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

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

       #include <sys/mman.h>

       void *mmap(void *addr, size_t length, int prot, int flags,
                  int fd, off_t offset);
       int munmap(void *addr, size_t length);

       See NOTES for information on feature test macro requirements.

       mmap()  creates a new mapping in the virtual address space of the call-
       ing process.  The starting address for the new mapping is specified  in
       addr.   The  length argument specifies the length of the mapping (which
       must be greater than 0).

       If addr is NULL, then the kernel chooses the address at which to create
       the  mapping;  this  is the most portable method of creating a new map-
       ping.  If addr is not NULL, then the kernel takes it as  a  hint  about
       where  to place the mapping; on Linux, the mapping will be created at a
       nearby page boundary.  The address of the new mapping  is  returned  as
       the result of the call.

       The contents of a file mapping (as opposed to an anonymous mapping; see
       MAP_ANONYMOUS below), are initialized using length  bytes  starting  at
       offset  offset  in  the  file (or other object) referred to by the file
       descriptor fd.  offset must be a multiple of the page size as  returned
       by sysconf(_SC_PAGE_SIZE).

       The  prot  argument describes the desired memory protection of the map-
       ping (and must not conflict with the open mode of  the  file).   It  is
       either  PROT_NONE  or  the  bitwise  OR of one or more of the following

       PROT_EXEC  Pages may be executed.

       PROT_READ  Pages may be read.

       PROT_WRITE Pages may be written.

       PROT_NONE  Pages may not be accessed.

       The flags argument determines whether updates to the mapping are  visi-
       ble to other processes mapping the same region, and whether updates are
       carried through to the underlying file.  This behavior is determined by
       including exactly one of the following values in flags:

              Share this mapping.  Updates to the mapping are visible to other
              processes mapping the same region, and (in  the  case  of  file-
              backed  mappings)  are  carried  through to the underlying file.
              (To precisely control when updates are carried  through  to  the
              underlying file requires the use of msync(2).)

              Create  a private copy-on-write mapping.  Updates to the mapping
              are not visible to other processes mapping the  same  file,  and
              are  not carried through to the underlying file.  It is unspeci-
              fied whether changes made to the file after the mmap() call  are
              visible in the mapped region.

       Both of these flags are described in POSIX.1-2001 and POSIX.1-2008.

       In addition, zero or more of the following values can be ORed in flags:

       MAP_32BIT (since Linux 2.4.20, 2.6)
              Put  the  mapping  into  the  first  2  Gigabytes of the process
              address space.  This flag  is  supported  only  on  x86-64,  for
              64-bit  programs.   It  was  added  to allow thread stacks to be
              allocated somewhere in the  first  2 GB  of  memory,  so  as  to
              improve  context-switch performance on some early 64-bit proces-
              sors.  Modern x86-64 processors no longer have this  performance
              problem,  so  use of this flag is not required on those systems.
              The MAP_32BIT flag is ignored when MAP_FIXED is set.

              Synonym for MAP_ANONYMOUS.  Deprecated.

              The mapping is not backed by any file; its contents are initial-
              ized  to zero.  The fd argument is ignored; however, some imple-
              mentations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is
              specified,  and  portable  applications should ensure this.  The
              offset argument should be zero.  The  use  of  MAP_ANONYMOUS  in
              conjunction  with  MAP_SHARED  is  supported on Linux only since
              kernel 2.4.

              This flag is ignored.  (Long ago, it signaled that  attempts  to
              write  to  the  underlying  file should fail with ETXTBUSY.  But
              this was a source of denial-of-service attacks.)

              This flag is ignored.

              Compatibility flag.  Ignored.

              Don't interpret addr as a hint: place  the  mapping  at  exactly
              that address.  addr must be suitably aligned: for most architec-
              tures a multiple of the page size is sufficient;  however,  some
              architectures may impose additional restrictions.  If the memory
              region specified by addr and len overlaps pages of any  existing
              mapping(s),  then the overlapped part of the existing mapping(s)
              will be discarded.  If the specified  address  cannot  be  used,
              mmap()  will  fail.  Software that aspires to be portable should
              use this option with care, keeping in mind that the exact layout
              of  a  process's  memory  mappings is allowed to change signifi-
              cantly between kernel versions, C library versions, and  operat-
              ing system releases.

              Furthermore,  this  option  is extremely hazardous (when used on
              its own), because it forcibly removes preexisting mappings, mak-
              ing  it  easy  for  a  multithreaded  process to corrupt its own
              address space.

              For example, thread A looks through /proc/<pid>/maps and locates
              an  available  address  range,  while  thread  B  simultaneously
              acquires part or all of that same address range.  Thread A  then
              calls  mmap(MAP_FIXED), effectively overwriting the mapping that
              thread B created.

              Thread B need not create a mapping  directly;  simply  making  a
              library call that, internally, uses dlopen(3) to load some other
              shared library, will suffice.  The dlopen(3) call will  map  the
              library  into  the process's address space.  Furthermore, almost
              any library call may be implemented in a way  that  adds  memory
              mappings to the address space, either with this technique, or by
              simply allocating memory.  Examples include  brk(2),  malloc(3),
              pthread_create(3),  and  the  PAM  libraries  <http://www.linux-

              This flag is used for stacks.  It indicates to the  kernel  vir-
              tual  memory  system  that the mapping should extend downward in
              memory.  The return address is one page lower  than  the  memory
              area  that  is actually created in the process's virtual address
              space.  Touching an address in the "guard" page below  the  map-
              ping  will cause the mapping to grow by a page.  This growth can
              be repeated until the mapping grows to within a page of the high
              end  of  the  next  lower  mapping,  at which point touching the
              "guard" page will result in a SIGSEGV signal.

       MAP_HUGETLB (since Linux 2.6.32)
              Allocate the mapping using "huge pages."  See the  Linux  kernel
              source  file Documentation/vm/hugetlbpage.txt for further infor-
              mation, as well as NOTES, below.

       MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux 3.8)
              Used in  conjunction  with  MAP_HUGETLB  to  select  alternative
              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
              support multiple hugetlb page sizes.

              More generally, the desired huge page size can be configured  by
              encoding  the  base-2  logarithm of the desired page size in the
              six bits at the offset MAP_HUGE_SHIFT.  (A value of zero in this
              bit  field provides the default huge page size; the default huge
              page size can be discovered vie the Hugepagesize  field  exposed
              by  /proc/meminfo.)   Thus,  the above two constants are defined

                  #define MAP_HUGE_2MB    (21 << MAP_HUGE_SHIFT)
                  #define MAP_HUGE_1GB    (30 << MAP_HUGE_SHIFT)

              The range of huge page sizes that are supported  by  the  system
              can  be  discovered  by  listing the subdirectories in /sys/ker-

       MAP_LOCKED (since Linux 2.5.37)
              Mark the mapped region to be locked in the same way as mlock(2).
              This  implementation  will  try to populate (prefault) the whole
              range but the mmap() call  doesn't  fail  with  ENOMEM  if  this
              fails.   Therefore  major  faults might happen later on.  So the
              semantic is not as strong as mlock(2).  One  should  use  mmap()
              plus  mlock(2)  when  major  faults are not acceptable after the
              initialization of the mapping.  The MAP_LOCKED flag  is  ignored
              in older kernels.

       MAP_NONBLOCK (since Linux 2.5.46)
              This  flag  is meaningful only in conjunction with MAP_POPULATE.
              Don't perform read-ahead: create page tables  entries  only  for
              pages that are already present in RAM.  Since Linux 2.6.23, this
              flag causes MAP_POPULATE to do nothing.  One day,  the  combina-
              tion of MAP_POPULATE and MAP_NONBLOCK may be reimplemented.

              Do  not reserve swap space for this mapping.  When swap space is
              reserved, one has the guarantee that it is  possible  to  modify
              the  mapping.   When  swap  space  is not reserved one might get
              SIGSEGV upon a write if no physical memory  is  available.   See
              also  the  discussion of the file /proc/sys/vm/overcommit_memory
              in proc(5).  In kernels before 2.6, this flag  had  effect  only
              for private writable mappings.

       MAP_POPULATE (since Linux 2.5.46)
              Populate  (prefault) page tables for a mapping.  For a file map-
              ping, this causes read-ahead on the file.   This  will  help  to
              reduce blocking on page faults later.  MAP_POPULATE is supported
              for private mappings only since Linux 2.6.23.

       MAP_STACK (since Linux 2.6.27)
              Allocate the mapping at an address suitable  for  a  process  or
              thread  stack.   This  flag is currently a no-op, but is used in
              the glibc threading implementation so that if some architectures
              require  special  treatment  for  stack allocations, support can
              later be transparently implemented for glibc.

       MAP_UNINITIALIZED (since Linux 2.6.33)
              Don't clear anonymous pages.  This flag is intended  to  improve
              performance  on  embedded devices.  This flag is honored only if
              the kernel was configured with the  CONFIG_MMAP_ALLOW_UNINITIAL-
              IZED  option.  Because of the security implications, that option
              is normally enabled only  on  embedded  devices  (i.e.,  devices
              where one has complete control of the contents of user memory).

       Of  the  above  flags,  only MAP_FIXED is specified in POSIX.1-2001 and
       POSIX.1-2008.  However, most systems also support MAP_ANONYMOUS (or its
       synonym MAP_ANON).

       Memory  mapped  by  mmap()  is  preserved across fork(2), with the same

       A file is mapped in multiples of the page size.  For a file that is not
       a  multiple  of  the  page  size,  the  remaining memory is zeroed when
       mapped, and writes to that region are not written out to the file.  The
       effect  of changing the size of the underlying file of a mapping on the
       pages that correspond to added  or  removed  regions  of  the  file  is

       The munmap() system call deletes the mappings for the specified address
       range, and causes further references to addresses within the  range  to
       generate  invalid  memory references.  The region is also automatically
       unmapped when the process is terminated.  On the  other  hand,  closing
       the file descriptor does not unmap the region.

       The  address  addr must be a multiple of the page size (but length need
       not be).  All pages containing  a  part  of  the  indicated  range  are
       unmapped,  and  subsequent  references  to  these  pages  will generate
       SIGSEGV.  It is not an error if the indicated range  does  not  contain
       any mapped pages.

       On success, mmap() returns a pointer to the mapped area.  On error, the
       value MAP_FAILED (that is, (void *) -1) is returned, and errno  is  set
       to indicate the cause of the error.

       On  success,  munmap() returns 0.  On failure, it returns -1, and errno
       is set to indicate the cause of the error (probably to EINVAL).

       EACCES A file descriptor refers to a non-regular file.  Or a file  map-
              ping  was  requested,  but  fd  is  not  open  for  reading.  Or
              MAP_SHARED was requested and PROT_WRITE is set, but  fd  is  not
              open in read/write (O_RDWR) mode.  Or PROT_WRITE is set, but the
              file is append-only.

       EAGAIN The file has been locked, or too much  memory  has  been  locked
              (see setrlimit(2)).

       EBADF  fd  is  not  a  valid file descriptor (and MAP_ANONYMOUS was not

       EINVAL We don't like addr, length, or offset (e.g., they are too large,
              or not aligned on a page boundary).

       EINVAL (since Linux 2.6.12) length was 0.

       EINVAL flags  contained neither MAP_PRIVATE or MAP_SHARED, or contained
              both of these values.

       ENFILE The system-wide limit on the total number of open files has been

       ENODEV The underlying filesystem of the specified file does not support
              memory mapping.

       ENOMEM No memory is available.

       ENOMEM The  process's  maximum  number  of  mappings  would  have  been
              exceeded.   This  error can also occur for munmap(), when unmap-
              ping a region in the middle of an existing mapping,  since  this
              results  in  two  smaller  mappings on either side of the region
              being unmapped.

       ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described  in
              getrlimit(2), would have been exceeded.

              On  32-bit  architecture  together with the large file extension
              (i.e., using 64-bit off_t): the number of pages used for  length
              plus  number  of  pages  used for offset would overflow unsigned
              long (32 bits).

       EPERM  The prot argument asks for PROT_EXEC but the mapped area belongs
              to a file on a filesystem that was mounted no-exec.

       EPERM  The operation was prevented by a file seal; see fcntl(2).

              MAP_DENYWRITE was set but the object specified by fd is open for

       Use of a mapped region can result in these signals:

              Attempted write into a region mapped as read-only.

       SIGBUS Attempted access to a portion of the buffer that does not corre-
              spond  to  the  file  (for  example, beyond the end of the file,
              including the case  where  another  process  has  truncated  the

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface          | Attribute     | Value   |
       |mmap(), munmap()   | Thread safety | MT-Safe |
       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.

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

       On  some  hardware  architectures  (e.g.,  i386),  PROT_WRITE   implies
       PROT_READ.   It  is  architecture  dependent  whether PROT_READ implies
       PROT_EXEC or not.  Portable programs should  always  set  PROT_EXEC  if
       they intend to execute code in the new mapping.

       The  portable  way  to create a mapping is to specify addr as 0 (NULL),
       and omit MAP_FIXED from flags.  In this case, the  system  chooses  the
       address  for  the  mapping; the address is chosen so as not to conflict
       with any existing mapping, and will not be 0.  If the MAP_FIXED flag is
       specified,  and  addr  is  0  (NULL), then the mapped address will be 0

       Certain flags constants are  defined  only  if  suitable  feature  test
       macros  are  defined  (possibly by default): _DEFAULT_SOURCE with glibc
       2.19 or later; or _BSD_SOURCE or _SVID_SOURCE in glibc  2.19  and  ear-
       lier.   (Employing  _GNU_SOURCE also suffices, and requiring that macro
       specifically would have been more logical, since these  flags  are  all
       Linux-specific.)  The relevant flags are: MAP_32BIT, MAP_ANONYMOUS (and
       the  synonym  MAP_ANON),   MAP_DENYWRITE,   MAP_EXECUTABLE,   MAP_FILE,

       An application can determine which pages of  a  mapping  are  currently
       resident in the buffer/page cache using mincore(2).

   Timestamps changes for file-backed mappings
       For file-backed mappings, the st_atime field for the mapped file may be
       updated at any time between the mmap() and the corresponding unmapping;
       the  first  reference  to a mapped page will update the field if it has
       not been already.

       The st_ctime and st_mtime field for a file mapped with  PROT_WRITE  and
       MAP_SHARED  will  be  updated  after  a write to the mapped region, and
       before a subsequent msync(2) with the MS_SYNC or MS_ASYNC flag, if  one

   Huge page (Huge TLB) mappings
       For mappings that employ huge pages, the requirements for the arguments
       of mmap() and munmap() differ somewhat from the requirements  for  map-
       pings that use the native system page size.

       For mmap(), offset must be a multiple of the underlying huge page size.
       The system automatically aligns length to be a multiple of the underly-
       ing huge page size.

       For munmap(), addr and length must both be a multiple of the underlying
       huge page size.

   C library/kernel differences
       This page describes the interface provided by the glibc mmap()  wrapper
       function.   Originally, this function invoked a system call of the same
       name.  Since kernel 2.4,  that  system  call  has  been  superseded  by
       mmap2(2),  and  nowadays  the  glibc  mmap()  wrapper  function invokes
       mmap2(2) with a suitably adjusted value for offset.

       On Linux, there are no guarantees  like  those  suggested  above  under
       MAP_NORESERVE.   By  default,  any  process can be killed at any moment
       when the system runs out of memory.

       In kernels before 2.6.7, the MAP_POPULATE flag has effect only if  prot
       is specified as PROT_NONE.

       SUSv3  specifies  that  mmap() should fail if length is 0.  However, in
       kernels before 2.6.12, mmap() succeeded in this case:  no  mapping  was
       created  and the call returned addr.  Since kernel 2.6.12, mmap() fails
       with the error EINVAL for this case.

       POSIX specifies that the system shall always zero fill any partial page
       at the end of the object and that system will never write any modifica-
       tion of the object beyond its end.  On Linux, when you  write  data  to
       such  partial  page  after the end of the object, the data stays in the
       page cache even after the file is closed and unmapped and  even  though
       the  data  is never written to the file itself, subsequent mappings may
       see the modified content.  In some cases, this could be fixed by  call-
       ing  msync(2)  before the unmap takes place; however, this doesn't work
       on tmpfs(5) (for example, when using the POSIX shared memory  interface
       documented in shm_overview(7)).

       The  following  program  prints part of the file specified in its first
       command-line argument to standard output.  The range  of  bytes  to  be
       printed  is  specified  via  offset and length values in the second and
       third command-line arguments.  The program creates a memory mapping  of
       the  required  pages  of  the file and then uses write(2) to output the
       desired bytes.

   Program source
       #include <sys/mman.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

       main(int argc, char *argv[])
           char *addr;
           int fd;
           struct stat sb;
           off_t offset, pa_offset;
           size_t length;
           ssize_t s;

           if (argc < 3 || argc > 4) {
               fprintf(stderr, "%s file offset [length]\n", argv[0]);

           fd = open(argv[1], O_RDONLY);
           if (fd == -1)

           if (fstat(fd, &sb) == -1)           /* To obtain file size */

           offset = atoi(argv[2]);
           pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
               /* offset for mmap() must be page aligned */

           if (offset >= sb.st_size) {
               fprintf(stderr, "offset is past end of file\n");

           if (argc == 4) {
               length = atoi(argv[3]);
               if (offset + length > sb.st_size)
                   length = sb.st_size - offset;
                       /* Can't display bytes past end of file */

           } else {    /* No length arg ==> display to end of file */
               length = sb.st_size - offset;

           addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
                       MAP_PRIVATE, fd, pa_offset);
           if (addr == MAP_FAILED)

           s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
           if (s != length) {
               if (s == -1)

               fprintf(stderr, "partial write");

           munmap(addr, length + offset - pa_offset);


       ftruncate(2), getpagesize(2),  memfd_create(2),  mincore(2),  mlock(2),
       mmap2(2),  mprotect(2), mremap(2), msync(2), remap_file_pages(2), setr-
       limit(2), shmat(2), userfaultfd(2), shm_open(3), shm_overview(7)

       The descriptions of the following files in  proc(5):  /proc/[pid]/maps,
       /proc/[pid]/map_files, and /proc/[pid]/smaps.

       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

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