1MMAP(2) Linux Programmer's Manual MMAP(2)
2
6 mmap, munmap - map or unmap files or devices into memory
7
9 #include <sys/mman.h>
10 void *mmap(void *addr, size_t length, int prot, int flags,
12 int fd, off_t offset);
13 int munmap(void *addr, size_t length);
14 See NOTES for information on feature test macro requirements.
16
18 mmap() creates a new mapping in the virtual address space of the call‐
19 ing process. The starting address for the new mapping is specified in
20 addr. The length argument specifies the length of the mapping (which
21 must be greater than 0).
22 If addr is NULL, then the kernel chooses the (page-aligned) address at
24 which to create the mapping; this is the most portable method of creat‐
25 ing a new mapping. If addr is not NULL, then the kernel takes it as a
26 hint about where to place the mapping; on Linux, the kernel will pick a
27 nearby page boundary (but always above or equal to the value specified
28 by /proc/sys/vm/mmap_min_addr) and attempt to create the mapping there.
29 If another mapping already exists there, the kernel picks a new address
30 that may or may not depend on the hint. The address of the new mapping
31 is returned as the result of the call.
32 The contents of a file mapping (as opposed to an anonymous mapping; see
34 MAP_ANONYMOUS below), are initialized using length bytes starting at
35 offset offset in the file (or other object) referred to by the file de‐
36 scriptor fd. offset must be a multiple of the page size as returned by
37 sysconf(_SC_PAGE_SIZE).
38 After the mmap() call has returned, the file descriptor, fd, can be
40 closed immediately without invalidating the mapping.
41 The prot argument describes the desired memory protection of the map‐
43 ping (and must not conflict with the open mode of the file). It is ei‐
44 ther PROT_NONE or the bitwise OR of one or more of the following flags:
45 PROT_EXEC Pages may be executed.
47 PROT_READ Pages may be read.
49 PROT_WRITE Pages may be written.
51 PROT_NONE Pages may not be accessed.
53 The flags argument
55 The flags argument determines whether updates to the mapping are visi‐
56 ble to other processes mapping the same region, and whether updates are
57 carried through to the underlying file. This behavior is determined by
58 including exactly one of the following values in flags:
59 MAP_SHARED
61 Share this mapping. Updates to the mapping are visible to other
62 processes mapping the same region, and (in the case of file-
63 backed mappings) are carried through to the underlying file.
64 (To precisely control when updates are carried through to the
65 underlying file requires the use of msync(2).)
66 MAP_SHARED_VALIDATE (since Linux 4.15)
68 This flag provides the same behavior as MAP_SHARED except that
69 MAP_SHARED mappings ignore unknown flags in flags. By contrast,
70 when creating a mapping using MAP_SHARED_VALIDATE, the kernel
71 verifies all passed flags are known and fails the mapping with
72 the error EOPNOTSUPP for unknown flags. This mapping type is
73 also required to be able to use some mapping flags (e.g.,
74 MAP_SYNC).
75 MAP_PRIVATE
77 Create a private copy-on-write mapping. Updates to the mapping
78 are not visible to other processes mapping the same file, and
79 are not carried through to the underlying file. It is unspeci‐
80 fied whether changes made to the file after the mmap() call are
81 visible in the mapped region.
82 Both MAP_SHARED and MAP_PRIVATE are described in POSIX.1-2001 and
84 POSIX.1-2008. MAP_SHARED_VALIDATE is a Linux extension.
85 In addition, zero or more of the following values can be ORed in flags:
87 MAP_32BIT (since Linux 2.4.20, 2.6)
89 Put the mapping into the first 2 Gigabytes of the process ad‐
90 dress space. This flag is supported only on x86-64, for 64-bit
91 programs. It was added to allow thread stacks to be allocated
92 somewhere in the first 2 GB of memory, so as to improve context-
93 switch performance on some early 64-bit processors. Modern
94 x86-64 processors no longer have this performance problem, so
95 use of this flag is not required on those systems. The
96 MAP_32BIT flag is ignored when MAP_FIXED is set.
97 MAP_ANON
99 Synonym for MAP_ANONYMOUS; provided for compatibility with other
100 implementations.
101 MAP_ANONYMOUS
103 The mapping is not backed by any file; its contents are initial‐
104 ized to zero. The fd argument is ignored; however, some imple‐
105 mentations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is
106 specified, and portable applications should ensure this. The
107 offset argument should be zero. The use of MAP_ANONYMOUS in
108 conjunction with MAP_SHARED is supported on Linux only since
109 kernel 2.4.
110 MAP_DENYWRITE
112 This flag is ignored. (Long ago—Linux 2.0 and earlier—it sig‐
113 naled that attempts to write to the underlying file should fail
114 with ETXTBSY. But this was a source of denial-of-service at‐
115 tacks.)
116 MAP_EXECUTABLE
118 This flag is ignored.
119 MAP_FILE
121 Compatibility flag. Ignored.
122 MAP_FIXED
124 Don't interpret addr as a hint: place the mapping at exactly
125 that address. addr must be suitably aligned: for most architec‐
126 tures a multiple of the page size is sufficient; however, some
127 architectures may impose additional restrictions. If the memory
128 region specified by addr and len overlaps pages of any existing
129 mapping(s), then the overlapped part of the existing mapping(s)
130 will be discarded. If the specified address cannot be used,
131 mmap() will fail.
132 Software that aspires to be portable should use the MAP_FIXED
134 flag with care, keeping in mind that the exact layout of a
135 process's memory mappings is allowed to change significantly be‐
136 tween kernel versions, C library versions, and operating system
137 releases. Carefully read the discussion of this flag in NOTES!
138 MAP_FIXED_NOREPLACE (since Linux 4.17)
140 This flag provides behavior that is similar to MAP_FIXED with
141 respect to the addr enforcement, but differs in that
142 MAP_FIXED_NOREPLACE never clobbers a preexisting mapped range.
143 If the requested range would collide with an existing mapping,
144 then this call fails with the error EEXIST. This flag can
145 therefore be used as a way to atomically (with respect to other
146 threads) attempt to map an address range: one thread will suc‐
147 ceed; all others will report failure.
148 Note that older kernels which do not recognize the
150 MAP_FIXED_NOREPLACE flag will typically (upon detecting a colli‐
151 sion with a preexisting mapping) fall back to a "non-MAP_FIXED"
152 type of behavior: they will return an address that is different
153 from the requested address. Therefore, backward-compatible
154 software should check the returned address against the requested
155 address.
156 MAP_GROWSDOWN
158 This flag is used for stacks. It indicates to the kernel vir‐
159 tual memory system that the mapping should extend downward in
160 memory. The return address is one page lower than the memory
161 area that is actually created in the process's virtual address
162 space. Touching an address in the "guard" page below the map‐
163 ping will cause the mapping to grow by a page. This growth can
164 be repeated until the mapping grows to within a page of the high
165 end of the next lower mapping, at which point touching the
166 "guard" page will result in a SIGSEGV signal.
167 MAP_HUGETLB (since Linux 2.6.32)
169 Allocate the mapping using "huge pages." See the Linux kernel
170 source file Documentation/admin-guide/mm/hugetlbpage.rst for
171 further information, as well as NOTES, below.
172 MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux 3.8)
174 Used in conjunction with MAP_HUGETLB to select alternative
175 hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
176 support multiple hugetlb page sizes.
177 More generally, the desired huge page size can be configured by
179 encoding the base-2 logarithm of the desired page size in the
180 six bits at the offset MAP_HUGE_SHIFT. (A value of zero in this
181 bit field provides the default huge page size; the default huge
182 page size can be discovered via the Hugepagesize field exposed
183 by /proc/meminfo.) Thus, the above two constants are defined
184 as:
185 #define MAP_HUGE_2MB (21 << MAP_HUGE_SHIFT)
187 #define MAP_HUGE_1GB (30 << MAP_HUGE_SHIFT)
188 The range of huge page sizes that are supported by the system
190 can be discovered by listing the subdirectories in /sys/ker‐
191 nel/mm/hugepages.
192 MAP_LOCKED (since Linux 2.5.37)
194 Mark the mapped region to be locked in the same way as mlock(2).
195 This implementation will try to populate (prefault) the whole
196 range but the mmap() call doesn't fail with ENOMEM if this
197 fails. Therefore major faults might happen later on. So the
198 semantic is not as strong as mlock(2). One should use mmap()
199 plus mlock(2) when major faults are not acceptable after the
200 initialization of the mapping. The MAP_LOCKED flag is ignored
201 in older kernels.
202 MAP_NONBLOCK (since Linux 2.5.46)
204 This flag is meaningful only in conjunction with MAP_POPULATE.
205 Don't perform read-ahead: create page tables entries only for
206 pages that are already present in RAM. Since Linux 2.6.23, this
207 flag causes MAP_POPULATE to do nothing. One day, the combina‐
208 tion of MAP_POPULATE and MAP_NONBLOCK may be reimplemented.
209 MAP_NORESERVE
211 Do not reserve swap space for this mapping. When swap space is
212 reserved, one has the guarantee that it is possible to modify
213 the mapping. When swap space is not reserved one might get
214 SIGSEGV upon a write if no physical memory is available. See
215 also the discussion of the file /proc/sys/vm/overcommit_memory
216 in proc(5). In kernels before 2.6, this flag had effect only
217 for private writable mappings.
218 MAP_POPULATE (since Linux 2.5.46)
220 Populate (prefault) page tables for a mapping. For a file map‐
221 ping, this causes read-ahead on the file. This will help to re‐
222 duce blocking on page faults later. MAP_POPULATE is supported
223 for private mappings only since Linux 2.6.23.
224 MAP_STACK (since Linux 2.6.27)
226 Allocate the mapping at an address suitable for a process or
227 thread stack.
228 This flag is currently a no-op on Linux. However, by employing
230 this flag, applications can ensure that they transparently ob‐
231 tain support if the flag is implemented in the future. Thus, it
232 is used in the glibc threading implementation to allow for the
233 fact that some architectures may (later) require special treat‐
234 ment for stack allocations. A further reason to employ this
235 flag is portability: MAP_STACK exists (and has an effect) on
236 some other systems (e.g., some of the BSDs).
237 MAP_SYNC (since Linux 4.15)
239 This flag is available only with the MAP_SHARED_VALIDATE mapping
240 type; mappings of type MAP_SHARED will silently ignore this
241 flag. This flag is supported only for files supporting DAX (di‐
242 rect mapping of persistent memory). For other files, creating a
243 mapping with this flag results in an EOPNOTSUPP error.
244 Shared file mappings with this flag provide the guarantee that
246 while some memory is mapped writable in the address space of the
247 process, it will be visible in the same file at the same offset
248 even after the system crashes or is rebooted. In conjunction
249 with the use of appropriate CPU instructions, this provides
250 users of such mappings with a more efficient way of making data
251 modifications persistent.
252 MAP_UNINITIALIZED (since Linux 2.6.33)
254 Don't clear anonymous pages. This flag is intended to improve
255 performance on embedded devices. This flag is honored only if
256 the kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIAL‐
257 IZED option. Because of the security implications, that option
258 is normally enabled only on embedded devices (i.e., devices
259 where one has complete control of the contents of user memory).
260 Of the above flags, only MAP_FIXED is specified in POSIX.1-2001 and
262 POSIX.1-2008. However, most systems also support MAP_ANONYMOUS (or its
263 synonym MAP_ANON).
264 munmap()
266 The munmap() system call deletes the mappings for the specified address
267 range, and causes further references to addresses within the range to
268 generate invalid memory references. The region is also automatically
269 unmapped when the process is terminated. On the other hand, closing
270 the file descriptor does not unmap the region.
271 The address addr must be a multiple of the page size (but length need
273 not be). All pages containing a part of the indicated range are un‐
274 mapped, and subsequent references to these pages will generate SIGSEGV.
275 It is not an error if the indicated range does not contain any mapped
276 pages.
277
279 On success, mmap() returns a pointer to the mapped area. On error, the
280 value MAP_FAILED (that is, (void *) -1) is returned, and errno is set
281 to indicate the cause of the error.
282 On success, munmap() returns 0. On failure, it returns -1, and errno
284 is set to indicate the cause of the error (probably to EINVAL).
285
287 EACCES A file descriptor refers to a non-regular file. Or a file map‐
288 ping was requested, but fd is not open for reading. Or
289 MAP_SHARED was requested and PROT_WRITE is set, but fd is not
290 open in read/write (O_RDWR) mode. Or PROT_WRITE is set, but the
291 file is append-only.
292 EAGAIN The file has been locked, or too much memory has been locked
294 (see setrlimit(2)).
295 EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not
297 set).
298 EEXIST MAP_FIXED_NOREPLACE was specified in flags, and the range cov‐
300 ered by addr and length clashes with an existing mapping.
301 EINVAL We don't like addr, length, or offset (e.g., they are too large,
303 or not aligned on a page boundary).
304 EINVAL (since Linux 2.6.12) length was 0.
306 EINVAL flags contained none of MAP_PRIVATE, MAP_SHARED or
308 MAP_SHARED_VALIDATE.
309 ENFILE The system-wide limit on the total number of open files has been
311 reached.
312 ENODEV The underlying filesystem of the specified file does not support
314 memory mapping.
315 ENOMEM No memory is available.
317 ENOMEM The process's maximum number of mappings would have been ex‐
319 ceeded. This error can also occur for munmap(), when unmapping
320 a region in the middle of an existing mapping, since this re‐
321 sults in two smaller mappings on either side of the region being
322 unmapped.
323 ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described in
325 getrlimit(2), would have been exceeded.
326 EOVERFLOW
328 On 32-bit architecture together with the large file extension
329 (i.e., using 64-bit off_t): the number of pages used for length
330 plus number of pages used for offset would overflow unsigned
331 long (32 bits).
332 EPERM The prot argument asks for PROT_EXEC but the mapped area belongs
334 to a file on a filesystem that was mounted no-exec.
335 EPERM The operation was prevented by a file seal; see fcntl(2).
337 ETXTBSY
339 MAP_DENYWRITE was set but the object specified by fd is open for
340 writing.
341 Use of a mapped region can result in these signals:
343 SIGSEGV
345 Attempted write into a region mapped as read-only.
346 SIGBUS Attempted access to a page of the buffer that lies beyond the
348 end of the mapped file. For an explanation of the treatment of
349 the bytes in the page that corresponds to the end of a mapped
350 file that is not a multiple of the page size, see NOTES.
351
353 For an explanation of the terms used in this section, see at‐
354 tributes(7).
355 ┌───────────────────┬───────────────┬─────────┐
357 │Interface │ Attribute │ Value │
358 ├───────────────────┼───────────────┼─────────┤
359 │mmap(), munmap() │ Thread safety │ MT-Safe │
360 └───────────────────┴───────────────┴─────────┘
362 POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.
363 On POSIX systems on which mmap(), msync(2), and munmap() are available,
365 _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0.
366 (See also sysconf(3).)
367
369 Memory mapped by mmap() is preserved across fork(2), with the same at‐
370 tributes.
371 A file is mapped in multiples of the page size. For a file that is not
373 a multiple of the page size, the remaining bytes in the partial page at
374 the end of the mapping are zeroed when mapped, and modifications to
375 that region are not written out to the file. The effect of changing
376 the size of the underlying file of a mapping on the pages that corre‐
377 spond to added or removed regions of the file is unspecified.
378 On some hardware architectures (e.g., i386), PROT_WRITE implies
380 PROT_READ. It is architecture dependent whether PROT_READ implies
381 PROT_EXEC or not. Portable programs should always set PROT_EXEC if
382 they intend to execute code in the new mapping.
383 The portable way to create a mapping is to specify addr as 0 (NULL),
385 and omit MAP_FIXED from flags. In this case, the system chooses the
386 address for the mapping; the address is chosen so as not to conflict
387 with any existing mapping, and will not be 0. If the MAP_FIXED flag is
388 specified, and addr is 0 (NULL), then the mapped address will be 0
389 (NULL).
390 Certain flags constants are defined only if suitable feature test
392 macros are defined (possibly by default): _DEFAULT_SOURCE with glibc
393 2.19 or later; or _BSD_SOURCE or _SVID_SOURCE in glibc 2.19 and ear‐
394 lier. (Employing _GNU_SOURCE also suffices, and requiring that macro
395 specifically would have been more logical, since these flags are all
396 Linux-specific.) The relevant flags are: MAP_32BIT, MAP_ANONYMOUS (and
397 the synonym MAP_ANON), MAP_DENYWRITE, MAP_EXECUTABLE, MAP_FILE,
398 MAP_GROWSDOWN, MAP_HUGETLB, MAP_LOCKED, MAP_NONBLOCK, MAP_NORESERVE,
399 MAP_POPULATE, and MAP_STACK.
400 An application can determine which pages of a mapping are currently
402 resident in the buffer/page cache using mincore(2).
403 Using MAP_FIXED safely
405 The only safe use for MAP_FIXED is where the address range specified by
406 addr and length was previously reserved using another mapping; other‐
407 wise, the use of MAP_FIXED is hazardous because it forcibly removes
408 preexisting mappings, making it easy for a multithreaded process to
409 corrupt its own address space.
410 For example, suppose that thread A looks through /proc/<pid>/maps in
412 order to locate an unused address range that it can map using
413 MAP_FIXED, while thread B simultaneously acquires part or all of that
414 same address range. When thread A subsequently employs
415 mmap(MAP_FIXED), it will effectively clobber the mapping that thread B
416 created. In this scenario, thread B need not create a mapping di‐
417 rectly; simply making a library call that, internally, uses dlopen(3)
418 to load some other shared library, will suffice. The dlopen(3) call
419 will map the library into the process's address space. Furthermore,
420 almost any library call may be implemented in a way that adds memory
421 mappings to the address space, either with this technique, or by simply
422 allocating memory. Examples include brk(2), malloc(3), pthread_cre‐
423 ate(3), and the PAM libraries ⟨http://www.linux-pam.org⟩.
424 Since Linux 4.17, a multithreaded program can use the MAP_FIXED_NORE‐
426 PLACE flag to avoid the hazard described above when attempting to cre‐
427 ate a mapping at a fixed address that has not been reserved by a preex‐
428 isting mapping.
429 Timestamps changes for file-backed mappings
431 For file-backed mappings, the st_atime field for the mapped file may be
432 updated at any time between the mmap() and the corresponding unmapping;
433 the first reference to a mapped page will update the field if it has
434 not been already.
435 The st_ctime and st_mtime field for a file mapped with PROT_WRITE and
437 MAP_SHARED will be updated after a write to the mapped region, and be‐
438 fore a subsequent msync(2) with the MS_SYNC or MS_ASYNC flag, if one
439 occurs.
440 Huge page (Huge TLB) mappings
442 For mappings that employ huge pages, the requirements for the arguments
443 of mmap() and munmap() differ somewhat from the requirements for map‐
444 pings that use the native system page size.
445 For mmap(), offset must be a multiple of the underlying huge page size.
447 The system automatically aligns length to be a multiple of the underly‐
448 ing huge page size.
449 For munmap(), addr, and length must both be a multiple of the underly‐
451 ing huge page size.
452 C library/kernel differences
454 This page describes the interface provided by the glibc mmap() wrapper
455 function. Originally, this function invoked a system call of the same
456 name. Since kernel 2.4, that system call has been superseded by
457 mmap2(2), and nowadays the glibc mmap() wrapper function invokes
458 mmap2(2) with a suitably adjusted value for offset.
459
461 On Linux, there are no guarantees like those suggested above under
462 MAP_NORESERVE. By default, any process can be killed at any moment
463 when the system runs out of memory.
464 In kernels before 2.6.7, the MAP_POPULATE flag has effect only if prot
466 is specified as PROT_NONE.
467 SUSv3 specifies that mmap() should fail if length is 0. However, in
469 kernels before 2.6.12, mmap() succeeded in this case: no mapping was
470 created and the call returned addr. Since kernel 2.6.12, mmap() fails
471 with the error EINVAL for this case.
472 POSIX specifies that the system shall always zero fill any partial page
474 at the end of the object and that system will never write any modifica‐
475 tion of the object beyond its end. On Linux, when you write data to
476 such partial page after the end of the object, the data stays in the
477 page cache even after the file is closed and unmapped and even though
478 the data is never written to the file itself, subsequent mappings may
479 see the modified content. In some cases, this could be fixed by call‐
480 ing msync(2) before the unmap takes place; however, this doesn't work
481 on tmpfs(5) (for example, when using the POSIX shared memory interface
482 documented in shm_overview(7)).
483
485 The following program prints part of the file specified in its first
486 command-line argument to standard output. The range of bytes to be
487 printed is specified via offset and length values in the second and
488 third command-line arguments. The program creates a memory mapping of
489 the required pages of the file and then uses write(2) to output the de‐
490 sired bytes.
491 Program source
493 #include <sys/mman.h>
494 #include <sys/stat.h>
495 #include <fcntl.h>
496 #include <stdio.h>
497 #include <stdlib.h>
498 #include <unistd.h>
499 #define handle_error(msg) \
501 do { perror(msg); exit(EXIT_FAILURE); } while (0)
502 int
504 main(int argc, char *argv[])
505 {
506 char *addr;
507 int fd;
508 struct stat sb;
509 off_t offset, pa_offset;
510 size_t length;
511 ssize_t s;
512 if (argc < 3 || argc > 4) {
514 fprintf(stderr, "%s file offset [length]\n", argv[0]);
515 exit(EXIT_FAILURE);
516 }
517 fd = open(argv[1], O_RDONLY);
519 if (fd == -1)
520 handle_error("open");
521 if (fstat(fd, &sb) == -1) /* To obtain file size */
523 handle_error("fstat");
524 offset = atoi(argv[2]);
526 pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
527 /* offset for mmap() must be page aligned */
528 if (offset >= sb.st_size) {
530 fprintf(stderr, "offset is past end of file\n");
531 exit(EXIT_FAILURE);
532 }
533 if (argc == 4) {
535 length = atoi(argv[3]);
536 if (offset + length > sb.st_size)
537 length = sb.st_size - offset;
538 /* Can't display bytes past end of file */
539 } else { /* No length arg ==> display to end of file */
541 length = sb.st_size - offset;
542 }
543 addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
545 MAP_PRIVATE, fd, pa_offset);
546 if (addr == MAP_FAILED)
547 handle_error("mmap");
548 s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
550 if (s != length) {
551 if (s == -1)
552 handle_error("write");
553 fprintf(stderr, "partial write");
555 exit(EXIT_FAILURE);
556 }
557 munmap(addr, length + offset - pa_offset);
559 close(fd);
560 exit(EXIT_SUCCESS);
562 }
563
565 ftruncate(2), getpagesize(2), memfd_create(2), mincore(2), mlock(2),
566 mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2), setr‐
567 limit(2), shmat(2), userfaultfd(2), shm_open(3), shm_overview(7)
568 The descriptions of the following files in proc(5): /proc/[pid]/maps,
570 /proc/[pid]/map_files, and /proc/[pid]/smaps.
571 B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.
573
575 This page is part of release 5.10 of the Linux man-pages project. A
576 description of the project, information about reporting bugs, and the
577 latest version of this page, can be found at
578 https://www.kernel.org/doc/man-pages/.
579Linux 2020-12-21 MMAP(2)