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  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 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 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 dis-
              couraged.

       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  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(2) 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)
              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  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).

       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  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(2), 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.

       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 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.  (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  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 (for example, when using POSIX shared memory  interface  docu-
       mented 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),  mpro-
       tect(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 4.05 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                             2016-03-15                           MMAP(2)

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