1MMAP(2)                    Linux Programmer's Manual                   MMAP(2)


6       mmap, munmap - map or unmap files or devices into memory


9       #include <sys/mman.h>
11       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);
15       See NOTES for information on feature test macro requirements.


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).
23       If addr is NULL, then the kernel chooses the address at which to create
24       the  mapping;  this  is the most portable method of creating a new map‐
25       ping.  If addr is not NULL, then the kernel takes it as  a  hint  about
26       where  to place the mapping; on Linux, the mapping will be created at a
27       nearby page boundary.  The address of the new mapping  is  returned  as
28       the result of the call.
30       The contents of a file mapping (as opposed to an anonymous mapping; see
31       MAP_ANONYMOUS below), are initialized using length  bytes  starting  at
32       offset  offset  in  the  file (or other object) referred to by the file
33       descriptor fd.  offset must be a multiple of the page size as  returned
34       by sysconf(_SC_PAGE_SIZE).
36       The  prot  argument describes the desired memory protection of the map‐
37       ping (and must not conflict with the open mode of  the  file).   It  is
38       either  PROT_NONE  or  the  bitwise  OR of one or more of the following
39       flags:
41       PROT_EXEC  Pages may be executed.
43       PROT_READ  Pages may be read.
45       PROT_WRITE Pages may be written.
47       PROT_NONE  Pages may not be accessed.
49       The flags argument determines whether updates to the mapping are  visi‐
50       ble to other processes mapping the same region, and whether updates are
51       carried through to the underlying file.  This behavior is determined by
52       including exactly one of the following values in flags:
54       MAP_SHARED
55              Share this mapping.  Updates to the mapping are visible to other
56              processes mapping the same region, and (in  the  case  of  file-
57              backed  mappings)  are  carried  through to the underlying file.
58              (To precisely control when updates are carried  through  to  the
59              underlying file requires the use of msync(2).)
61       MAP_PRIVATE
62              Create  a private copy-on-write mapping.  Updates to the mapping
63              are not visible to other processes mapping the  same  file,  and
64              are  not carried through to the underlying file.  It is unspeci‐
65              fied whether changes made to the file after the mmap() call  are
66              visible in the mapped region.
68       Both of these flags are described in POSIX.1-2001 and POSIX.1-2008.
70       In addition, zero or more of the following values can be ORed in flags:
72       MAP_32BIT (since Linux 2.4.20, 2.6)
73              Put  the  mapping  into  the  first  2  Gigabytes of the process
74              address space.  This flag  is  supported  only  on  x86-64,  for
75              64-bit  programs.   It  was  added  to allow thread stacks to be
76              allocated somewhere in the  first  2 GB  of  memory,  so  as  to
77              improve  context-switch performance on some early 64-bit proces‐
78              sors.  Modern x86-64 processors no longer have this  performance
79              problem,  so  use of this flag is not required on those systems.
80              The MAP_32BIT flag is ignored when MAP_FIXED is set.
82       MAP_ANON
83              Synonym for MAP_ANONYMOUS.  Deprecated.
86              The mapping is not backed by any file; its contents are initial‐
87              ized  to zero.  The fd argument is ignored; however, some imple‐
88              mentations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is
89              specified,  and  portable  applications should ensure this.  The
90              offset argument should be zero.  The  use  of  MAP_ANONYMOUS  in
91              conjunction  with  MAP_SHARED  is  supported on Linux only since
92              kernel 2.4.
95              This flag is ignored.  (Long ago, it signaled that  attempts  to
96              write  to  the  underlying  file should fail with ETXTBUSY.  But
97              this was a source of denial-of-service attacks.)
100              This flag is ignored.
102       MAP_FILE
103              Compatibility flag.  Ignored.
105       MAP_FIXED
106              Don't interpret addr as a hint: place  the  mapping  at  exactly
107              that address.  addr must be suitably aligned: for most architec‐
108              tures a multiple of the page size is sufficient;  however,  some
109              architectures may impose additional restrictions.  If the memory
110              region specified by addr and len overlaps pages of any  existing
111              mapping(s),  then the overlapped part of the existing mapping(s)
112              will be discarded.  If the specified  address  cannot  be  used,
113              mmap()  will  fail.  Software that aspires to be portable should
114              use this option with care, keeping in mind that the exact layout
115              of  a  process's  memory  mappings is allowed to change signifi‐
116              cantly between kernel versions, C library versions, and  operat‐
117              ing system releases.
119              Furthermore,  this  option  is extremely hazardous (when used on
120              its own), because it forcibly removes preexisting mappings, mak‐
121              ing  it  easy  for  a  multithreaded  process to corrupt its own
122              address space.
124              For example, thread A looks through /proc/<pid>/maps and locates
125              an  available  address  range,  while  thread  B  simultaneously
126              acquires part or all of that same address range.  Thread A  then
127              calls  mmap(MAP_FIXED), effectively overwriting the mapping that
128              thread B created.
130              Thread B need not create a mapping  directly;  simply  making  a
131              library call that, internally, uses dlopen(3) to load some other
132              shared library, will suffice.  The dlopen(3) call will  map  the
133              library  into  the process's address space.  Furthermore, almost
134              any library call may be implemented in a way  that  adds  memory
135              mappings to the address space, either with this technique, or by
136              simply allocating memory.  Examples include  brk(2),  malloc(3),
137              pthread_create(3),  and  the  PAM  libraries  ⟨http://www.linux-
138              pam.org⟩.
141              This flag is used for stacks.  It indicates to the  kernel  vir‐
142              tual  memory  system  that the mapping should extend downward in
143              memory.  The return address is one page lower  than  the  memory
144              area  that  is actually created in the process's virtual address
145              space.  Touching an address in the "guard" page below  the  map‐
146              ping  will cause the mapping to grow by a page.  This growth can
147              be repeated until the mapping grows to within a page of the high
148              end  of  the  next  lower  mapping,  at which point touching the
149              "guard" page will result in a SIGSEGV signal.
151       MAP_HUGETLB (since Linux 2.6.32)
152              Allocate the mapping using "huge pages."  See the  Linux  kernel
153              source  file Documentation/vm/hugetlbpage.txt for further infor‐
154              mation, as well as NOTES, below.
156       MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux 3.8)
157              Used in  conjunction  with  MAP_HUGETLB  to  select  alternative
158              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
159              support multiple hugetlb page sizes.
161              More generally, the desired huge page size can be configured  by
162              encoding  the  base-2  logarithm of the desired page size in the
163              six bits at the offset MAP_HUGE_SHIFT.  (A value of zero in this
164              bit  field provides the default huge page size; the default huge
165              page size can be discovered vie the Hugepagesize  field  exposed
166              by  /proc/meminfo.)   Thus,  the above two constants are defined
167              as:
169                  #define MAP_HUGE_2MB    (21 << MAP_HUGE_SHIFT)
170                  #define MAP_HUGE_1GB    (30 << MAP_HUGE_SHIFT)
172              The range of huge page sizes that are supported  by  the  system
173              can  be  discovered  by  listing the subdirectories in /sys/ker‐
174              nel/mm/hugepages.
176       MAP_LOCKED (since Linux 2.5.37)
177              Mark the mapped region to be locked in the same way as mlock(2).
178              This  implementation  will  try to populate (prefault) the whole
179              range but the mmap() call  doesn't  fail  with  ENOMEM  if  this
180              fails.   Therefore  major  faults might happen later on.  So the
181              semantic is not as strong as mlock(2).  One  should  use  mmap()
182              plus  mlock(2)  when  major  faults are not acceptable after the
183              initialization of the mapping.  The MAP_LOCKED flag  is  ignored
184              in older kernels.
186       MAP_NONBLOCK (since Linux 2.5.46)
187              This  flag  is meaningful only in conjunction with MAP_POPULATE.
188              Don't perform read-ahead: create page tables  entries  only  for
189              pages that are already present in RAM.  Since Linux 2.6.23, this
190              flag causes MAP_POPULATE to do nothing.  One day,  the  combina‐
191              tion of MAP_POPULATE and MAP_NONBLOCK may be reimplemented.
194              Do  not reserve swap space for this mapping.  When swap space is
195              reserved, one has the guarantee that it is  possible  to  modify
196              the  mapping.   When  swap  space  is not reserved one might get
197              SIGSEGV upon a write if no physical memory  is  available.   See
198              also  the  discussion of the file /proc/sys/vm/overcommit_memory
199              in proc(5).  In kernels before 2.6, this flag  had  effect  only
200              for private writable mappings.
202       MAP_POPULATE (since Linux 2.5.46)
203              Populate  (prefault) page tables for a mapping.  For a file map‐
204              ping, this causes read-ahead on the file.   This  will  help  to
205              reduce blocking on page faults later.  MAP_POPULATE is supported
206              for private mappings only since Linux 2.6.23.
208       MAP_STACK (since Linux 2.6.27)
209              Allocate the mapping at an address suitable  for  a  process  or
210              thread  stack.   This  flag is currently a no-op, but is used in
211              the glibc threading implementation so that if some architectures
212              require  special  treatment  for  stack allocations, support can
213              later be transparently implemented for glibc.
215       MAP_UNINITIALIZED (since Linux 2.6.33)
216              Don't clear anonymous pages.  This flag is intended  to  improve
217              performance  on  embedded devices.  This flag is honored only if
218              the kernel was configured with the  CONFIG_MMAP_ALLOW_UNINITIAL‐
219              IZED  option.  Because of the security implications, that option
220              is normally enabled only  on  embedded  devices  (i.e.,  devices
221              where one has complete control of the contents of user memory).
223       Of  the  above  flags,  only MAP_FIXED is specified in POSIX.1-2001 and
224       POSIX.1-2008.  However, most systems also support MAP_ANONYMOUS (or its
225       synonym MAP_ANON).
227       Memory  mapped  by  mmap()  is  preserved across fork(2), with the same
228       attributes.
230       A file is mapped in multiples of the page size.  For a file that is not
231       a  multiple  of  the  page  size,  the  remaining memory is zeroed when
232       mapped, and writes to that region are not written out to the file.  The
233       effect  of changing the size of the underlying file of a mapping on the
234       pages that correspond to added  or  removed  regions  of  the  file  is
235       unspecified.
237   munmap()
238       The munmap() system call deletes the mappings for the specified address
239       range, and causes further references to addresses within the  range  to
240       generate  invalid  memory references.  The region is also automatically
241       unmapped when the process is terminated.  On the  other  hand,  closing
242       the file descriptor does not unmap the region.
244       The  address  addr must be a multiple of the page size (but length need
245       not be).  All pages containing  a  part  of  the  indicated  range  are
246       unmapped,  and  subsequent  references  to  these  pages  will generate
247       SIGSEGV.  It is not an error if the indicated range  does  not  contain
248       any mapped pages.


251       On success, mmap() returns a pointer to the mapped area.  On error, the
252       value MAP_FAILED (that is, (void *) -1) is returned, and errno  is  set
253       to indicate the cause of the error.
255       On  success,  munmap() returns 0.  On failure, it returns -1, and errno
256       is set to indicate the cause of the error (probably to EINVAL).


259       EACCES A file descriptor refers to a non-regular file.  Or a file  map‐
260              ping  was  requested,  but  fd  is  not  open  for  reading.  Or
261              MAP_SHARED was requested and PROT_WRITE is set, but  fd  is  not
262              open in read/write (O_RDWR) mode.  Or PROT_WRITE is set, but the
263              file is append-only.
265       EAGAIN The file has been locked, or too much  memory  has  been  locked
266              (see setrlimit(2)).
268       EBADF  fd  is  not  a  valid file descriptor (and MAP_ANONYMOUS was not
269              set).
271       EINVAL We don't like addr, length, or offset (e.g., they are too large,
272              or not aligned on a page boundary).
274       EINVAL (since Linux 2.6.12) length was 0.
276       EINVAL flags  contained neither MAP_PRIVATE or MAP_SHARED, or contained
277              both of these values.
279       ENFILE The system-wide limit on the total number of open files has been
280              reached.
282       ENODEV The underlying filesystem of the specified file does not support
283              memory mapping.
285       ENOMEM No memory is available.
287       ENOMEM The  process's  maximum  number  of  mappings  would  have  been
288              exceeded.   This  error can also occur for munmap(), when unmap‐
289              ping a region in the middle of an existing mapping,  since  this
290              results  in  two  smaller  mappings on either side of the region
291              being unmapped.
293       ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described  in
294              getrlimit(2), would have been exceeded.
296       EOVERFLOW
297              On  32-bit  architecture  together with the large file extension
298              (i.e., using 64-bit off_t): the number of pages used for  length
299              plus  number  of  pages  used for offset would overflow unsigned
300              long (32 bits).
302       EPERM  The prot argument asks for PROT_EXEC but the mapped area belongs
303              to a file on a filesystem that was mounted no-exec.
305       EPERM  The operation was prevented by a file seal; see fcntl(2).
307       ETXTBSY
308              MAP_DENYWRITE was set but the object specified by fd is open for
309              writing.
311       Use of a mapped region can result in these signals:
313       SIGSEGV
314              Attempted write into a region mapped as read-only.
316       SIGBUS Attempted access to a portion of the buffer that does not corre‐
317              spond  to  the  file  (for  example, beyond the end of the file,
318              including the case  where  another  process  has  truncated  the
319              file).


322       For   an   explanation   of   the  terms  used  in  this  section,  see
323       attributes(7).
325       ┌───────────────────┬───────────────┬─────────┐
326Interface          Attribute     Value   
327       ├───────────────────┼───────────────┼─────────┤
328mmap(), munmap()   │ Thread safety │ MT-Safe │
329       └───────────────────┴───────────────┴─────────┘


331       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.


334       On POSIX systems on which mmap(), msync(2), and munmap() are available,
335       _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0.
336       (See also sysconf(3).)


339       On  some  hardware  architectures  (e.g.,  i386),  PROT_WRITE   implies
340       PROT_READ.   It  is  architecture  dependent  whether PROT_READ implies
341       PROT_EXEC or not.  Portable programs should  always  set  PROT_EXEC  if
342       they intend to execute code in the new mapping.
344       The  portable  way  to create a mapping is to specify addr as 0 (NULL),
345       and omit MAP_FIXED from flags.  In this case, the  system  chooses  the
346       address  for  the  mapping; the address is chosen so as not to conflict
347       with any existing mapping, and will not be 0.  If the MAP_FIXED flag is
348       specified,  and  addr  is  0  (NULL), then the mapped address will be 0
349       (NULL).
351       Certain flags constants are  defined  only  if  suitable  feature  test
352       macros  are  defined  (possibly by default): _DEFAULT_SOURCE with glibc
353       2.19 or later; or _BSD_SOURCE or _SVID_SOURCE in glibc  2.19  and  ear‐
354       lier.   (Employing  _GNU_SOURCE also suffices, and requiring that macro
355       specifically would have been more logical, since these  flags  are  all
356       Linux-specific.)  The relevant flags are: MAP_32BIT, MAP_ANONYMOUS (and
357       the  synonym  MAP_ANON),   MAP_DENYWRITE,   MAP_EXECUTABLE,   MAP_FILE,
359       MAP_POPULATE, and MAP_STACK.
361       An application can determine which pages of  a  mapping  are  currently
362       resident in the buffer/page cache using mincore(2).
364   Timestamps changes for file-backed mappings
365       For file-backed mappings, the st_atime field for the mapped file may be
366       updated at any time between the mmap() and the corresponding unmapping;
367       the  first  reference  to a mapped page will update the field if it has
368       not been already.
370       The st_ctime and st_mtime field for a file mapped with  PROT_WRITE  and
371       MAP_SHARED  will  be  updated  after  a write to the mapped region, and
372       before a subsequent msync(2) with the MS_SYNC or MS_ASYNC flag, if  one
373       occurs.
375   Huge page (Huge TLB) mappings
376       For mappings that employ huge pages, the requirements for the arguments
377       of mmap() and munmap() differ somewhat from the requirements  for  map‐
378       pings that use the native system page size.
380       For mmap(), offset must be a multiple of the underlying huge page size.
381       The system automatically aligns length to be a multiple of the underly‐
382       ing huge page size.
384       For munmap(), addr and length must both be a multiple of the underlying
385       huge page size.
387   C library/kernel differences
388       This page describes the interface provided by the glibc mmap()  wrapper
389       function.   Originally, this function invoked a system call of the same
390       name.  Since kernel 2.4,  that  system  call  has  been  superseded  by
391       mmap2(2),  and  nowadays  the  glibc  mmap()  wrapper  function invokes
392       mmap2(2) with a suitably adjusted value for offset.


395       On Linux, there are no guarantees  like  those  suggested  above  under
396       MAP_NORESERVE.   By  default,  any  process can be killed at any moment
397       when the system runs out of memory.
399       In kernels before 2.6.7, the MAP_POPULATE flag has effect only if  prot
400       is specified as PROT_NONE.
402       SUSv3  specifies  that  mmap() should fail if length is 0.  However, in
403       kernels before 2.6.12, mmap() succeeded in this case:  no  mapping  was
404       created  and the call returned addr.  Since kernel 2.6.12, mmap() fails
405       with the error EINVAL for this case.
407       POSIX specifies that the system shall always zero fill any partial page
408       at the end of the object and that system will never write any modifica‐
409       tion of the object beyond its end.  On Linux, when you  write  data  to
410       such  partial  page  after the end of the object, the data stays in the
411       page cache even after the file is closed and unmapped and  even  though
412       the  data  is never written to the file itself, subsequent mappings may
413       see the modified content.  In some cases, this could be fixed by  call‐
414       ing  msync(2)  before the unmap takes place; however, this doesn't work
415       on tmpfs(5) (for example, when using the POSIX shared memory  interface
416       documented in shm_overview(7)).


419       The  following  program  prints part of the file specified in its first
420       command-line argument to standard output.  The range  of  bytes  to  be
421       printed  is  specified  via  offset and length values in the second and
422       third command-line arguments.  The program creates a memory mapping  of
423       the  required  pages  of  the file and then uses write(2) to output the
424       desired bytes.
426   Program source
427       #include <sys/mman.h>
428       #include <sys/stat.h>
429       #include <fcntl.h>
430       #include <stdio.h>
431       #include <stdlib.h>
432       #include <unistd.h>
434       #define handle_error(msg) \
435           do { perror(msg); exit(EXIT_FAILURE); } while (0)
437       int
438       main(int argc, char *argv[])
439       {
440           char *addr;
441           int fd;
442           struct stat sb;
443           off_t offset, pa_offset;
444           size_t length;
445           ssize_t s;
447           if (argc < 3 || argc > 4) {
448               fprintf(stderr, "%s file offset [length]\n", argv[0]);
449               exit(EXIT_FAILURE);
450           }
452           fd = open(argv[1], O_RDONLY);
453           if (fd == -1)
454               handle_error("open");
456           if (fstat(fd, &sb) == -1)           /* To obtain file size */
457               handle_error("fstat");
459           offset = atoi(argv[2]);
460           pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
461               /* offset for mmap() must be page aligned */
463           if (offset >= sb.st_size) {
464               fprintf(stderr, "offset is past end of file\n");
465               exit(EXIT_FAILURE);
466           }
468           if (argc == 4) {
469               length = atoi(argv[3]);
470               if (offset + length > sb.st_size)
471                   length = sb.st_size - offset;
472                       /* Can't display bytes past end of file */
474           } else {    /* No length arg ==> display to end of file */
475               length = sb.st_size - offset;
476           }
478           addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
479                       MAP_PRIVATE, fd, pa_offset);
480           if (addr == MAP_FAILED)
481               handle_error("mmap");
483           s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
484           if (s != length) {
485               if (s == -1)
486                   handle_error("write");
488               fprintf(stderr, "partial write");
489               exit(EXIT_FAILURE);
490           }
492           munmap(addr, length + offset - pa_offset);
493           close(fd);
495           exit(EXIT_SUCCESS);
496       }


499       ftruncate(2), getpagesize(2),  memfd_create(2),  mincore(2),  mlock(2),
500       mmap2(2),  mprotect(2), mremap(2), msync(2), remap_file_pages(2), setr‐
501       limit(2), shmat(2), userfaultfd(2), shm_open(3), shm_overview(7)
503       The descriptions of the following files in  proc(5):  /proc/[pid]/maps,
504       /proc/[pid]/map_files, and /proc/[pid]/smaps.
506       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.


509       This  page  is  part of release 4.15 of the Linux man-pages project.  A
510       description of the project, information about reporting bugs,  and  the
511       latest     version     of     this    page,    can    be    found    at
512       https://www.kernel.org/doc/man-pages/.
516Linux                             2017-12-18                           MMAP(2)