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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 | .\" |
93015253 | 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15 | .\" manual page may be incorrect or out-of-date.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 | .\" |
67d2c687 | 40 | .TH MMAP 2 2015-05-07 "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 : |
ca90e95a | 118 | .TP |
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. | |
72e8bdae MK |
123 | (To precisely control when updates are carried through |
124 | to the underlying file requires the use of | |
125 | .BR msync (2).) | |
fea681da MK |
126 | .TP |
127 | .B MAP_PRIVATE | |
128 | Create a private copy-on-write mapping. | |
a62f5121 MK |
129 | Updates to the mapping are not visible to other processes |
130 | mapping the same file, and are not carried through to | |
131 | the underlying file. | |
fea681da | 132 | It is unspecified whether changes made to the file after the |
1a956089 | 133 | .BR mmap () |
fea681da MK |
134 | call are visible in the mapped region. |
135 | .LP | |
a62f5121 MK |
136 | Both of these flags are described in POSIX.1-2001. |
137 | ||
138 | In addition, zero or more of the following values can be ORed in | |
139 | .IR flags : | |
fea681da | 140 | .TP |
c368e7ca MK |
141 | .BR MAP_32BIT " (since Linux 2.4.20, 2.6)" |
142 | Put the mapping into the first 2 Gigabytes of the process address space. | |
33a0ccb2 | 143 | This flag is supported only on x86-64, for 64-bit programs. |
c368e7ca MK |
144 | It was added to allow thread stacks to be allocated somewhere |
145 | in the first 2GB of memory, | |
146 | so as to improve context-switch performance on some early | |
147 | 64-bit processors. | |
148 | .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 | |
149 | Modern x86-64 processors no longer have this performance problem, | |
150 | so use of this flag is not required on those systems. | |
151 | The | |
83314009 | 152 | .B MAP_32BIT |
c368e7ca | 153 | flag is ignored when |
a62f5121 | 154 | .B MAP_FIXED |
83314009 | 155 | is set. |
fea681da | 156 | .TP |
a62f5121 | 157 | .B MAP_ANON |
c13182ef MK |
158 | Synonym for |
159 | .BR MAP_ANONYMOUS . | |
a62f5121 MK |
160 | Deprecated. |
161 | .TP | |
fea681da | 162 | .B MAP_ANONYMOUS |
5e8cde2f | 163 | The mapping is not backed by any file; |
d9bfdb9c | 164 | its contents are initialized to zero. |
5e8cde2f | 165 | The |
fea681da MK |
166 | .I fd |
167 | and | |
168 | .I offset | |
a62f5121 | 169 | arguments are ignored; |
c13182ef | 170 | however, some implementations require |
a62f5121 | 171 | .I fd |
c13182ef MK |
172 | to be \-1 if |
173 | .B MAP_ANONYMOUS | |
174 | (or | |
175 | .BR MAP_ANON ) | |
a62f5121 MK |
176 | is specified, |
177 | and portable applications should ensure this. | |
c13182ef | 178 | The use of |
a62f5121 | 179 | .B MAP_ANONYMOUS |
c13182ef | 180 | in conjunction with |
51ffcca0 | 181 | .B MAP_SHARED |
33a0ccb2 | 182 | is supported on Linux only since kernel 2.4. |
fea681da | 183 | .TP |
83314009 MK |
184 | .B MAP_DENYWRITE |
185 | This flag is ignored. | |
186 | .\" Introduced in 1.1.36, removed in 1.3.24. | |
d9bfdb9c | 187 | (Long ago, it signaled that attempts to write to the underlying file |
83314009 MK |
188 | should fail with |
189 | .BR ETXTBUSY . | |
190 | But this was a source of denial-of-service attacks.) | |
191 | .TP | |
192 | .B MAP_EXECUTABLE | |
193 | This flag is ignored. | |
194 | .\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link. | |
d9bfdb9c | 195 | .\" (Long ago, it signaled that the underlying file is an executable. |
83314009 MK |
196 | .\" However, that information was not really used anywhere.) |
197 | .\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of | |
198 | .\" MAP_DENYWRITE? | |
199 | .TP | |
fea681da | 200 | .B MAP_FILE |
c13182ef MK |
201 | Compatibility flag. |
202 | Ignored. | |
988db661 | 203 | .\" On some systems, this was required as the opposite of |
83314009 | 204 | .\" MAP_ANONYMOUS -- mtk, 1 May 2007 |
fea681da | 205 | .TP |
51ffcca0 | 206 | .B MAP_FIXED |
83314009 | 207 | Don't interpret |
14f5ae6d | 208 | .I addr |
83314009 | 209 | as a hint: place the mapping at exactly that address. |
14f5ae6d | 210 | .I addr |
83314009 MK |
211 | must be a multiple of the page size. |
212 | If the memory region specified by | |
14f5ae6d | 213 | .I addr |
83314009 MK |
214 | and |
215 | .I len | |
216 | overlaps pages of any existing mapping(s), then the overlapped | |
217 | part of the existing mapping(s) will be discarded. | |
218 | If the specified address cannot be used, | |
219 | .BR mmap () | |
220 | will fail. | |
221 | Because requiring a fixed address for a mapping is less portable, | |
222 | the use of this option is discouraged. | |
fea681da | 223 | .TP |
83314009 MK |
224 | .B MAP_GROWSDOWN |
225 | Used for stacks. | |
226 | Indicates to the kernel virtual memory system that the mapping | |
5fab2e7c | 227 | should extend downward in memory. |
83314009 | 228 | .TP |
76a34baa MK |
229 | .BR MAP_HUGETLB " (since Linux 2.6.32)" |
230 | Allocate the mapping using "huge pages." | |
66a9882e | 231 | See the Linux kernel source file |
76a34baa MK |
232 | .I Documentation/vm/hugetlbpage.txt |
233 | for further information. | |
234 | .TP | |
5d2038b6 MK |
235 | .BR MAP_HUGE_2MB ", " MAP_HUGE_1GB " (since Linux 3.8)" |
236 | .\" See https://lwn.net/Articles/533499/ | |
237 | Used in conjunction with | |
238 | .B MAP_HUGETLB | |
239 | to select alternative hugetlb page sizes (respectively, 2 MB and 1 GB) | |
240 | on systems that support multiple hugetlb page sizes. | |
241 | ||
242 | More generally, the desired huge page size can be configured by encoding | |
243 | the base-2 logarithm of the desired page size in the six bits at the offset | |
244 | .BR MAP_HUGE_SHIFT . | |
245 | (A value of zero in this bit field provides the default huge page size; | |
246 | the default huge page size can be discovered vie the | |
247 | .I Hugepagesize | |
248 | field exposed by | |
249 | .IR /proc/meminfo .) | |
250 | Thus, the above two constants are defined as: | |
251 | ||
252 | .nf | |
253 | .in +4n | |
254 | #define MAP_HUGE_2MB (21 << MAP_HUGE_SHIFT) | |
255 | #define MAP_HUGE_1GB (30 << MAP_HUGE_SHIFT) | |
256 | .in | |
257 | .fi | |
258 | ||
259 | The range of huge page sizes that are supported by the system | |
260 | can be discovered by listing the subdirectories in | |
261 | .IR /sys/kernel/mm/hugepages . | |
262 | .TP | |
83314009 | 263 | .BR MAP_LOCKED " (since Linux 2.5.37)" |
7e3786bc | 264 | Mark the mmaped region to be locked in the same way as |
74d32233 | 265 | .BR mlock (2). |
7e3786bc MH |
266 | This implementation will try to populate (prefault) the whole range but |
267 | the mmap call doesn't fail with | |
268 | .B ENOMEM | |
911f1c7a MK |
269 | if this fails. |
270 | Therefore major faults might happen later on. | |
271 | So the semantic is not as strong as | |
7e3786bc | 272 | .BR mlock (2). |
911f1c7a | 273 | One should use |
7e3786bc | 274 | .BR mmap (2) |
911f1c7a | 275 | plus |
7e3786bc | 276 | .BR mlock (2) |
911f1c7a MK |
277 | when major faults are not acceptable after the initialization of the mapping. |
278 | The | |
279 | .BR MAP_LOCKED | |
280 | flag is ignored in older kernels. | |
83314009 | 281 | .\" If set, the mapped pages will not be swapped out. |
fea681da MK |
282 | .TP |
283 | .BR MAP_NONBLOCK " (since Linux 2.5.46)" | |
51ffcca0 MK |
284 | Only meaningful in conjunction with |
285 | .BR MAP_POPULATE . | |
c13182ef | 286 | Don't perform read-ahead: |
33a0ccb2 | 287 | create page tables entries only for pages |
51ffcca0 | 288 | that are already present in RAM. |
7c40de08 | 289 | Since Linux 2.6.23, this flag causes |
f38fa944 MK |
290 | .BR MAP_POPULATE |
291 | to do nothing. | |
487c2f05 | 292 | One day, the combination of |
f38fa944 MK |
293 | .BR MAP_POPULATE |
294 | and | |
295 | .BR MAP_NONBLOCK | |
3b777aff | 296 | may be reimplemented. |
83314009 MK |
297 | .TP |
298 | .B MAP_NORESERVE | |
299 | Do not reserve swap space for this mapping. | |
300 | When swap space is reserved, one has the guarantee | |
301 | that it is possible to modify the mapping. | |
8bd58774 MK |
302 | When swap space is not reserved one might get |
303 | .B SIGSEGV | |
304 | upon a write | |
83314009 MK |
305 | if no physical memory is available. |
306 | See also the discussion of the file | |
307 | .I /proc/sys/vm/overcommit_memory | |
308 | in | |
309 | .BR proc (5). | |
33a0ccb2 | 310 | In kernels before 2.6, this flag had effect only for |
83314009 MK |
311 | private writable mappings. |
312 | .TP | |
313 | .BR MAP_POPULATE " (since Linux 2.5.46)" | |
f38fa944 MK |
314 | Populate (prefault) page tables for a mapping. |
315 | For a file mapping, this causes read-ahead on the file. | |
bbebbb6d | 316 | This will help to reduce blocking on page faults later. |
f38fa944 | 317 | .BR MAP_POPULATE |
33a0ccb2 | 318 | is supported for private mappings only since Linux 2.6.23. |
e6205b0c MK |
319 | .TP |
320 | .BR MAP_STACK " (since Linux 2.6.27)" | |
321 | Allocate the mapping at an address suitable for a process | |
322 | or thread stack. | |
323 | This flag is currently a no-op, | |
324 | but is used in the glibc threading implementation so that | |
325 | if some architectures require special treatment for stack allocations, | |
326 | support can later be transparently implemented for glibc. | |
67b59ff5 | 327 | .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 |
e6205b0c MK |
328 | .\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7 |
329 | .\" http://thread.gmane.org/gmane.linux.kernel/720412 | |
330 | .\" "pthread_create() slow for many threads; also time to revisit 64b | |
331 | .\" context switch optimization?" | |
12062404 MK |
332 | .TP |
333 | .BR MAP_UNINITIALIZED " (since Linux 2.6.33)" | |
334 | Don't clear anonymous pages. | |
335 | This flag is intended to improve performance on embedded devices. | |
33a0ccb2 | 336 | This flag is honored only if the kernel was configured with the |
12062404 MK |
337 | .B CONFIG_MMAP_ALLOW_UNINITIALIZED |
338 | option. | |
339 | Because of the security implications, | |
340 | that option is normally enabled only on embedded devices | |
341 | (i.e., devices where one has complete control of the contents of user memory). | |
a62f5121 | 342 | .LP |
7c7adcbe MK |
343 | Of the above flags, only |
344 | .B MAP_FIXED | |
345 | is specified in POSIX.1-2001. | |
346 | However, most systems also support | |
347 | .B MAP_ANONYMOUS | |
348 | (or its synonym | |
349 | .BR MAP_ANON ). | |
350 | .LP | |
2d6cfc1a MK |
351 | Some systems document the additional flags |
352 | .BR MAP_AUTOGROW , | |
353 | .BR MAP_AUTORESRV , | |
354 | .BR MAP_COPY , | |
355 | and | |
356 | .BR MAP_LOCAL . | |
fea681da | 357 | .LP |
fea681da | 358 | Memory mapped by |
1a956089 | 359 | .BR mmap () |
fea681da MK |
360 | is preserved across |
361 | .BR fork (2), | |
362 | with the same attributes. | |
363 | .LP | |
c13182ef MK |
364 | A file is mapped in multiples of the page size. |
365 | For a file that is not | |
fea681da | 366 | a multiple of the page size, the remaining memory is zeroed when mapped, |
c13182ef MK |
367 | and writes to that region are not written out to the file. |
368 | The effect of | |
fea681da MK |
369 | changing the size of the underlying file of a mapping on the pages that |
370 | correspond to added or removed regions of the file is unspecified. | |
de5f7e28 | 371 | .SS munmap() |
fea681da | 372 | The |
1a956089 | 373 | .BR munmap () |
fea681da MK |
374 | system call deletes the mappings for the specified address range, and |
375 | causes further references to addresses within the range to generate | |
c13182ef MK |
376 | invalid memory references. |
377 | The region is also automatically unmapped | |
378 | when the process is terminated. | |
379 | On the other hand, closing the file | |
fea681da MK |
380 | descriptor does not unmap the region. |
381 | .LP | |
382 | The address | |
14f5ae6d | 383 | .I addr |
0e824bcb MK |
384 | must be a multiple of the page size (but |
385 | .I length | |
386 | need not be). | |
c13182ef | 387 | All pages containing a part |
fea681da | 388 | of the indicated range are unmapped, and subsequent references |
8bd58774 MK |
389 | to these pages will generate |
390 | .BR SIGSEGV . | |
c13182ef | 391 | It is not an error if the |
fea681da | 392 | indicated range does not contain any mapped pages. |
de5f7e28 | 393 | .SS Timestamps changes for file-backed mappings |
fea681da | 394 | For file-backed mappings, the |
8478ee02 | 395 | .I st_atime |
fea681da | 396 | field for the mapped file may be updated at any time between the |
1a956089 | 397 | .BR mmap () |
fea681da MK |
398 | and the corresponding unmapping; the first reference to a mapped |
399 | page will update the field if it has not been already. | |
400 | .LP | |
401 | The | |
8478ee02 | 402 | .I st_ctime |
fea681da | 403 | and |
8478ee02 | 404 | .I st_mtime |
c13182ef MK |
405 | field for a file mapped with |
406 | .B PROT_WRITE | |
407 | and | |
408 | .B MAP_SHARED | |
51ffcca0 | 409 | will be updated after |
fea681da | 410 | a write to the mapped region, and before a subsequent |
0bfa087b | 411 | .BR msync (2) |
c13182ef MK |
412 | with the |
413 | .B MS_SYNC | |
414 | or | |
0daa9e92 | 415 | .B MS_ASYNC |
51ffcca0 | 416 | flag, if one occurs. |
47297adb | 417 | .SH RETURN VALUE |
fea681da | 418 | On success, |
1a956089 | 419 | .BR mmap () |
fea681da MK |
420 | returns a pointer to the mapped area. |
421 | On error, the value | |
422 | .B MAP_FAILED | |
c13182ef | 423 | (that is, |
009df872 | 424 | .IR "(void\ *)\ \-1" ) |
5e8cde2f | 425 | is returned, and |
fea681da | 426 | .I errno |
80691a91 MK |
427 | is set to indicate the cause of the error. |
428 | ||
fea681da | 429 | On success, |
1a956089 | 430 | .BR munmap () |
80691a91 MK |
431 | returns 0. |
432 | On failure, it returns \-1, and | |
fea681da | 433 | .I errno |
80691a91 | 434 | is set to indicate the cause of the error (probably to |
51ffcca0 | 435 | .BR EINVAL ). |
fea681da MK |
436 | .SH ERRORS |
437 | .TP | |
438 | .B EACCES | |
439 | A file descriptor refers to a non-regular file. | |
5e7c71f6 | 440 | Or a file mapping was requested, but |
fea681da MK |
441 | .I fd |
442 | is not open for reading. | |
c13182ef MK |
443 | Or |
444 | .B MAP_SHARED | |
445 | was requested and | |
446 | .B PROT_WRITE | |
51ffcca0 | 447 | is set, but |
fea681da | 448 | .I fd |
682edefb MK |
449 | is not open in read/write |
450 | .RB ( O_RDWR ) | |
451 | mode. | |
c13182ef MK |
452 | Or |
453 | .B PROT_WRITE | |
51ffcca0 | 454 | is set, but the file is append-only. |
fea681da MK |
455 | .TP |
456 | .B EAGAIN | |
83cd3686 MK |
457 | The file has been locked, or too much memory has been locked (see |
458 | .BR setrlimit (2)). | |
fea681da MK |
459 | .TP |
460 | .B EBADF | |
461 | .I fd | |
c13182ef | 462 | is not a valid file descriptor (and |
51ffcca0 MK |
463 | .B MAP_ANONYMOUS |
464 | was not set). | |
fea681da MK |
465 | .TP |
466 | .B EINVAL | |
467 | We don't like | |
14f5ae6d | 468 | .IR addr , |
62a04c81 | 469 | .IR length , |
fea681da | 470 | or |
0daa9e92 | 471 | .I offset |
62a04c81 MK |
472 | (e.g., they are too large, or not aligned on a page boundary). |
473 | .TP | |
474 | .B EINVAL | |
f99fc197 | 475 | (since Linux 2.6.12) |
fea681da | 476 | .I length |
62a04c81 MK |
477 | was 0. |
478 | .TP | |
479 | .B EINVAL | |
480 | .I flags | |
481 | contained neither | |
482 | .B MAP_PRIVATE | |
fea681da | 483 | or |
62a04c81 MK |
484 | .BR MAP_SHARED , |
485 | or contained both of these values. | |
fea681da MK |
486 | .TP |
487 | .B ENFILE | |
488 | .\" This is for shared anonymous segments | |
489 | .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() | |
490 | The system limit on the total number of open files has been reached. | |
491 | .\" .TP | |
492 | .\" .B ENOEXEC | |
493 | .\" A file could not be mapped for reading. | |
494 | .TP | |
495 | .B ENODEV | |
9ee4a2b6 | 496 | The underlying filesystem of the specified file does not support |
fea681da MK |
497 | memory mapping. |
498 | .TP | |
499 | .B ENOMEM | |
74309bed MK |
500 | No memory is available. |
501 | .TP | |
502 | .B ENOMEM | |
503 | The process's maximum number of mappings would have been exceeded. | |
c0b89788 MK |
504 | This error can also occur for |
505 | .BR munmap (2), | |
506 | when unmapping a region in the middle of an existing mapping, | |
507 | since this results in two smaller mappings on either side of | |
508 | the region being unmapped. | |
fea681da MK |
509 | .TP |
510 | .B EPERM | |
511 | The | |
512 | .I prot | |
513 | argument asks for | |
514 | .B PROT_EXEC | |
9ee4a2b6 | 515 | but the mapped area belongs to a file on a filesystem that |
fea681da MK |
516 | was mounted no-exec. |
517 | .\" (Since 2.4.25 / 2.6.0.) | |
518 | .TP | |
fbab10e5 MK |
519 | .B EPERM |
520 | The operation was prevented by a file seal; see | |
521 | .BR fcntl (2). | |
522 | .TP | |
fea681da | 523 | .B ETXTBSY |
c13182ef | 524 | .B MAP_DENYWRITE |
51ffcca0 | 525 | was set but the object specified by |
fea681da MK |
526 | .I fd |
527 | is open for writing. | |
da3ce098 CH |
528 | .TP |
529 | .B EOVERFLOW | |
42b437ca MK |
530 | On 32-bit architecture together with the large file extension |
531 | (i.e., using 64-bit | |
532 | .IR off_t ): | |
533 | the number of pages used for | |
534 | .I length | |
535 | plus number of pages used for | |
536 | .I offset | |
537 | would overflow | |
538 | .I "unsigned long" | |
539 | (32 bits). | |
fea681da MK |
540 | .LP |
541 | Use of a mapped region can result in these signals: | |
542 | .TP | |
543 | .B SIGSEGV | |
1e321034 | 544 | Attempted write into a region mapped as read-only. |
fea681da MK |
545 | .TP |
546 | .B SIGBUS | |
547 | Attempted access to a portion of the buffer that does not correspond | |
548 | to the file (for example, beyond the end of the file, including the | |
549 | case where another process has truncated the file). | |
8fddf95a MS |
550 | .SH ATTRIBUTES |
551 | For an explanation of the terms used in this section, see | |
552 | .BR attributes (7). | |
553 | .TS | |
554 | allbox; | |
555 | lbw18 lb lb | |
556 | l l l. | |
557 | Interface Attribute Value | |
558 | T{ | |
559 | .BR mmap (), | |
560 | .BR munmap () | |
561 | T} Thread safety MT-Safe | |
562 | .TE | |
47297adb | 563 | .SH CONFORMING TO |
2b2581ee MK |
564 | SVr4, 4.4BSD, POSIX.1-2001. |
565 | .\" SVr4 documents additional error codes ENXIO and ENODEV. | |
566 | .\" SUSv2 documents additional error codes EMFILE and EOVERFLOW. | |
fea681da MK |
567 | .SH AVAILABILITY |
568 | On POSIX systems on which | |
1a956089 | 569 | .BR mmap (), |
9af134cd | 570 | .BR msync (2), |
fea681da | 571 | and |
1a956089 | 572 | .BR munmap () |
fea681da MK |
573 | are available, |
574 | .B _POSIX_MAPPED_FILES | |
6387216b MK |
575 | is defined in \fI<unistd.h>\fP to a value greater than 0. |
576 | (See also | |
fea681da | 577 | .BR sysconf (3).) |
97c1eac8 | 578 | .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L. |
fea681da MK |
579 | .\" -1: unavailable, 0: ask using sysconf(). |
580 | .\" glibc defines it to 1. | |
a1d5f77c | 581 | .SH NOTES |
34ccb744 | 582 | On some hardware architectures (e.g., i386), |
0daa9e92 | 583 | .B PROT_WRITE |
f3edaabb MK |
584 | implies |
585 | .BR PROT_READ . | |
a1d5f77c MK |
586 | It is architecture dependent whether |
587 | .B PROT_READ | |
588 | implies | |
589 | .B PROT_EXEC | |
590 | or not. | |
591 | Portable programs should always set | |
592 | .B PROT_EXEC | |
593 | if they intend to execute code in the new mapping. | |
80d17cfa MK |
594 | |
595 | The portable way to create a mapping is to specify | |
596 | .I addr | |
597 | as 0 (NULL), and omit | |
598 | .B MAP_FIXED | |
599 | from | |
600 | .IR flags . | |
601 | In this case, the system chooses the address for the mapping; | |
46cdb997 | 602 | the address is chosen so as not to conflict with any existing mapping, |
80d17cfa MK |
603 | and will not be 0. |
604 | If the | |
605 | .B MAP_FIXED | |
606 | flag is specified, and | |
607 | .I addr | |
491cd2f0 | 608 | is 0 (NULL), then the mapped address will be 0 (NULL). |
45e97e2a MK |
609 | |
610 | Certain | |
611 | .I flags | |
612 | constants are defined only if either | |
613 | .BR _BSD_SOURCE | |
614 | or | |
615 | .BR _SVID_SOURCE | |
616 | is defined. | |
617 | (Requiring | |
618 | .BR _GNU_SOURCE | |
619 | also suffices, | |
620 | and requiring that macro specifically would have been more logical, | |
76c637e1 | 621 | since these flags are all Linux-specific.) |
45e97e2a MK |
622 | The relevant flags are: |
623 | .BR MAP_32BIT , | |
624 | .BR MAP_ANONYMOUS | |
625 | (and the synonym | |
626 | .BR MAP_ANON ), | |
627 | .BR MAP_DENYWRITE , | |
628 | .BR MAP_EXECUTABLE , | |
629 | .BR MAP_FILE , | |
630 | .BR MAP_GROWSDOWN , | |
631 | .BR MAP_HUGETLB , | |
632 | .BR MAP_LOCKED , | |
633 | .BR MAP_NONBLOCK , | |
634 | .BR MAP_NORESERVE , | |
635 | .BR MAP_POPULATE , | |
636 | and | |
637 | .BR MAP_STACK . | |
35c189fb | 638 | .\" |
0722a578 | 639 | .SS C library/kernel differences |
35c189fb MK |
640 | This page describes the interface provided by the glibc |
641 | .BR mmap () | |
642 | wrapper function. | |
643 | Originally, this function invoked a system call of the same name. | |
644 | Since kernel 2.4, that system call has been superseded by | |
645 | .BR mmap2 (2), | |
646 | and nowadays | |
647 | .\" Since around glibc 2.1/2.2, depending on the platform. | |
648 | the glibc | |
649 | .BR mmap () | |
650 | wrapper function invokes | |
651 | .BR mmap2 (2) | |
652 | with a suitably adjusted value for | |
653 | .IR offset . | |
fea681da | 654 | .SH BUGS |
c13182ef MK |
655 | On Linux there are no guarantees like those suggested above under |
656 | .BR MAP_NORESERVE . | |
dbc53ca8 | 657 | By default, any process can be killed |
fea681da | 658 | at any moment when the system runs out of memory. |
dbc53ca8 MK |
659 | |
660 | In kernels before 2.6.7, the | |
661 | .B MAP_POPULATE | |
33a0ccb2 | 662 | flag has effect only if |
dbc53ca8 MK |
663 | .I prot |
664 | is specified as | |
665 | .BR PROT_NONE . | |
c8f3e580 | 666 | |
c13182ef | 667 | SUSv3 specifies that |
c8f3e580 MK |
668 | .BR mmap () |
669 | should fail if | |
670 | .I length | |
671 | is 0. | |
672 | However, in kernels before 2.6.12, | |
673 | .BR mmap () | |
674 | succeeded in this case: no mapping was created and the call returned | |
14f5ae6d | 675 | .IR addr . |
c8f3e580 MK |
676 | Since kernel 2.6.12, |
677 | .BR mmap () | |
678 | fails with the error | |
679 | .B EINVAL | |
680 | for this case. | |
2e43522f | 681 | |
a780f17b MK |
682 | POSIX specifies that the system shall always |
683 | zero fill any partial page at the end | |
b072a788 | 684 | of the object and that system will never write any modification of the |
a780f17b MK |
685 | object beyond its end. |
686 | On Linux, when you write data to such partial page after the end | |
b072a788 | 687 | of the object, the data stays in the page cache even after the file |
a780f17b MK |
688 | is closed and unmapped |
689 | and even though the data is never written to the file itself, | |
690 | subsequent mappings may see the modified content. | |
691 | In some cases, this could be fixed by calling | |
692 | .BR msync (2) | |
693 | before the unmap takes place; | |
694 | however, this doesn't work on tmpfs | |
695 | (for example, when using POSIX shared memory interface documented in | |
696 | .BR shm_overview (7)). | |
74fa61b7 | 697 | .SH EXAMPLE |
2e001ad4 MK |
698 | .\" FIXME . Add an example here that uses an anonymous shared region for |
699 | .\" IPC between parent and child. | |
74fa61b7 MK |
700 | .PP |
701 | The following program prints part of the file specified in | |
702 | its first command-line argument to standard output. | |
703 | The range of bytes to be printed is specified via offset and length | |
704 | values in the second and third command-line arguments. | |
705 | The program creates a memory mapping of the required | |
706 | pages of the file and then uses | |
707 | .BR write (2) | |
708 | to output the desired bytes. | |
f30b7415 | 709 | .SS Program source |
74fa61b7 | 710 | .nf |
74fa61b7 MK |
711 | #include <sys/mman.h> |
712 | #include <sys/stat.h> | |
713 | #include <fcntl.h> | |
714 | #include <stdio.h> | |
715 | #include <stdlib.h> | |
716 | #include <unistd.h> | |
717 | ||
4407d3d8 MK |
718 | #define handle_error(msg) \\ |
719 | do { perror(msg); exit(EXIT_FAILURE); } while (0) | |
720 | ||
74fa61b7 MK |
721 | int |
722 | main(int argc, char *argv[]) | |
723 | { | |
724 | char *addr; | |
725 | int fd; | |
726 | struct stat sb; | |
727 | off_t offset, pa_offset; | |
728 | size_t length; | |
729 | ssize_t s; | |
730 | ||
fbbfa7ce | 731 | if (argc < 3 || argc > 4) { |
74fa61b7 MK |
732 | fprintf(stderr, "%s file offset [length]\\n", argv[0]); |
733 | exit(EXIT_FAILURE); | |
734 | } | |
735 | ||
736 | fd = open(argv[1], O_RDONLY); | |
4407d3d8 | 737 | if (fd == \-1) |
8568021d | 738 | handle_error("open"); |
74fa61b7 | 739 | |
4407d3d8 MK |
740 | if (fstat(fd, &sb) == \-1) /* To obtain file size */ |
741 | handle_error("fstat"); | |
74fa61b7 MK |
742 | |
743 | offset = atoi(argv[2]); | |
744 | pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) \- 1); | |
745 | /* offset for mmap() must be page aligned */ | |
746 | ||
747 | if (offset >= sb.st_size) { | |
748 | fprintf(stderr, "offset is past end of file\\n"); | |
749 | exit(EXIT_FAILURE); | |
750 | } | |
751 | ||
752 | if (argc == 4) { | |
753 | length = atoi(argv[3]); | |
754 | if (offset + length > sb.st_size) | |
755 | length = sb.st_size \- offset; | |
f81fb444 | 756 | /* Can\(aqt display bytes past end of file */ |
5b6adad1 | 757 | |
74fa61b7 MK |
758 | } else { /* No length arg ==> display to end of file */ |
759 | length = sb.st_size \- offset; | |
760 | } | |
761 | ||
762 | addr = mmap(NULL, length + offset \- pa_offset, PROT_READ, | |
763 | MAP_PRIVATE, fd, pa_offset); | |
4407d3d8 MK |
764 | if (addr == MAP_FAILED) |
765 | handle_error("mmap"); | |
74fa61b7 MK |
766 | |
767 | s = write(STDOUT_FILENO, addr + offset \- pa_offset, length); | |
768 | if (s != length) { | |
769 | if (s == \-1) | |
4407d3d8 MK |
770 | handle_error("write"); |
771 | ||
772 | fprintf(stderr, "partial write"); | |
74fa61b7 MK |
773 | exit(EXIT_FAILURE); |
774 | } | |
775 | ||
776 | exit(EXIT_SUCCESS); | |
c54ed37e | 777 | } |
74fa61b7 | 778 | .fi |
47297adb | 779 | .SH SEE ALSO |
fea681da | 780 | .BR getpagesize (2), |
c4d76cd9 | 781 | .BR memfd_create (2), |
f75c3a3b | 782 | .BR mincore (2), |
fea681da MK |
783 | .BR mlock (2), |
784 | .BR mmap2 (2), | |
54504ac3 | 785 | .BR mprotect (2), |
fea681da MK |
786 | .BR mremap (2), |
787 | .BR msync (2), | |
931e4e25 | 788 | .BR remap_file_pages (2), |
83cd3686 | 789 | .BR setrlimit (2), |
7921f13b | 790 | .BR shmat (2), |
f93af9c6 MK |
791 | .BR shm_open (3), |
792 | .BR shm_overview (7) | |
173fe7e7 | 793 | |
0bf14b87 MK |
794 | The descriptions of the following files in |
795 | .BR proc (5): | |
796 | .IR /proc/[pid]/maps , | |
797 | .IR /proc/[pid]/map_files , | |
798 | and | |
799 | .IR /proc/[pid]/smaps . | |
800 | ||
fea681da MK |
801 | B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. |
802 | .\" | |
803 | .\" Repeat after me: private read-only mappings are 100% equivalent to | |
804 | .\" shared read-only mappings. No ifs, buts, or maybes. -- Linus |