mmap(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 (page-aligned) address  at
       which to create the mapping; this is the most portable method of creat-
       ing 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 de-
       scriptor 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 ei-
       ther 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_SHARED_VALIDATE (since Linux 4.15)
              This flag provides the same behavior as MAP_SHARED  except  that
              MAP_SHARED mappings ignore unknown flags in flags.  By contrast,
              when creating a mapping using  MAP_SHARED_VALIDATE,  the  kernel
              verifies  all  passed flags are known and fails the mapping with
              the error EOPNOTSUPP for unknown flags.  This  mapping  type  is
              also  required  to  be  able  to  use  some mapping flags (e.g.,
              MAP_SYNC).

       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 MAP_SHARED and  MAP_PRIVATE  are  described  in  POSIX.1-2001  and
       POSIX.1-2008.  MAP_SHARED_VALIDATE is a Linux extension.

       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 ad-
              dress 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 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 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--Linux 2.0 and earlier--it sig-
              naled  that attempts to write to the underlying file should fail
              with ETXTBUSY.  But this was a source of  denial-of-service  at-
              tacks.)

       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 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  the  MAP_FIXED
              flag  with  care,  keeping  in  mind  that the exact layout of a
              process's memory mappings is allowed to change significantly be-
              tween  kernel versions, C library versions, and operating system
              releases.  Carefully read the discussion of this flag in NOTES!

       MAP_FIXED_NOREPLACE (since Linux 4.17)
              This flag provides behavior that is similar  to  MAP_FIXED  with
              respect   to   the   addr   enforcement,  but  differs  in  that
              MAP_FIXED_NOREPLACE never clobbers a preexisting  mapped  range.
              If  the  requested range would collide with an existing mapping,
              then this call fails with  the  error  EEXIST.   This  flag  can
              therefore  be used as a way to atomically (with respect to other
              threads) attempt to map an address range: one thread  will  suc-
              ceed; all others will report failure.

              Note   that   older   kernels   which   do   not  recognize  the
              MAP_FIXED_NOREPLACE flag will typically (upon detecting a colli-
              sion  with a preexisting mapping) fall back to a "non-MAP_FIXED"
              type of behavior: they will return an address that is  different
              from  the  requested  address.   Therefore,  backward-compatible
              software should check the returned address against the requested
              address.

       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 via 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 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.

       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 re-
              duce 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_SYNC (since Linux 4.15)
              This flag is available only with the MAP_SHARED_VALIDATE mapping
              type; mappings of type  MAP_SHARED  will  silently  ignore  this
              flag.  This flag is supported only for files supporting DAX (di-
              rect mapping of persistent memory).  For other files, creating a
              mapping with this flag results in an EOPNOTSUPP error.

              Shared  file  mappings with this flag provide the guarantee that
              while some memory is writably mapped in the address space of the
              process,  it will be visible in the same file at the same offset
              even after the system crashes or is  rebooted.   In  conjunction
              with  the  use  of  appropriate  CPU instructions, this provides
              users of such mappings with a more efficient way of making  data
              modifications persistent.

       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  at-
       tributes.

       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 un-
       specified.

   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 un-
       mapped, 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).

       EEXIST MAP_FIXED_NOREPLACE  was  specified in flags, and the range cov-
              ered by addr and length is clashes with an existing mapping.

       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  ex-
              ceeded.   This error can also occur for munmap(), when unmapping
              a region in the middle of an existing mapping,  since  this  re-
              sults 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, in-
              cluding the case where another process has truncated the file).

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(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).

   Using MAP_FIXED safely
       The only safe use for MAP_FIXED is where the address range specified by
       addr  and  length was previously reserved using another mapping; other-
       wise, the use of MAP_FIXED is hazardous  because  it  forcibly  removes
       preexisting  mappings,  making  it  easy for a multithreaded process to
       corrupt its own address space.

       For example, suppose that thread A looks through  /proc/<pid>/maps  and
       in  order  to  locate  an  unused  address  range that it can map using
       MAP_FIXED, while thread B simultaneously acquires part or all  of  that
       same    address    range.    When   thread   A   subsequently   employs
       mmap(MAP_FIXED), it will effectively clobber the mapping that thread  B
       created.   In  this  scenario,  thread  B need not create a mapping di-
       rectly; 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_cre-
       ate(3), and the PAM libraries <http://www.linux-pam.org>.

       Since Linux 4.17, a multithreaded program can use  the  MAP_FIXED_NORE-
       PLACE  flag to avoid the hazard described above when attempting to cre-
       ate a mapping at a fixed address that has not been reserved by a preex-
       isting mapping.

   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 be-
       fore 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 de-
       sired 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 4.16 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                             2018-04-30                           MMAP(2)

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