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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 calling 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 mapping.  If addr is not NULL, then the ker-
       nel  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  mapping  (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  visible  to  other  pro-
       cesses  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 mapping 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  con-
	      text-switch  performance on some early 64-bit processors.  Modern x86-64 processors
	      no longer have this performance problem, 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 initialized 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 under-
	      lying 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 porta-
	      ble, the use of this option is discouraged.

       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 Documen-
	      tation/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: 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_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  mapping,  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_UNINITIALIZED 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.  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 map-
       ping 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 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.

   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 val-
	      ues.

       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 mappings 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 correspond  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.  Origi-
       nally, this function invoked a system call of the same name.  Since kernel 2.4, that  sys-
       tem  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  archi-
       tecture	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_POPU-
       LATE, 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 modification 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 con-
       tent.  In some cases, this could be fixed by  calling  msync(2)	before	the  unmap  takes
       place;  however,  this  doesn't work on tmpfs (for example, when using POSIX shared memory
       interface documented 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  map-
       ping 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);
       }

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 3.55 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					    2013-04-17					  MMAP(2)
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