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

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

       #include <sys/mman.h>

       #ifdef _POSIX_MAPPED_FILES

       void  *  mmap(void *start, size_t length, int prot , int flags, int fd,
       off_t offset);

       int munmap(void *start, size_t length);


       The mmap function asks to map length bytes starting  at  offset  offset
       from  the  file  (or  other object) specified by the file descriptor fd
       into memory, preferably at address start.  This  latter  address  is  a
       hint  only,  and is usually specified as 0.  The actual place where the
       object is mapped is returned by mmap, and is never 0.

       The prot argument describes the desired memory protection (and must not
       conflict  with the open mode of the file). It is either PROT_NONE or is
       the bitwise OR of one or more of the other PROT_* 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 parameter specifies the type of the  mapped  object,  mapping
       options  and  whether modifications made to the mapped copy of the page
       are private to the process or are to be shared with  other  references.
       It has bits

       MAP_FIXED  Do  not  select  a different address than the one specified.
                  If the specified address cannot be used, mmap will fail.  If
                  MAP_FIXED  is  specified,  start  must  be a multiple of the
                  pagesize.  Use of this option is discouraged.

       MAP_SHARED Share this mapping with all other processes  that  map  this
                  object.   Storing  to the region is equivalent to writing to
                  the file.  The  file  may  not  actually  be  updated  until
                  msync(2) or munmap(2) are called.

                  Create  a  private  copy-on-write  mapping.   Stores  to the
                  region do not affect the original file.  It  is  unspecified
                  whether  changes  made  to  the file after the mmap call are
                  visible in the mapped region.

       You must specify exactly one of MAP_SHARED and MAP_PRIVATE.

       The above three flags are described in POSIX.1b (formerly POSIX.4)  and
       SUSv2.  Linux also knows about the following non-standard flags:

              This  flag is ignored.  (Long ago, it signalled 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.

              (Used  together  with  MAP_PRIVATE.)  Do  not reserve swap space
              pages for this mapping. When swap space is reserved, one has the
              guarantee  that  it  is possible to modify this private copy-on-
              write region.  When it is not reserved  one  might  get  SIGSEGV
              upon a write when no memory is available.

              This flag is ignored.

              Used for stacks. Indicates to the kernel VM system that the map-
              ping should extend downwards in memory.

              The mapping is not backed by any file; the fd and  offset  argu-
              ments  are ignored.  This flag in conjunction with MAP_SHARED is
              implemented since Linux 2.4.

              Alias for MAP_ANONYMOUS. Deprecated.

              Compatibility flag. Ignored.

       Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
       MAP_COPY, and MAP_LOCAL.

       fd  should  be a valid file descriptor, unless MAP_ANONYMOUS is set, in
       which case the argument is ignored.

       offset should be a multiple of the page size as  returned  by  getpage-

       Memory  mapped  by  mmap  is  preserved  across  fork(2), with the same

       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

       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  start 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.

       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() with the MS_SYNC or MS_ASYNC flag, if one

       On success, mmap returns a pointer  to  the  mapped  area.   On  error,
       MAP_FAILED  (-1)  is returned, and errno is set appropriately.  On suc-
       cess, munmap returns 0, on failure -1, and errno is  set  (probably  to

       EBADF  fd  is  not  a  valid file descriptor (and MAP_ANONYMOUS was not

       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

       EINVAL We don't like start or length or offset.  (E.g.,  they  are  too
              large, or not aligned on a PAGESIZE boundary.)

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

       EAGAIN The file has been locked, or too much memory has been locked.

       ENOMEM No memory is available, or the process's maximum number of  map-
              pings would have been exceeded.

       ENODEV The underlying filesystem of the specified file does not support
              memory mapping.

       Use of a mapped region can result in these signals:

              Attempted write into a region specified to mmap 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

       SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD, SUSv2.  SVr4 documents addi-
       tional error codes ENXIO and ENODEV.  SUSv2 documents additional  error
       codes EMFILE and EOVERFLOW.

       getpagesize(2),   mmap2(2),   mremap(2),  msync(2),  shm_open(2),  B.O.
       Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

Linux 2.3.51                      2000-03-25                           MMAP(2)