The mmap function creates a new mapped file region, a new private
    region, or a new shared memory region.

    Your application must ensure correct synchronization when using
    mmap in conjunction with any other file access method, such as
    read and write, and standard input/output.

    Before calling mmap, the calling application must also ensure
    that all bytes in the range [off, off+len] are written to the
    file (using the fsync function, for example). If this requirement
    is not met, mmap fails with errno set to ENXIO (No such device or
    address).

    The addr and len arguments specify the requested starting address
    and length, in bytes, for the new region. The address is a
    multiple of the page size returned by sysconf(_SC_PAGE_SIZE).

    If the len argument is not a multiple of the page size returned
    by sysconf(_SC_PAGE_SIZE), then the result of any reference to
    an address between the end of the region and the end of the page
    containing the end of the region is undefined.

    The flags argument specifies attributes of the mapped region.
    Values for flags are constructed by a bitwise-inclusive OR
    operation on the flags from the following list of symbolic names
    defined in the <mman.h> header file:

    MAP_FILE         Create a mapped file region.
    MAP_ANONYMOUS    Create an unnamed memory region.
    MAP_VARIABLE     Place region at the computed address.
    MAP_FIXED        Place region at fixed address.
    MAP_SHARED       Share changes.
    MAP_PRIVATE      Changes are private.

    The MAP_FILE and MAP_ANONYMOUS flags control whether the region
    you want to map is a mapped file region or an anonymous shared
    memory region. One of these flags must be selected.

    If MAP_FILE is set in the flags argument:

    o  A new mapped file region is created, mapping the file
       associated with the filedes argument.

    o  The off argument specifies the file byte offset where the
       mapping starts. This offset must be a multiple of the page
       size returned by sysconf(_SC_PAGE_SIZE).

    o  If the end of the mapped file region is beyond the end of the
       file, the result of any reference to an address in the mapped
       file region corresponding to an offset beyond the end of the
       file is unspecified.

    If MAP_ANONYMOUS is set in the flags argument:

    o  A new memory region is created and initialized to all zeros.

    o  The filedes argument is ignored.

    The new region is placed at the requested address if the
    requested address is not null and it is possible to place the
    region at this address. When the requested address is null or
    the region cannot be placed at the requested address, the MAP_
    VARIABLE and MAP_FIXED flags control the placement of the region.
    One of these flags must be selected.

    If MAP_VARIABLE is set in the flags argument:

    o  If the requested address is null or if it is not possible for
       the system to place the region at the requested address, the
       region is placed at an address selected by the system.

    If MAP_FIXED is set in the flags argument:

    o  If the requested address is not null, the mmap function
       succeeds even if the requested address is already part of
       another region. (If the address is within an existing region,
       the effect on the pages within that region and within the area
       of the overlap produced by the two regions is the same as if
       they were unmapped. In other words, whatever is mapped between
       addr and addr + len is unmapped.)

    o  If the requested address is null and MAP_FIXED is specified,
       the results are undefined.

    The MAP_PRIVATE and MAP_SHARED flags control the visibility of
    modifications to the mapped file or shared memory region. One of
    these flags must be selected.

    If MAP_SHARED is set in the flags argument:

    o  If the region is a mapped region, modifications to the region
       are visible to other processes that mapped the same region
       using MAP_SHARED.

    o  If the region is a mapped file region, modifications to the
       region are written to the file. (Note that the modifications
       are not immediately written to the file because of buffer
       cache delay; that is, the write to the file does not occur
       until there is a need to reuse the buffer cache. If the
       modifications must be written to the file immediately, use
       the msync function to ensure that this is done.)

    If MAP_PRIVATE is set in the flags argument:

    o  Modifications to the mapped region by the calling process are
       not visible to other processes that mapped the same region
       using either MAP_PRIVATE or MAP_SHARED.

    o  Modifications to the mapped region by the calling process are
       not written to the file.

    It is unspecified whether modifications by processes that mapped
    the region using MAP_SHARED are visible to other processes that
    mapped the same region using MAP_PRIVATE.

    The prot argument specifies access permissions for the mapped
    region. Specify one of the following:

    PROT_NONE      No access
    PROT_READ      Read-only
    PROT_WRITE     Read/Write access

    After the successful completion of the mmap function, you can
    close the filedes argument without effect on the mapped region or
    on the contents of the mapped file. Each mapped region creates a
    file reference, similar to an open file descriptor, that prevents
    the file data from being deallocated.

                                   NOTE

       The following rules apply to OpenVMS specific file
       references:

       o  Because of the additional file reference, if filedes is
          not opened for file sharing, mmap reopens it with file
          sharing enabled.

       o  The additional file reference that remains for mapped
          regions implies that a later open, fopen, or create call
          to the file that is mapped must specify file sharing.

    Modifications made to the file using the write function are
    visible to mapped regions, and modifications to a mapped region
    are visible with the read function.

                                   NOTE

       Beginning with OpenVMS Version 7.2, while processing a MAP_
       SHARED request, the mmap function constructs the flags
       argument of the SYS$CRMPSC service as a bitwise inclusive
       OR of those bits it sets by itself to fulfill the MAP_
       SHARED request and those bits specified by the caller in
       the optional argument.

       By default, for MAP_SHARED the mmap function creates a
       temporary group global section. The optional mmap argument
       provides the caller with direct access to the features of
       the SYS$CRMPSC system service.

       Using the optional argument, the caller can create, for
       example, a system global section (SEC$M_SYSGBL bit) or
       permanent global section (SEC$M_PERM bit). For example,
       to create a system permanent global section, the caller
       can specify (SEC$M_SYSGBL | SEC$M_PERM) in the optional
       argument.

       The mmap function does not check or set any privileges.
       It is the responsibility of the caller to set appropriate
       privileges, such as SYSGBL privilege for SEC$M_SYSGBL, and
       PRMGBL for SEC$M_PERM, before calling mmap with the optional
       argument.

    See also read, write, open, fopen, creat, and sysconf.