mmap

Table of Contents

About MMAP

This is a utility library providing access to the mmap family of functions in a portable way. It should work on Posix and Windows systems. mmap allows you to directly map a file into the address space of your process without having to manually read it into memory sequentially. Typically this is much more efficient for files that are larger than a few Kb.

Supported operations

The library offers access to the following functions:

• mmap
• munmap
• msync
• mprotect

It also provides a convenience macro called with-mmap to perform safe, local mappings of files.

(mmap:with-mmap (addr fd size #p"/etc/lsb-release")
  (with-output-to-string (out)
    (loop for i from 0 below size
          for char = (code-char (cffi:mem-aref addr :char i))
          do (write-char char out))))

If you're on a system where mmap is supported, :mmap will be in *features*.

System Information

Definition Index

• MMAP

  • ORG.SHIRAKUMO.FRAF.TRIAL.MMAP

  No documentation provided.

  • EXTERNAL CONDITION

    MMAP-ERROR

    Source

    Error signalled if the mmap attempt fails for some reason.
    
    Possible reasons include, but are not limited to:
    - File not found
    - File access denied
    - Out of memory
    - Out of address space
    - Mapping not allowed
    - Invalid combination of flags
    
    See MMAP
    See CODE
    See MESSAGE

  • EXTERNAL FUNCTION

    MADVISE

    • ADDR
    • SIZE
    • ADVICE

    Source

    Gives hints about the usage patterns of the memory to better tune mapping behaviour.
    
    The values passed to this function must be the ones retrieved from a call
    to MMAP.
    
    The following advice hints are supported:
    
     :NORMAL          --- [POSIX] This is the default.
     :SEQUENTIAL      --- [POSIX] Expect memory to be addressed sequentially.
     :RANDOM          --- [POSIX] Expect memory to be addressed randomly.
     :WILL-NEED       --- [POSIX] Expect the memory to be used very soon.
     :DONT-NEED       --- [POSIX] Expect the memory to not be needed any
                                  time soon. This will most likely cause
                                  pages to be offloaded until they are
                                  accessed again.
     :FREE            --- [LINUX] The pages in the specified range are no
                                  longer needed and can be freed at any
                                  time, for instance to make space in case
                                  of memory pressure.
     :REMOVE          --- [LINUX] Free the given pages and the associated
                                  backing store.
     :DONT-FORK       --- [LINUX] Don't make changes available in children.
     :DO-FORK         --- [LINUX] Undo :DONT-FORK behaviour.
     :MERGEABLE       --- [LINUX] The pages in the specified range may be
                                  merged with ones with identical content.
     :UNMERGEABLE     --- [LINUX] Undo :MERGEABLE behaviour.
     :HUGE-PAGE       --- [LINUX] Enable transparent huge pages for the
                                  specified page range.
     :NO-HUGE-PAGE    --- [LINUX] Ensure that the memory in the given
                                  range is not backed by transparent huge
                                  pages.
     :DONT-DUMP       --- [LINUX] The pages in the specified range should
                                  be excluded from core dumps.
     :DO-DUMP         --- [LINUX] Undo :DONT-DUMP behaviour.
     :WIPE-ON-FORK    --- [LINUX] Memory in the given range is zeroed out
                                  for children.
     :KEEP-ON-FORK    --- [LINUX] Undo :WIPE-ON-FORK behaviour.
     :COLD            --- [LINUX] Deactivate the given range of
                                  pages. This makes them a more likely
                                  target for reclamation in the presence
                                  of memory pressure.
     :PAGEOUT         --- [LINUX] The pages in the specified range should
                                  be reclaimed and their data flushed out.
    
    This function returns nothing useful.
    
    This function may signal an MMAP-ERROR in case the operating system notices
    a problem.
    
    See MMAP
    See MMAP-ERROR
    See https://pubs.opengroup.org/onlinepubs/007904875/functions/posix_madvise.html
    See https://man7.org/linux/man-pages/man2/madvise.2.html

  • EXTERNAL FUNCTION

    MMAP

    • PATH
    • &KEY
    • OPEN
    • PROTECTION
    • MMAP
    • SIZE
    • OFFSET

    Source

    Map the given path or number of bytes into the address space.
    
    PATH can be either a pathname designator, FD, or NIL. If it is NIL, an
    anonymous file is mapped and the MMAP flag list must include the flag
    :ANONYMOUS. If it is a path or an open POSIX file descriptor, then the
    contents of the given file on the file system are mapped into the
    address space. The file contents can then be read, written, or
    executed depending on the given flags as if normal memory was
    accessed. If the file is NIL or its size cannot be automatically
    determined, you must pass a valid SIZE. You may optionally pass an
    OFFSET (in bytes) into the file from which the mapping begins.
    
    [POSIX] PATH may also be the symbol :ANONYMOUS, in which case an anonymous
    file descriptor is created and returned for you. This can be useful when
    sharing a file descriptor with another process without needing to involve
    a disk-backed file.
    
    If the map attempt fails, an error of type MMAP-ERROR is signalled.
    If the call succeeds, three values are returned:
    
      PTR  --- A CFFI:FOREIGN-POINTER that points to the start of the place in
               memory where the file contents have been mapped. The contents
               should be placed in increasing address order, unless the flag
               :GROWS-DOWN is active.
      FD   --- An opaque file descriptor. You should not touch this.
      SIZE --- The size of the region of memory that has been mapped in bytes.
    
    All three values need to be passed on to MUNMAP completely unchanged. Any
    change could cause severe issues.
    
    The three options OPEN, PROTECTION, and MMAP are lists of flags. Not all of
    those flags are portable, some are only allowed on Linux, some only on non-
    Windows systems. To indicate support, the flags are marked as EVERY, POSIX
    (non-Windows), LINUX, or WINDOWS.
    
    OPEN
     :READ          --- [EVERY] Opens the file for read access.
     :WRITE         --- [EVERY] Opens the file for write access.
     :CREATE        --- [EVERY] Creates the file if it does not exist yet.
     :ENSURE-CREATE --- [EVERY] Creates the file if it does not exist yet and
                                errors if it does.
     :TRUNCATE      --- [EVERY] Truncates the file and replaces it if it exists.
     :DIRECT        --- [EVERY] Causes system buffers to be bypassed.
     :FILE-SYNC     --- [EVERY] Causes writes to the file to be flushed asap.
     :DATA-SYNC     --- [POSIX] Similar to FILE-SYNC, but uses data integrity
                                semantics rather than file integrity semantics.
     :DONT-CLAIM-TTY--- [POSIX] If the file is a tty and the process does not
                                already have a controlling tty, this file will
                                not become the process' controlling tty.
     :NON-BLOCK     --- [POSIX] Attempt to open the file in non-blocking mode,
                                causing operations on the fd to return asap.
     :NO-FOLLOW     --- [LINUX] Errors if the file is a symlink.
     :ASYNC         --- [LINUX] Enable signal driven IO.
     :DIRECTORY     --- [LINUX] Errors if the file is not a directory.
     :LARGE-FILE    --- [LINUX] Allows opening files with size not representable
                                by a 32 bit unsigned integer.
    
    PROTECTION
     :READ          --- [EVERY] Allows reading from the memory region. The OPEN
                                flag :READ is required for this protection mode.
                                This flag is required on windows.
     :WRITE         --- [EVERY] Allows writing to the memory region.
     :EXEC          --- [EVERY] Allows executing code in the memory region.
     :NONE          --- [POSIX] Prevents accessing the memory region.
    
    MMAP
     :PRIVATE       --- [EVERY] The underlying file is not changed if the memory
                                area is written to. Copy-on-write is employed to
                                ensure separation.
     :SHARED        --- [EVERY] The underlying file is changed if the memory
                                area is written to and the change will be
                                visible to other processes. In this case the
                                OPEN flag :WRITE must be specified.
     :ANONYMOUS     --- [LINUX/WINDOWS] The path should be a number of bytes to
                                map to memory. The memory region is then mapped
                                against an "anonymous" file.
     :NO-RESERVE    --- [LINUX] Don't reserve swap for this mapping. If memory
                                runs out, a segfault will be generated instead.
     :LOCKED        --- [LINUX] Locks the region to RAM, preventing it from
                                being swapped out.
     :GROWS-DOWN    --- [LINUX] Causes the memory region to be mapped with a
                                decreasing address, like in a stack.
     :POPULATE      --- [LINUX] Pre-populate the memory region with the file
                                contents, which can help performance.
     :NON-BLOCK     --- [LINUX] Only useful with :POPULATE -- do not perform a
                                read-ahead.
    
    The default values for the flags are:
     :OPEN (:READ) :PROTECTION (:READ) :MMAP (:PRIVATE)
    
    Note that if you are intending to use MPROTECT to change the protection of
    the mapped file at a later date, you need to call MMAP with the maximal
    combination of protection flags first. If this is not the protection that
    you want to start out with, call MPROTECT with the correct combination
    immediately after. For instance, if you would like to start with (:READ) and
    later want to change it to (:READ :WRITE), call MMAP with (:READ :WRITE),
    and immediately after call MPROTECT with (:READ).
    
    See MUNMAP
    See WITH-MMAP
    See MMAP-ERROR
    See http://pubs.opengroup.org/onlinepubs/7908799/xsh/mmap.html
    See http://pubs.opengroup.org/onlinepubs/009604499/functions/stat.html
    See http://man7.org/linux/man-pages/man2/mmap.2.html
    See http://man7.org/linux/man-pages/man2/stat.2.html
    See https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-createfilew
    See https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-getfilesize
    See https://docs.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-createfilemappinga
    See https://msdn.microsoft.com/en-us/library/windows/desktop/aa366761(v=vs.85).aspx

  • EXTERNAL FUNCTION

    MPROTECT

    • ADDR
    • SIZE
    • PROTECTION

    Source

    Changes the access protection of the mapped memory region.
    
    The values passed to this function must be the ones retrieved from a call
    to MMAP.
    
    The following protection flags are supported:
    
     :READ          --- [EVERY] Allows reading from the memory region. The OPEN
                                flag :READ is required for this protection mode.
                                This flag is required on windows.
     :WRITE         --- [EVERY] Allows writing to the memory region.
     :EXEC          --- [EVERY] Allows executing code in the memory region.
     :NONE          --- [POSIX] Prevents accessing the memory region.
    
    This function returns nothing useful.
    
    This function may signal an MMAP-ERROR in case the operating system notices
    a problem.
    
    See MMAP
    See MMAP-ERROR
    See http://pubs.opengroup.org/onlinepubs/9699919799/functions/mprotect.html
    See http://man7.org/linux/man-pages/man2/mprotect.2.html
    See https://msdn.microsoft.com/en-us/library/windows/desktop/aa366898(v=vs.85).aspx

  • EXTERNAL FUNCTION

    MSYNC

    • ADDR
    • FD
    • SIZE
    • &KEY
    • FLAGS

    Source

    Causes writes to the mapped file area to be written to disk.
    
    The values passed to this function must be the ones retrieved from a call
    to MMAP.
    
    The following flags are supported:
    
     :SYNC          --- [EVERY] Writing is synchronous. A call to this function
                                will not return until the data is flushed to
                                disk.
     :ASYNC         --- [EVERY] Writing is asynchronous and a call will return
                                immediately.
     :INVALIDATE    --- [POSIX] Asks to invalidate other mappings of the same
                                file, ensuring the view is synchronised.
    
    This function returns nothing useful.
    
    This function may signal an MMAP-ERROR in case the operating system notices
    a problem.
    
    See MMAP
    See MMAP-ERROR
    See http://pubs.opengroup.org/onlinepubs/000095399/functions/msync.html
    See http://man7.org/linux/man-pages/man2/msync.2.html
    See https://msdn.microsoft.com/en-us/library/windows/desktop/aa366563(v=vs.85).aspx
    See https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-flushfilebuffers

  • EXTERNAL FUNCTION

    MUNMAP

    • ADDR
    • FD
    • SIZE

    Source

    Unmaps the memory region, freeing the address space and its file.
    
    The values passed to this function must be the ones retrieved from a call
    to MMAP. Calling MUNMAP with the same values more than once will lead to
    undefined consequences and may very well corrupt your system to crash. The
    same goes for calling MUNMAP with values not directly returned by MMAP,
    calling it with changed values returned by MMAP, or attempting to
    dereference the PTR after a call to MUNMAP.
    
    This function returns nothing useful.
    
    On POSIX systems you may pass NIL for the FD argument, in which case
    the file descriptor is not closed. It is then your responsibility to
    close it appropriately at a later point.
    
    This function may signal an MMAP-ERROR in case the operating system notices
    a problem.
    
    See MMAP
    See MMAP-ERROR
    See WITH-MMAP
    See http://pubs.opengroup.org/onlinepubs/9699919799/functions/mprotect.html
    See http://man7.org/linux/man-pages/man2/mprotect.2.html
    See https://msdn.microsoft.com/en-us/library/windows/desktop/aa366882(v=vs.85).aspx
    See https://msdn.microsoft.com/en-us/library/windows/desktop/ms724211(v=vs.85).aspx

  • EXTERNAL GENERIC-FUNCTION

    CODE

    • CONDITION

    The OS-specific error code returned for the mmap failure.
    
    See MMAP-ERROR

  • EXTERNAL GENERIC-FUNCTION

    MESSAGE

    • CONDITION

    The (hopefully) user-readable error message for the mmap failure.
    
    See MMAP-ERROR

  • EXTERNAL MACRO

    WITH-MMAP

    • ADDR
    • FD
    • SIZE
    • PATH
    • &REST
    • ARGS
    • &KEY
    • DONT-CLOSE
    • &ALLOW-OTHER-KEYS
    • &BODY
    • BODY

    Source

    Map the file or number of bytes to a memory region within the body.
    
    This is a convenience macro that calls MMAP with the given arguments,
    binds the results to the variables ADDR, FD, and SIZE, and automatically
    ensures that MUNMAP is called with the correct values when the body is
    exited.
    
    If the flag DONT-CLOSE is set, WITH-MMAP will not free the file
    descriptor on unwind. This is useful primarily if you pass in an FD
    for the path yourself and are either not responsible for closing it,
    or would like to continue using it for other purposes.
    
    It is safe to change the ADDR, FD, and SIZE bindings, though probably not
    very good style to do so. It is NOT safe to save the ADDR and SIZE values
    somewhere and use them outside of the dynamic scope of the body. Attempting
    to do so is very likely going to burn your process to the ground.
    
    See MMAP
    See MUNMAP