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 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 re-implemented.
              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).

       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
       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

       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).)

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
       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 */

               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 3.23 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                             2009-09-26                           MMAP(2)
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