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 (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
       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  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.

       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.

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

       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 the 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);
           }

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       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.

COLOPHON
       This page is part of release 4.14 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                             2017-09-15                           MMAP(2)

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