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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);

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  only  supported  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 only supported on Linux 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 downwards in memory.

       MAP_HUGETLB (since Linux 2.6.32)
	      Allocate	the mapping using "huge pages."	 See the kernel source
	      file Documentation/vm/hugetlbpage.txt for further information.

       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: 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 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  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.

       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 only honored 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  file system of the specified file does not sup‐
	      port 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 file system that was mounted no-exec.

       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).

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
       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 address will be 0
       (NULL).

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 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.

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.

       #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);
       } /* main */

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

COLOPHON
       This  page  is  part of release 3.27 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				  2010-06-20			       MMAP(2)
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