Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
MMAP(2)                   FreeBSD System Calls Manual                  MMAP(2)

     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

     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

     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

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

     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

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

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

     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

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

     [EINVAL]           An invalid value was passed in the prot argument.

     [EINVAL]           An undefined option was set in the flags argument.

     [EINVAL]           Both MAP_PRIVATE and MAP_SHARED were specified.

     [EINVAL]           None of MAP_ANON, MAP_PRIVATE, MAP_SHARED, or
                        MAP_STACK was specified.  At least one of these flags
                        must be included.

     [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

     [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

     [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.

FreeBSD 11.0-PRERELEASE       September 17, 2014       FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help