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MMAP(2) 		   Linux Programmers Manual		      MMAP(2)

       mmap, munmap - map or unmap files or devices into memory


       void *mmap(void *addr, size_t length, int prot, int flags,
		  int fd, off_t offset);
       int munmap(void *addr, size_t length);

       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
       the  next  higher  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

       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.

		  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:

	      Put the mapping into the first 2GB of the process address space.
	      Ignored when MAP_FIXED is set.  This flag is currently only sup
	      ported on x86-64, for 64-bit programs.

	      Synonym for MAP_ANONYMOUS.  Deprecated.

	      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 only supported on Linux since kernel 2.4.

	      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.)

	      This flag is ignored.

	      Compatibility flag.  Ignored.

	      Dont  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

	      Used for stacks.	Indicates to the kernel virtual memory	system
	      that the mapping should extend downwards in memory.

       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.  Dont perform
	      read-ahead:  only  create page tables entries 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 re-implemented.

	      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 only had effect
	      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
	      only supported for private mappings since Linux 2.6.23.

       Of  the above flags, only MAP_FIXED is specified in POSIX.1-2001.  How
       ever,  most  systems  also  support  MAP_ANONYMOUS  (or	 its   synonym

       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

       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

       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

       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).

       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

       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

       EINVAL We dont 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

       ENODEV The  underlying  file system of the specified file does not sup
	      port memory mapping.

       ENOMEM No memory is available, or the processs 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 file system that was mounted no-exec.

	      MAP_DENYWRITE was set but the object specified by fd is open for

       Use of a mapped region can result in these signals:

	      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

       SVr4, 4.4BSD, POSIX.1-2001.

       On POSIX systems on which mmap(), msync(2) and munmap() are  available,
       _POSIX_MAPPED_FILES is defined in  to a value greater than 0.
       (See also sysconf(3).)

       Since kernel 2.4, this system call has  been  superseded  by  mmap2(2).
       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 adddress will be 0

       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 only has effect 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.

       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.


       #define handle_error(msg) \
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

       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]);

	   fd = open(argv[1], O_RDONLY);
	   if (fd == -1)

	   if (fstat(fd, &sb) == -1)	       /* To obtain file size */

	   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");

	   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)

	   s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
	   if (s != length) {
	       if (s == -1)

	       fprintf(stderr, "partial write");

       } /* main */

       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),
       B.O. Gallmeister, POSIX.4, OReilly, pp. 128-129 and 389-391.

       This  page  is  part of release 3.05 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				  2008-06-05			       MMAP(2)

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