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).

       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(),  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), 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 4.10 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				  2016-12-12			       MMAP(2)