munmap(2)



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

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 call-
       ing 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  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
       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 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:

       MAP_SHARED
              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).)

       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  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 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 prob-
              lem, 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 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.

       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 dis-
              couraged.

       MAP_GROWSDOWN
              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
              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/ker-
              nel/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()  plus
              mlock(2) when major faults are not acceptable after the initial-
              ization 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.

       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 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
       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  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
              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(), 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.

       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).

       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.

       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 corre-
              spond 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).

       +-------------------+---------------+---------+
       |Interface          | Attribute     | Value   |
       +-------------------+---------------+---------+
       |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 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,
       MAP_GROWSDOWN, MAP_HUGETLB,  MAP_LOCKED,  MAP_NONBLOCK,  MAP_NORESERVE,
       MAP_POPULATE, and MAP_STACK.

       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
       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 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.

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 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 POSIX shared memory interface doc-
       umented 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);
           }

           munmap(addr, length + offset - pa_offset);
           close(fd);

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       getpagesize(2),  memfd_create(2), mincore(2), mlock(2), mmap2(2), mpro-
       tect(2),  mremap(2),   msync(2),   remap_file_pages(2),   setrlimit(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.

COLOPHON
       This page is part of release 4.11 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-05-03                           MMAP(2)

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