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MMAP(2)                    Linux Programmer's Manual                   MMAP(2)



NAME
       mmap, mmap64, 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);
       void *mmap64(void *addr, size_t length, int prot, int flags,
                  int fd, off64_t offset);
       int munmap(void *addr, size_t length);

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 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 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 processes  mapping  the  same
       region, and whether updates are carried through to the underlying file.  This behavior is determined by includ-
       ing 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 only supported 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 context-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  argu-
              ments  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 map-
              ping(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 discouraged.

       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_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 only had effect 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 only supported for
              private mappings since Linux 2.6.23.

       Of the above flags, only MAP_FIXED is specified in POSIX.1-2001.  However, most systems also support MAP_ANONY-
       MOUS (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.

   mmap64()
       The mmap64() system call operates in exactly the same way as mmap(), except that the final  argument  specifies
       the offset as a 64-bit off64_t. This  enables  applications to aceess the large files.

   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 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 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  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 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
       Since  kernel  2.4, this system call has been superseded by mmap2(2).  Nowadays, the glibc mmap() wrapper func-
       tion 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 (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), setr-
       limit(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 3.24 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-09-26                           MMAP(2)