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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 <&lt;sys/mman.h>&gt;

       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
       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
       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
              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.

       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_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 re-implemented.

       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.

       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 &lt;unistd.h&gt; 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  adddress  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 3.05 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)