the synonym | |
575 | .BR MAP_ANON ), | |
576 | .BR MAP_DENYWRITE , | |
577 | .BR MAP_EXECUTABLE , | |
578 | .BR MAP_FILE , | |
579 | .BR MAP_GROWSDOWN , | |
580 | .BR MAP_HUGETLB , | |
581 | .BR MAP_LOCKED , | |
582 | .BR MAP_NONBLOCK , | |
583 | .BR MAP_NORESERVE , | |
584 | .BR MAP_POPULATE , | |
585 | and | |
586 | .BR MAP_STACK . | |
fea681da | 587 | .SH BUGS |
c13182ef MK |
588 | On Linux there are no guarantees like those suggested above under |
589 | .BR MAP_NORESERVE . | |
dbc53ca8 | 590 | By default, any process can be killed |
fea681da | 591 | at any moment when the system runs out of memory. |
dbc53ca8 MK |
592 | |
593 | In kernels before 2.6.7, the | |
594 | .B MAP_POPULATE | |
595 | flag only has effect if | |
596 | .I prot | |
597 | is specified as | |
598 | .BR PROT_NONE . | |
c8f3e580 | 599 | |
c13182ef | 600 | SUSv3 specifies that |
c8f3e580 MK |
601 | .BR mmap () |
602 | should fail if | |
603 | .I length | |
604 | is 0. | |
605 | However, in kernels before 2.6.12, | |
606 | .BR mmap () | |
607 | succeeded in this case: no mapping was created and the call returned | |
14f5ae6d | 608 | .IR addr . |
c8f3e580 MK |
609 | Since kernel 2.6.12, |
610 | .BR mmap () | |
611 | fails with the error | |
612 | .B EINVAL | |
613 | for this case. | |
2e43522f | 614 | |
a780f17b MK |
615 | POSIX specifies that the system shall always |
616 | zero fill any partial page at the end | |
b072a788 | 617 | of the object and that system will never write any modification of the |
a780f17b MK |
618 | object beyond its end. |
619 | On Linux, when you write data to such partial page after the end | |
b072a788 | 620 | of the object, the data stays in the page cache even after the file |
a780f17b MK |
621 | is closed and unmapped |
622 | and even though the data is never written to the file itself, | |
623 | subsequent mappings may see the modified content. | |
624 | In some cases, this could be fixed by calling | |
625 | .BR msync (2) | |
626 | before the unmap takes place; | |
627 | however, this doesn't work on tmpfs | |
628 | (for example, when using POSIX shared memory interface documented in | |
629 | .BR shm_overview (7)). | |
74fa61b7 | 630 | .SH EXAMPLE |
2e001ad4 MK |
631 | .\" FIXME . Add an example here that uses an anonymous shared region for |
632 | .\" IPC between parent and child. | |
74fa61b7 MK |
633 | .PP |
634 | The following program prints part of the file specified in | |
635 | its first command-line argument to standard output. | |
636 | The range of bytes to be printed is specified via offset and length | |
637 | values in the second and third command-line arguments. | |
638 | The program creates a memory mapping of the required | |
639 | pages of the file and then uses | |
640 | .BR write (2) | |
641 | to output the desired bytes. | |
642 | .nf | |
643 | ||
644 | #include <sys/mman.h> | |
645 | #include <sys/stat.h> | |
646 | #include <fcntl.h> | |
647 | #include <stdio.h> | |
648 | #include <stdlib.h> | |
649 | #include <unistd.h> | |
650 | ||
4407d3d8 MK |
651 | #define handle_error(msg) \\ |
652 | do { perror(msg); exit(EXIT_FAILURE); } while (0) | |
653 | ||
74fa61b7 MK |
654 | int |
655 | main(int argc, char *argv[]) | |
656 | { | |
657 | char *addr; | |
658 | int fd; | |
659 | struct stat sb; | |
660 | off_t offset, pa_offset; | |
661 | size_t length; | |
662 | ssize_t s; | |
663 | ||
fbbfa7ce | 664 | if (argc < 3 || argc > 4) { |
74fa61b7 MK |
665 | fprintf(stderr, "%s file offset [length]\\n", argv[0]); |
666 | exit(EXIT_FAILURE); | |
667 | } | |
668 | ||
669 | fd = open(argv[1], O_RDONLY); | |
4407d3d8 | 670 | if (fd == \-1) |
8568021d | 671 | handle_error("open"); |
74fa61b7 | 672 | |
4407d3d8 MK |
673 | if (fstat(fd, &sb) == \-1) /* To obtain file size */ |
674 | handle_error("fstat"); | |
74fa61b7 MK |
675 | |
676 | offset = atoi(argv[2]); | |
677 | pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) \- 1); | |
678 | /* offset for mmap() must be page aligned */ | |
679 | ||
680 | if (offset >= sb.st_size) { | |
681 | fprintf(stderr, "offset is past end of file\\n"); | |
682 | exit(EXIT_FAILURE); | |
683 | } | |
684 | ||
685 | if (argc == 4) { | |
686 | length = atoi(argv[3]); | |
687 | if (offset + length > sb.st_size) | |
688 | length = sb.st_size \- offset; | |
f81fb444 | 689 | /* Can\(aqt display bytes past end of file */ |
5b6adad1 | 690 | |
74fa61b7 MK |
691 | } else { /* No length arg ==> display to end of file */ |
692 | length = sb.st_size \- offset; | |
693 | } | |
694 | ||
695 | addr = mmap(NULL, length + offset \- pa_offset, PROT_READ, | |
696 | MAP_PRIVATE, fd, pa_offset); | |
4407d3d8 MK |
697 | if (addr == MAP_FAILED) |
698 | handle_error("mmap"); | |
74fa61b7 MK |
699 | |
700 | s = write(STDOUT_FILENO, addr + offset \- pa_offset, length); | |
701 | if (s != length) { | |
702 | if (s == \-1) | |
4407d3d8 MK |
703 | handle_error("write"); |
704 | ||
705 | fprintf(stderr, "partial write"); | |
74fa61b7 MK |
706 | exit(EXIT_FAILURE); |
707 | } | |
708 | ||
709 | exit(EXIT_SUCCESS); | |
c54ed37e | 710 | } |
74fa61b7 | 711 | .fi |
47297adb | 712 | .SH SEE ALSO |
fea681da | 713 | .BR getpagesize (2), |
f75c3a3b | 714 | .BR mincore (2), |
fea681da MK |
715 | .BR mlock (2), |
716 | .BR mmap2 (2), | |
54504ac3 | 717 | .BR mprotect (2), |
fea681da MK |
718 | .BR mremap (2), |
719 | .BR msync (2), | |
931e4e25 | 720 | .BR remap_file_pages (2), |
83cd3686 | 721 | .BR setrlimit (2), |
7921f13b | 722 | .BR shmat (2), |
f93af9c6 MK |
723 | .BR shm_open (3), |
724 | .BR shm_overview (7) | |
173fe7e7 | 725 | |
fea681da MK |
726 | B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. |
727 | .\" | |
728 | .\" Repeat after me: private read-only mappings are 100% equivalent to | |
729 | .\" shared read-only mappings. No ifs, buts, or maybes. -- Linus |