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NAME

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

SYNOPSIS

       #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.

DESCRIPTION

       mmap()  creates  a  new  mapping  in  the  virtual address space of the
       calling process.  The starting address for the new mapping is specified
       in addr.  The length argument specifies the length of the mapping.

       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
       mapping.  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
       mapping  (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
       flags:

       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
       visible 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:

       MAP_SHARED
              Share this mapping.  Updates to the mapping are visible to other
              processes  that  map  this  file, and 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).)

       MAP_PRIVATE
              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
              unspecified 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 2GB of memory, so as to improve
              context-switch  performance  on  some  early  64-bit processors.
              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.

       MAP_ANON
              Synonym for MAP_ANONYMOUS.  Deprecated.

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

       MAP_DENYWRITE
              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.)

       MAP_EXECUTABLE
              This flag is ignored.

       MAP_FILE
              Compatibility flag.  Ignored.

       MAP_FIXED
              Don't interpret addr as a hint: place  the  mapping  at  exactly
              that address.  addr must be a multiple of the page size.  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.  Because requiring a fixed address
              for a mapping is less  portable,  the  use  of  this  option  is
              discouraged.

       MAP_GROWSDOWN
              Used  for stacks.  Indicates to the kernel virtual memory system
              that the mapping should extend downward in memory.

       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
              information, 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
              as:

                  #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/kernel/mm/hugepages.

       MAP_LOCKED (since Linux 2.5.37)
              Mark the mmaped 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(2)  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)
              Only meaningful 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  combination  of
              MAP_POPULATE and MAP_NONBLOCK may be reimplemented.

       MAP_NORESERVE
              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
              mapping, 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_UNINITIALIZED 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).

       Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
       MAP_COPY, and MAP_LOCAL.

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

       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
       unspecified.

   munmap()
       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.

RETURN VALUE

       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).

ERRORS

       EACCES A file descriptor refers to  a  non-regular  file.   Or  a  file
              mapping  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
              set).

       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
              reached.

       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(2),  when
              unmapping a region in the middle of an existing  mapping,  since
              this  results  in  two  smaller  mappings  on either side of the
              region being unmapped.

       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).

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

       EOVERFLOW
              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).

       Use of a mapped region can result in these signals:

       SIGSEGV
              Attempted write into a region mapped as read-only.

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

ATTRIBUTES

       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       ┌───────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├───────────────────┼───────────────┼─────────┤
       │mmap(), munmap()   │ Thread safety │ MT-Safe │
       └───────────────────┴───────────────┴─────────┘

CONFORMING TO

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

AVAILABILITY

       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).)

NOTES

       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
       (NULL).

       Certain  flags  constants  are  defined  only  if either _BSD_SOURCE or
       _SVID_SOURCE is defined.  (Requiring  _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,  MAP_GROWSDOWN,   MAP_HUGETLB,   MAP_LOCKED,
       MAP_NONBLOCK, MAP_NORESERVE, MAP_POPULATE, and MAP_STACK.

   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
       occurs.

   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
       mappings 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
       underlying 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.

BUGS

       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
       modification 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  calling msync(2) before the unmap takes place; however, this
       doesn't work on tmpfs (for example,  when  using  POSIX  shared  memory
       interface documented in shm_overview(7)).

EXAMPLE

       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)

       int
       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]);
               exit(EXIT_FAILURE);
           }

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

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

           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");
               exit(EXIT_FAILURE);
           }

           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)
               handle_error("mmap");

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

               fprintf(stderr, "partial write");
               exit(EXIT_FAILURE);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       getpagesize(2),  memfd_create(2),   mincore(2),   mlock(2),   mmap2(2),
       mprotect(2),  mremap(2),  msync(2),  remap_file_pages(2), setrlimit(2),
       shmat(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.

COLOPHON

       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.