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MMAP(2)			   Linux Programmer's Manual		       MMAP(2)

       mmap, mmap64, munmap - map or unmap files or devices into memory

       #include <sys/mman.h>

       void *mmap(void *addr, size_t length, int prot, int flags,
		  int fd, off_t offset);
       void *mmap64(void *addr, size_t length, int prot, int flags,
		  int fd, off64_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  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

       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:

       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.

	      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.

	      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‐

	      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.

	      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

       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.

       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  mmap64()  system  call operates in exactly the same way as mmap(),
       except that the	final  argument	 specifies  the	 offset	 as  a	64-bit
       off64_t. This  enables  applications to aceess the large files.

       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 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

       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.

	      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 <unistd.h> 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  address  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.

       #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)

       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, O'Reilly, pp. 128-129 and 389-391.

       This page is part of release 3.24 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2009-09-26			       MMAP(2)

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