Provided by: freebsd-manpages_10.1~RC1-1_all bug


     mmap — allocate memory, or map files or devices into memory


     Standard C Library (libc, -lc)


     #include <sys/mman.h>

     void *
     mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);


     The mmap() system call causes the pages starting at addr and continuing for at most len
     bytes to be mapped from the object described by fd, starting at byte offset offset.  If len
     is not a multiple of the pagesize, the mapped region may extend past the specified range.
     Any such extension beyond the end of the mapped object will be zero-filled.

     If addr is non-zero, it is used as a hint to the system.  (As a convenience to the system,
     the actual address of the region may differ from the address supplied.)  If addr is zero, an
     address will be selected by the system.  The actual starting address of the region is
     returned.  A successful mmap deletes any previous mapping in the allocated address range.

     The protections (region accessibility) are specified in the prot argument by or'ing the
     following values:

     PROT_NONE   Pages may not be accessed.
     PROT_READ   Pages may be read.
     PROT_WRITE  Pages may be written.
     PROT_EXEC   Pages may be executed.

     The flags argument 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.  Sharing, mapping type and options are specified in the flags
     argument by or'ing the following values:

     MAP_32BIT          Request a region in the first 2GB of the current process's address space.
                        If a suitable region cannot be found, mmap() will fail.  This flag is
                        only available on 64-bit platforms.

     MAP_ALIGNED(n)     Align the region on a requested boundary.  If a suitable region cannot be
                        found, mmap() will fail.  The n argument specifies the binary logarithm
                        of the desired alignment.

     MAP_ALIGNED_SUPER  Align the region to maximize the potential use of large (“super”) pages.
                        If a suitable region cannot be found, mmap() will fail.  The system will
                        choose a suitable page size based on the size of mapping.  The page size
                        used as well as the alignment of the region may both be affected by
                        properties of the file being mapped.  In particular, the physical address
                        of existing pages of a file may require a specific alignment.  The region
                        is not guaranteed to be aligned on any specific boundary.

     MAP_ANON           Map anonymous memory not associated with any specific file.  The file
                        descriptor used for creating MAP_ANON must be -1.  The offset argument
                        must be 0.

     MAP_ANONYMOUS      This flag is identical to MAP_ANON and is provided for compatibility.

     MAP_EXCL           This flag can only be used in combination with MAP_FIXED.  Please see the
                        definition of MAP_FIXED for the description of its effect.

     MAP_FIXED          Do not permit the system to select a different address than the one
                        specified.  If the specified address cannot be used, mmap() will fail.
                        If MAP_FIXED is specified, addr must be a multiple of the pagesize.  If
                        MAP_EXCL is not specified, a successfull MAP_FIXED request replaces any
                        previous mappings for the process' pages in the range from addr to addr +
                        len.  In contrast, if MAP_EXCL is specified, the request will fail if a
                        mapping already exists within the range.

     MAP_HASSEMAPHORE   Notify the kernel that the region may contain semaphores and that special
                        handling may be necessary.

     MAP_INHERIT        This flag never operated as advertised and is no longer supported.
                        Please refer to minherit(2) for further information.

     MAP_NOCORE         Region is not included in a core file.

     MAP_NOSYNC         Causes data dirtied via this VM map to be flushed to physical media only
                        when necessary (usually by the pager) rather than gratuitously.
                        Typically this prevents the update daemons from flushing pages dirtied
                        through such maps and thus allows efficient sharing of memory across
                        unassociated processes using a file-backed shared memory map.  Without
                        this option any VM pages you dirty may be flushed to disk every so often
                        (every 30-60 seconds usually) which can create performance problems if
                        you do not need that to occur (such as when you are using shared file-
                        backed mmap regions for IPC purposes).  Note that VM/file system
                        coherency is maintained whether you use MAP_NOSYNC or not.  This option
                        is not portable across UNIX platforms (yet), though some may implement
                        the same behavior by default.

                        WARNING!  Extending a file with ftruncate(2), thus creating a big hole,
                        and then filling the hole by modifying a shared mmap() can lead to severe
                        file fragmentation.  In order to avoid such fragmentation you should
                        always pre-allocate the file's backing store by write()ing zero's into
                        the newly extended area prior to modifying the area via your mmap().  The
                        fragmentation problem is especially sensitive to MAP_NOSYNC pages,
                        because pages may be flushed to disk in a totally random order.

                        The same applies when using MAP_NOSYNC to implement a file-based shared
                        memory store.  It is recommended that you create the backing store by
                        write()ing zero's to the backing file rather than ftruncate()ing it.  You
                        can test file fragmentation by observing the KB/t (kilobytes per
                        transfer) results from an “iostat 1” while reading a large file
                        sequentially, e.g. using “dd if=filename of=/dev/null bs=32k”.

                        The fsync(2) system call will flush all dirty data and metadata
                        associated with a file, including dirty NOSYNC VM data, to physical
                        media.  The sync(8) command and sync(2) system call generally do not
                        flush dirty NOSYNC VM data.  The msync(2) system call is usually not
                        needed since BSD implements a coherent file system buffer cache.
                        However, it may be used to associate dirty VM pages with file system
                        buffers and thus cause them to be flushed to physical media sooner rather
                        than later.

     MAP_PREFAULT_READ  Immediately update the calling process's lowest-level virtual address
                        translation structures, such as its page table, so that every memory
                        resident page within the region is mapped for read access.  Ordinarily
                        these structures are updated lazily.  The effect of this option is to
                        eliminate any soft faults that would otherwise occur on the initial read
                        accesses to the region.  Although this option does not preclude prot from
                        including PROT_WRITE, it does not eliminate soft faults on the initial
                        write accesses to the region.

     MAP_PRIVATE        Modifications are private.

     MAP_SHARED         Modifications are shared.

     MAP_STACK          MAP_STACK implies MAP_ANON, and offset of 0.  The fd argument must be -1
                        and prot must include at least PROT_READ and PROT_WRITE.  This option
                        creates a memory region that grows to at most len bytes in size, starting
                        from the stack top and growing down.  The stack top is the starting
                        address returned by the call, plus len bytes.  The bottom of the stack at
                        maximum growth is the starting address returned by the call.

     The close(2) system call does not unmap pages, see munmap(2) for further information.

     The current design does not allow a process to specify the location of swap space.  In the
     future we may define an additional mapping type, MAP_SWAP, in which the file descriptor
     argument specifies a file or device to which swapping should be done.


     Although this implementation does not impose any alignment restrictions on the offset
     argument, a portable program must only use page-aligned values.

     Large page mappings require that the pages backing an object be aligned in matching blocks
     in both the virtual address space and RAM.  The system will automatically attempt to use
     large page mappings when mapping an object that is already backed by large pages in RAM by
     aligning the mapping request in the virtual address space to match the alignment of the
     large physical pages.  The system may also use large page mappings when mapping portions of
     an object that are not yet backed by pages in RAM.  The MAP_ALIGNED_SUPER flag is an
     optimization that will align the mapping request to the size of a large page similar to
     MAP_ALIGNED, except that the system will override this alignment if an object already uses
     large pages so that the mapping will be consistent with the existing large pages.  This flag
     is mostly useful for maximizing the use of large pages on the first mapping of objects that
     do not yet have pages present in RAM.


     Upon successful completion, mmap() returns a pointer to the mapped region.  Otherwise, a
     value of MAP_FAILED is returned and errno is set to indicate the error.


     The mmap() system call will fail if:

     [EACCES]           The flag PROT_READ was specified as part of the prot argument and fd was
                        not open for reading.  The flags MAP_SHARED and PROT_WRITE were specified
                        as part of the flags and prot argument and fd was not open for writing.

     [EBADF]            The fd argument is not a valid open file descriptor.

     [EINVAL]           MAP_FIXED was specified and the addr argument was not page aligned, or
                        part of the desired address space resides out of the valid address space
                        for a user process.

     [EINVAL]           Both MAP_FIXED and MAP_32BIT were specified and part of the desired
                        address space resides outside of the first 2GB of user address space.

     [EINVAL]           The len argument was equal to zero.

     [EINVAL]           MAP_ALIGNED was specified and the desired alignment was either larger
                        than the virtual address size of the machine or smaller than a page.

     [EINVAL]           MAP_ANON was specified and the fd argument was not -1.

     [EINVAL]           MAP_ANON was specified and the offset argument was not 0.

     [EINVAL]           Both MAP_FIXED and MAP_EXCL were specified, but the requested region is
                        already used by a mapping.

     [EINVAL]           MAP_EXCL was specified, but MAP_FIXED was not.

     [ENODEV]           MAP_ANON has not been specified and fd did not reference a regular or
                        character special file.

     [ENOMEM]           MAP_FIXED was specified and the addr argument was not available.
                        MAP_ANON was specified and insufficient memory was available.


     madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2),
     getpagesize(3), getpagesizes(3)


     The len argument is limited to the maximum file size or available userland address space.
     Files may not be able to be made more than 1TB large on 32 bit systems due to file systems
     restrictions and bugs, but address space is far more restrictive.  Larger files may be
     possible on 64 bit systems.

     The previous documented limit of 2GB was a documentation bug.  That limit has not existed
     since FreeBSD 2.2.