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1 | .\" Hey Emacs! This file is -*- nroff -*- source. |
2 | .\" | |
a62f5121 | 3 | .\" Copyright (C) 1996 Andries Brouwer <aeb@cwi.nl> |
c11b1abf | 4 | .\" and Copyright (C) 2006, 2007 Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da MK |
5 | .\" |
6 | .\" Permission is granted to make and distribute verbatim copies of this | |
7 | .\" manual provided the copyright notice and this permission notice are | |
8 | .\" preserved on all copies. | |
9 | .\" | |
10 | .\" Permission is granted to copy and distribute modified versions of this | |
11 | .\" manual under the conditions for verbatim copying, provided that the | |
12 | .\" entire resulting derived work is distributed under the terms of a | |
13 | .\" permission notice identical to this one. | |
c13182ef | 14 | .\" |
fea681da MK |
15 | .\" Since the Linux kernel and libraries are constantly changing, this |
16 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
17 | .\" responsibility for errors or omissions, or for damages resulting from | |
18 | .\" the use of the information contained herein. The author(s) may not | |
19 | .\" have taken the same level of care in the production of this manual, | |
20 | .\" which is licensed free of charge, as they might when working | |
21 | .\" professionally. | |
c13182ef | 22 | .\" |
fea681da MK |
23 | .\" Formatted or processed versions of this manual, if unaccompanied by |
24 | .\" the source, must acknowledge the copyright and authors of this work. | |
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 | .\" |
76a34baa | 40 | .TH MMAP 2 2009-09-26 "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 |
fea681da | 52 | .SH DESCRIPTION |
1a956089 | 53 | .BR mmap () |
c13182ef | 54 | creates a new mapping in the virtual address space of |
5e8cde2f MK |
55 | the calling process. |
56 | The starting address for the new mapping is specified in | |
14f5ae6d | 57 | .IR addr . |
5e8cde2f MK |
58 | The |
59 | .I length | |
60 | argument specifies the length of the mapping. | |
61 | ||
62 | If | |
14f5ae6d | 63 | .I addr |
5e8cde2f MK |
64 | is NULL, |
65 | then the kernel chooses the address at which to create the mapping; | |
66 | this is the most portable method of creating a new mapping. | |
c13182ef | 67 | If |
14f5ae6d | 68 | .I addr |
5e8cde2f MK |
69 | is not NULL, |
70 | then the kernel takes it as a hint about where to place the mapping; | |
6aa7db0a MK |
71 | on Linux, the mapping will be created at a nearby page boundary. |
72 | .\" Before Linux 2.6.24, the address was rounded up to the next page | |
29328361 | 73 | .\" boundary; since 2.6.24, it is rounded down! |
5e8cde2f MK |
74 | The address of the new mapping is returned as the result of the call. |
75 | ||
76 | The contents of a file mapping (as opposed to an anonymous mapping; see | |
77 | .B MAP_ANONYMOUS | |
d9bfdb9c | 78 | below), are initialized using |
fea681da MK |
79 | .I length |
80 | bytes starting at offset | |
81 | .I offset | |
5e8cde2f MK |
82 | in the file (or other object) referred to by the file descriptor |
83 | .IR fd . | |
a62f5121 MK |
84 | .I offset |
85 | must be a multiple of the page size as returned by | |
86 | .IR sysconf(_SC_PAGE_SIZE) . | |
fea681da MK |
87 | .LP |
88 | The | |
89 | .I prot | |
c13182ef | 90 | argument describes the desired memory protection of the mapping |
5e8cde2f MK |
91 | (and must not conflict with the open mode of the file). |
92 | It is either | |
fea681da | 93 | .B PROT_NONE |
a62f5121 | 94 | or the bitwise OR of one or more of the following flags: |
fea681da MK |
95 | .TP 1.1i |
96 | .B PROT_EXEC | |
97 | Pages may be executed. | |
98 | .TP | |
99 | .B PROT_READ | |
100 | Pages may be read. | |
101 | .TP | |
102 | .B PROT_WRITE | |
103 | Pages may be written. | |
104 | .TP | |
105 | .B PROT_NONE | |
106 | Pages may not be accessed. | |
107 | .LP | |
108 | The | |
109 | .I flags | |
a62f5121 MK |
110 | argument determines whether updates to the mapping |
111 | are visible to other processes mapping the same region, | |
ba7cb080 | 112 | and whether updates are carried through to the underlying file. |
d9bfdb9c | 113 | This behavior is determined by including exactly one |
a62f5121 | 114 | of the following values in |
5e8cde2f | 115 | .IR flags : |
fea681da | 116 | .TP 1.1i |
fea681da | 117 | .B MAP_SHARED |
c13182ef | 118 | Share this mapping. |
a62f5121 MK |
119 | Updates to the mapping are visible to other processes that map this file, |
120 | and are carried through to the underlying file. | |
fea681da MK |
121 | The file may not actually be updated until |
122 | .BR msync (2) | |
123 | or | |
2777b1ca | 124 | .BR munmap () |
a62f5121 | 125 | is called. |
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. | |
143 | This flag is only supported on x86-64, for 64-bit programs. | |
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 |
74f3f90b | 182 | is only supported on Linux 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 | |
227 | should extend downwards in memory. | |
228 | .TP | |
76a34baa MK |
229 | .BR MAP_HUGETLB " (since Linux 2.6.32)" |
230 | Allocate the mapping using "huge pages." | |
231 | See the kernel source file | |
232 | .I Documentation/vm/hugetlbpage.txt | |
233 | for further information. | |
234 | .TP | |
83314009 MK |
235 | .BR MAP_LOCKED " (since Linux 2.5.37)" |
236 | Lock the pages of the mapped region into memory in the manner of | |
74d32233 | 237 | .BR mlock (2). |
83314009 MK |
238 | This flag is ignored in older kernels. |
239 | .\" If set, the mapped pages will not be swapped out. | |
fea681da MK |
240 | .TP |
241 | .BR MAP_NONBLOCK " (since Linux 2.5.46)" | |
51ffcca0 MK |
242 | Only meaningful in conjunction with |
243 | .BR MAP_POPULATE . | |
c13182ef MK |
244 | Don't perform read-ahead: |
245 | only create page tables entries for pages | |
51ffcca0 | 246 | that are already present in RAM. |
7c40de08 | 247 | Since Linux 2.6.23, this flag causes |
f38fa944 MK |
248 | .BR MAP_POPULATE |
249 | to do nothing. | |
250 | One day the combination of | |
251 | .BR MAP_POPULATE | |
252 | and | |
253 | .BR MAP_NONBLOCK | |
254 | may be re-implemented. | |
83314009 MK |
255 | .TP |
256 | .B MAP_NORESERVE | |
257 | Do not reserve swap space for this mapping. | |
258 | When swap space is reserved, one has the guarantee | |
259 | that it is possible to modify the mapping. | |
8bd58774 MK |
260 | When swap space is not reserved one might get |
261 | .B SIGSEGV | |
262 | upon a write | |
83314009 MK |
263 | if no physical memory is available. |
264 | See also the discussion of the file | |
265 | .I /proc/sys/vm/overcommit_memory | |
266 | in | |
267 | .BR proc (5). | |
268 | In kernels before 2.6, this flag only had effect for | |
269 | private writable mappings. | |
270 | .TP | |
271 | .BR MAP_POPULATE " (since Linux 2.5.46)" | |
f38fa944 MK |
272 | Populate (prefault) page tables for a mapping. |
273 | For a file mapping, this causes read-ahead on the file. | |
83314009 | 274 | Later accesses to the mapping will not be blocked by page faults. |
f38fa944 MK |
275 | .BR MAP_POPULATE |
276 | is only supported for private mappings since Linux 2.6.23. | |
fea681da | 277 | .LP |
a62f5121 MK |
278 | Of the above flags, only |
279 | .B MAP_FIXED | |
280 | is specified in POSIX.1-2001. | |
281 | However, most systems also support | |
282 | .B MAP_ANONYMOUS | |
c13182ef | 283 | (or its synonym |
a62f5121 | 284 | .BR MAP_ANON ). |
e6205b0c MK |
285 | .TP |
286 | .BR MAP_STACK " (since Linux 2.6.27)" | |
287 | Allocate the mapping at an address suitable for a process | |
288 | or thread stack. | |
289 | This flag is currently a no-op, | |
290 | but is used in the glibc threading implementation so that | |
291 | if some architectures require special treatment for stack allocations, | |
292 | support can later be transparently implemented for glibc. | |
67b59ff5 | 293 | .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 |
e6205b0c MK |
294 | .\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7 |
295 | .\" http://thread.gmane.org/gmane.linux.kernel/720412 | |
296 | .\" "pthread_create() slow for many threads; also time to revisit 64b | |
297 | .\" context switch optimization?" | |
a62f5121 | 298 | .LP |
2d6cfc1a MK |
299 | Some systems document the additional flags |
300 | .BR MAP_AUTOGROW , | |
301 | .BR MAP_AUTORESRV , | |
302 | .BR MAP_COPY , | |
303 | and | |
304 | .BR MAP_LOCAL . | |
fea681da | 305 | .LP |
fea681da | 306 | Memory mapped by |
1a956089 | 307 | .BR mmap () |
fea681da MK |
308 | is preserved across |
309 | .BR fork (2), | |
310 | with the same attributes. | |
311 | .LP | |
c13182ef MK |
312 | A file is mapped in multiples of the page size. |
313 | For a file that is not | |
fea681da | 314 | a multiple of the page size, the remaining memory is zeroed when mapped, |
c13182ef MK |
315 | and writes to that region are not written out to the file. |
316 | The effect of | |
fea681da MK |
317 | changing the size of the underlying file of a mapping on the pages that |
318 | correspond to added or removed regions of the file is unspecified. | |
de5f7e28 | 319 | .SS munmap() |
fea681da | 320 | The |
1a956089 | 321 | .BR munmap () |
fea681da MK |
322 | system call deletes the mappings for the specified address range, and |
323 | causes further references to addresses within the range to generate | |
c13182ef MK |
324 | invalid memory references. |
325 | The region is also automatically unmapped | |
326 | when the process is terminated. | |
327 | On the other hand, closing the file | |
fea681da MK |
328 | descriptor does not unmap the region. |
329 | .LP | |
330 | The address | |
14f5ae6d | 331 | .I addr |
c13182ef MK |
332 | must be a multiple of the page size. |
333 | All pages containing a part | |
fea681da | 334 | of the indicated range are unmapped, and subsequent references |
8bd58774 MK |
335 | to these pages will generate |
336 | .BR SIGSEGV . | |
c13182ef | 337 | It is not an error if the |
fea681da | 338 | indicated range does not contain any mapped pages. |
de5f7e28 | 339 | .SS Timestamps changes for file-backed mappings |
fea681da | 340 | For file-backed mappings, the |
8478ee02 | 341 | .I st_atime |
fea681da | 342 | field for the mapped file may be updated at any time between the |
1a956089 | 343 | .BR mmap () |
fea681da MK |
344 | and the corresponding unmapping; the first reference to a mapped |
345 | page will update the field if it has not been already. | |
346 | .LP | |
347 | The | |
8478ee02 | 348 | .I st_ctime |
fea681da | 349 | and |
8478ee02 | 350 | .I st_mtime |
c13182ef MK |
351 | field for a file mapped with |
352 | .B PROT_WRITE | |
353 | and | |
354 | .B MAP_SHARED | |
51ffcca0 | 355 | will be updated after |
fea681da | 356 | a write to the mapped region, and before a subsequent |
0bfa087b | 357 | .BR msync (2) |
c13182ef MK |
358 | with the |
359 | .B MS_SYNC | |
360 | or | |
0daa9e92 | 361 | .B MS_ASYNC |
51ffcca0 | 362 | flag, if one occurs. |
fea681da MK |
363 | .SH "RETURN VALUE" |
364 | On success, | |
1a956089 | 365 | .BR mmap () |
fea681da MK |
366 | returns a pointer to the mapped area. |
367 | On error, the value | |
368 | .B MAP_FAILED | |
c13182ef | 369 | (that is, |
009df872 | 370 | .IR "(void\ *)\ \-1" ) |
5e8cde2f | 371 | is returned, and |
fea681da MK |
372 | .I errno |
373 | is set appropriately. | |
374 | On success, | |
1a956089 | 375 | .BR munmap () |
fea681da MK |
376 | returns 0, on failure \-1, and |
377 | .I errno | |
c13182ef | 378 | is set (probably to |
51ffcca0 | 379 | .BR EINVAL ). |
fea681da MK |
380 | .SH ERRORS |
381 | .TP | |
382 | .B EACCES | |
383 | A file descriptor refers to a non-regular file. | |
c13182ef MK |
384 | Or |
385 | .B MAP_PRIVATE | |
51ffcca0 | 386 | was requested, but |
fea681da MK |
387 | .I fd |
388 | is not open for reading. | |
c13182ef MK |
389 | Or |
390 | .B MAP_SHARED | |
391 | was requested and | |
392 | .B PROT_WRITE | |
51ffcca0 | 393 | is set, but |
fea681da | 394 | .I fd |
682edefb MK |
395 | is not open in read/write |
396 | .RB ( O_RDWR ) | |
397 | mode. | |
c13182ef MK |
398 | Or |
399 | .B PROT_WRITE | |
51ffcca0 | 400 | is set, but the file is append-only. |
fea681da MK |
401 | .TP |
402 | .B EAGAIN | |
83cd3686 MK |
403 | The file has been locked, or too much memory has been locked (see |
404 | .BR setrlimit (2)). | |
fea681da MK |
405 | .TP |
406 | .B EBADF | |
407 | .I fd | |
c13182ef | 408 | is not a valid file descriptor (and |
51ffcca0 MK |
409 | .B MAP_ANONYMOUS |
410 | was not set). | |
fea681da MK |
411 | .TP |
412 | .B EINVAL | |
413 | We don't like | |
14f5ae6d | 414 | .IR addr , |
62a04c81 | 415 | .IR length , |
fea681da | 416 | or |
0daa9e92 | 417 | .I offset |
62a04c81 MK |
418 | (e.g., they are too large, or not aligned on a page boundary). |
419 | .TP | |
420 | .B EINVAL | |
f99fc197 | 421 | (since Linux 2.6.12) |
fea681da | 422 | .I length |
62a04c81 MK |
423 | was 0. |
424 | .TP | |
425 | .B EINVAL | |
426 | .I flags | |
427 | contained neither | |
428 | .B MAP_PRIVATE | |
fea681da | 429 | or |
62a04c81 MK |
430 | .BR MAP_SHARED , |
431 | or contained both of these values. | |
fea681da MK |
432 | .TP |
433 | .B ENFILE | |
434 | .\" This is for shared anonymous segments | |
435 | .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() | |
436 | The system limit on the total number of open files has been reached. | |
437 | .\" .TP | |
438 | .\" .B ENOEXEC | |
439 | .\" A file could not be mapped for reading. | |
440 | .TP | |
441 | .B ENODEV | |
24d01c53 | 442 | The underlying file system of the specified file does not support |
fea681da MK |
443 | memory mapping. |
444 | .TP | |
445 | .B ENOMEM | |
446 | No memory is available, or the process's maximum number of mappings would | |
447 | have been exceeded. | |
448 | .TP | |
449 | .B EPERM | |
450 | The | |
451 | .I prot | |
452 | argument asks for | |
453 | .B PROT_EXEC | |
24d01c53 | 454 | but the mapped area belongs to a file on a file system that |
fea681da MK |
455 | was mounted no-exec. |
456 | .\" (Since 2.4.25 / 2.6.0.) | |
457 | .TP | |
458 | .B ETXTBSY | |
c13182ef | 459 | .B MAP_DENYWRITE |
51ffcca0 | 460 | was set but the object specified by |
fea681da MK |
461 | .I fd |
462 | is open for writing. | |
463 | .LP | |
464 | Use of a mapped region can result in these signals: | |
465 | .TP | |
466 | .B SIGSEGV | |
1e321034 | 467 | Attempted write into a region mapped as read-only. |
fea681da MK |
468 | .TP |
469 | .B SIGBUS | |
470 | Attempted access to a portion of the buffer that does not correspond | |
471 | to the file (for example, beyond the end of the file, including the | |
472 | case where another process has truncated the file). | |
2b2581ee MK |
473 | .SH "CONFORMING TO" |
474 | SVr4, 4.4BSD, POSIX.1-2001. | |
475 | .\" SVr4 documents additional error codes ENXIO and ENODEV. | |
476 | .\" SUSv2 documents additional error codes EMFILE and EOVERFLOW. | |
fea681da MK |
477 | .SH AVAILABILITY |
478 | On POSIX systems on which | |
1a956089 | 479 | .BR mmap (), |
0bfa087b | 480 | .BR msync (2) |
fea681da | 481 | and |
1a956089 | 482 | .BR munmap () |
fea681da MK |
483 | are available, |
484 | .B _POSIX_MAPPED_FILES | |
6387216b MK |
485 | is defined in \fI<unistd.h>\fP to a value greater than 0. |
486 | (See also | |
fea681da | 487 | .BR sysconf (3).) |
97c1eac8 | 488 | .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L. |
fea681da MK |
489 | .\" -1: unavailable, 0: ask using sysconf(). |
490 | .\" glibc defines it to 1. | |
a1d5f77c | 491 | .SH NOTES |
5636a29a MK |
492 | Since kernel 2.4, this system call has been superseded by |
493 | .BR mmap2 (2). | |
494 | Nowadays, | |
495 | .\" Since around glibc 2.1/2.2, depending on the platform. | |
496 | the glibc | |
497 | .BR mmap () | |
498 | wrapper function invokes | |
499 | .BR mmap2 (2) | |
500 | with a suitably adjusted value for | |
501 | .IR offset . | |
502 | ||
34ccb744 | 503 | On some hardware architectures (e.g., i386), |
0daa9e92 | 504 | .B PROT_WRITE |
f3edaabb MK |
505 | implies |
506 | .BR PROT_READ . | |
a1d5f77c MK |
507 | It is architecture dependent whether |
508 | .B PROT_READ | |
509 | implies | |
510 | .B PROT_EXEC | |
511 | or not. | |
512 | Portable programs should always set | |
513 | .B PROT_EXEC | |
514 | if they intend to execute code in the new mapping. | |
80d17cfa MK |
515 | |
516 | The portable way to create a mapping is to specify | |
517 | .I addr | |
518 | as 0 (NULL), and omit | |
519 | .B MAP_FIXED | |
520 | from | |
521 | .IR flags . | |
522 | In this case, the system chooses the address for the mapping; | |
46cdb997 | 523 | the address is chosen so as not to conflict with any existing mapping, |
80d17cfa MK |
524 | and will not be 0. |
525 | If the | |
526 | .B MAP_FIXED | |
527 | flag is specified, and | |
528 | .I addr | |
491cd2f0 | 529 | is 0 (NULL), then the mapped address will be 0 (NULL). |
fea681da | 530 | .SH BUGS |
c13182ef MK |
531 | On Linux there are no guarantees like those suggested above under |
532 | .BR MAP_NORESERVE . | |
dbc53ca8 | 533 | By default, any process can be killed |
fea681da | 534 | at any moment when the system runs out of memory. |
dbc53ca8 MK |
535 | |
536 | In kernels before 2.6.7, the | |
537 | .B MAP_POPULATE | |
538 | flag only has effect if | |
539 | .I prot | |
540 | is specified as | |
541 | .BR PROT_NONE . | |
c8f3e580 | 542 | |
c13182ef | 543 | SUSv3 specifies that |
c8f3e580 MK |
544 | .BR mmap () |
545 | should fail if | |
546 | .I length | |
547 | is 0. | |
548 | However, in kernels before 2.6.12, | |
549 | .BR mmap () | |
550 | succeeded in this case: no mapping was created and the call returned | |
14f5ae6d | 551 | .IR addr . |
c8f3e580 MK |
552 | Since kernel 2.6.12, |
553 | .BR mmap () | |
554 | fails with the error | |
555 | .B EINVAL | |
556 | for this case. | |
74fa61b7 | 557 | .SH EXAMPLE |
2e001ad4 MK |
558 | .\" FIXME . Add an example here that uses an anonymous shared region for |
559 | .\" IPC between parent and child. | |
74fa61b7 MK |
560 | .PP |
561 | The following program prints part of the file specified in | |
562 | its first command-line argument to standard output. | |
563 | The range of bytes to be printed is specified via offset and length | |
564 | values in the second and third command-line arguments. | |
565 | The program creates a memory mapping of the required | |
566 | pages of the file and then uses | |
567 | .BR write (2) | |
568 | to output the desired bytes. | |
569 | .nf | |
570 | ||
571 | #include <sys/mman.h> | |
572 | #include <sys/stat.h> | |
573 | #include <fcntl.h> | |
574 | #include <stdio.h> | |
575 | #include <stdlib.h> | |
576 | #include <unistd.h> | |
577 | ||
4407d3d8 MK |
578 | #define handle_error(msg) \\ |
579 | do { perror(msg); exit(EXIT_FAILURE); } while (0) | |
580 | ||
74fa61b7 MK |
581 | int |
582 | main(int argc, char *argv[]) | |
583 | { | |
584 | char *addr; | |
585 | int fd; | |
586 | struct stat sb; | |
587 | off_t offset, pa_offset; | |
588 | size_t length; | |
589 | ssize_t s; | |
590 | ||
fbbfa7ce | 591 | if (argc < 3 || argc > 4) { |
74fa61b7 MK |
592 | fprintf(stderr, "%s file offset [length]\\n", argv[0]); |
593 | exit(EXIT_FAILURE); | |
594 | } | |
595 | ||
596 | fd = open(argv[1], O_RDONLY); | |
4407d3d8 | 597 | if (fd == \-1) |
8568021d | 598 | handle_error("open"); |
74fa61b7 | 599 | |
4407d3d8 MK |
600 | if (fstat(fd, &sb) == \-1) /* To obtain file size */ |
601 | handle_error("fstat"); | |
74fa61b7 MK |
602 | |
603 | offset = atoi(argv[2]); | |
604 | pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) \- 1); | |
605 | /* offset for mmap() must be page aligned */ | |
606 | ||
607 | if (offset >= sb.st_size) { | |
608 | fprintf(stderr, "offset is past end of file\\n"); | |
609 | exit(EXIT_FAILURE); | |
610 | } | |
611 | ||
612 | if (argc == 4) { | |
613 | length = atoi(argv[3]); | |
614 | if (offset + length > sb.st_size) | |
615 | length = sb.st_size \- offset; | |
f81fb444 | 616 | /* Can\(aqt display bytes past end of file */ |
5b6adad1 | 617 | |
74fa61b7 MK |
618 | } else { /* No length arg ==> display to end of file */ |
619 | length = sb.st_size \- offset; | |
620 | } | |
621 | ||
622 | addr = mmap(NULL, length + offset \- pa_offset, PROT_READ, | |
623 | MAP_PRIVATE, fd, pa_offset); | |
4407d3d8 MK |
624 | if (addr == MAP_FAILED) |
625 | handle_error("mmap"); | |
74fa61b7 MK |
626 | |
627 | s = write(STDOUT_FILENO, addr + offset \- pa_offset, length); | |
628 | if (s != length) { | |
629 | if (s == \-1) | |
4407d3d8 MK |
630 | handle_error("write"); |
631 | ||
632 | fprintf(stderr, "partial write"); | |
74fa61b7 MK |
633 | exit(EXIT_FAILURE); |
634 | } | |
635 | ||
636 | exit(EXIT_SUCCESS); | |
637 | } /* main */ | |
638 | .fi | |
fea681da MK |
639 | .SH "SEE ALSO" |
640 | .BR getpagesize (2), | |
f75c3a3b | 641 | .BR mincore (2), |
fea681da MK |
642 | .BR mlock (2), |
643 | .BR mmap2 (2), | |
54504ac3 | 644 | .BR mprotect (2), |
fea681da MK |
645 | .BR mremap (2), |
646 | .BR msync (2), | |
931e4e25 | 647 | .BR remap_file_pages (2), |
83cd3686 | 648 | .BR setrlimit (2), |
7921f13b | 649 | .BR shmat (2), |
f93af9c6 MK |
650 | .BR shm_open (3), |
651 | .BR shm_overview (7) | |
fea681da MK |
652 | .br |
653 | B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. | |
654 | .\" | |
655 | .\" Repeat after me: private read-only mappings are 100% equivalent to | |
656 | .\" shared read-only mappings. No ifs, buts, or maybes. -- Linus |