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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.   The  file	may  not  actually  be
		  updated until msync(2) or munmap() is called.

       MAP_PRIVATE
		  Create a private copy-on-write mapping.  Updates to the map‐
		  ping are not visible to other	 processes  mapping  the  same
		  file,	 and  are  not carried through to the underlying file.
		  It is unspecified 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.

       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.

       MAP_LOCKED (since Linux 2.5.37)
	      Lock the pages of the mapped region into memory in the manner of
	      mlock(2).	 This 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_POP‐
	      ULATE 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.	 Later accesses to the
	      mapping will not be blocked by  page  faults.   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.	  How‐
       ever,   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.	All pages con‐
       taining a part of the indicated range are unmapped, and subsequent ref‐
       erences	to  these  pages will generate SIGSEGV.	 It is not an error if
       the indicated range does not contain any mapped pages.

   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.

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
       appropriately.  On success, munmap() returns  0,	 on  failure  -1,  and
       errno is set (probably to EINVAL).

ERRORS
       EACCES A	 file descriptor refers to a non-regular file.	Or MAP_PRIVATE
	      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  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, or the process's maximum number of map‐
	      pings would have been exceeded.

       EPERM  The prot argument asks for PROT_EXEC but the mapped area belongs
	      to a file on a filesystem that was mounted no-exec.

       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).

CONFORMING TO
       SVr4, 4.4BSD, POSIX.1-2001.

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
       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.

       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  either  _BSD_SOURCE  or
       _SVID_SOURCE  is	 defined.   (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.

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), mincore(2), mlock(2), mmap2(2), mprotect(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 3.63 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2014-01-05			       MMAP(2)
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