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

       mmap, mmap64, munmap - map or unmap files or devices into memory

       #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);

       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 map-
       ping; 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_ANONY-
       MOUS 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 bit-
       wise 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 including exactly one of the  fol-
       lowing values in flags:

       MAP_SHARED Share  this  mapping.   Updates to the mapping are visible to other pro-
                  cesses 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 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.

              Synonym for MAP_ANONYMOUS.  Deprecated.

              The mapping is not backed by any file; its contents are initialized 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  con-
              junction with MAP_SHARED is only supported on Linux 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 discouraged.

              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 Docu-
              mentation/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.

              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 mem-
              ory is available.  See also the discussion of the file /proc/sys/vm/overcom-
              mit_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_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 implemen-
              tation  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.

       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   appli-
       cations to aceess the large files.

       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 termi-
       nated.  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.

       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).

       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 setr-

       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

       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.

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

       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 correspond to the
              file (for example, beyond the end of the  file,  including  the  case  where
              another process has truncated the file).

       SVr4, 4.4BSD, POSIX.1-2001.

       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

       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

       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  pro-
       grams  should  always  set PROT_EXEC if they intend to execute code in the new map-

       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 map-
       ping; 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).

       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

       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.

       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 pro-
       gram 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)

       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");

       } /* main */

       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.

       This page is part of release 3.24 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2009-09-26                           MMAP(2)

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