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           #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.


           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-
                  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 supported on Linux only 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 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-
           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
           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
           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


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


           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)).


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

    Linux 2014-01-05 MMAP(2)


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