mmap


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

              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 downward 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.
              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.

       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).)

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

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-06-20                           MMAP(2)
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