Doc/lib/libmmap.tex

   1 \section{\module{mmap} ---
   2 Memory-mapped file support}
   3
   4 \declaremodule{builtin}{mmap}
   5 \modulesynopsis{Interface to memory-mapped files for \UNIX\ and Windows.}
   6
   7 Memory-mapped file objects behave like both strings and like
   8 file objects.  Unlike normal string objects, however, these are
   9 mutable.  You can use mmap objects in most places where strings
  10 are expected; for example, you can use the \module{re} module to
  11 search through a memory-mapped file.  Since they're mutable, you can
  12 change a single character by doing \code{obj[\var{index}] = 'a'}, or
  13 change a substring by assigning to a slice:
  14 \code{obj[\var{i1}:\var{i2}] = '...'}.  You can also read and write
  15 data starting at the current file position, and \method{seek()}
  16 through the file to different positions.
  17
  18 A memory-mapped file is created by the \function{mmap()} function,
  19 which is different on \UNIX{} and on Windows.  In either case you must
  20 provide a file descriptor for a file opened for update.
  21 If you wish to map an existing Python file object, use its
  22 \method{fileno()} method to obtain the correct value for the
  23 \var{fileno} parameter.  Otherwise, you can open the file using the
  24 \function{os.open()} function, which returns a file descriptor
  25 directly (the file still needs to be closed when done).
  26
  27 For both the \UNIX{} and Windows versions of the function,
  28 \var{access} may be specified as an optional keyword parameter.
  29 \var{access} accepts one of three values: \constant{ACCESS_READ},
  30 \constant{ACCESS_WRITE}, or \constant{ACCESS_COPY} to specify
  31 readonly, write-through or copy-on-write memory respectively.
  32 \var{access} can be used on both \UNIX{} and Windows.  If
  33 \var{access} is not specified, Windows mmap returns a write-through
  34 mapping.  The initial memory values for all three access types are
  35 taken from the specified file.  Assignment to an
  36 \constant{ACCESS_READ} memory map raises a \exception{TypeError}
  37 exception.  Assignment to an \constant{ACCESS_WRITE} memory map
  38 affects both memory and the underlying file.  Assigment to an
  39 \constant{ACCESS_COPY} memory map affects memory but does not update
  40 the underlying file.
  41
  42 \begin{funcdesc}{mmap}{fileno, length\optional{, tagname\optional{, access}}}
  43   \strong{(Windows version)} Maps \var{length} bytes from the file
  44   specified by the file handle \var{fileno}, and returns a mmap
  45   object.  If \var{length} is \code{0}, the maximum length of the map
  46   will be the current size of the file when \function{mmap()} is
  47   called.
  48
  49   \var{tagname}, if specified and not \code{None}, is a string giving
  50   a tag name for the mapping.  Windows allows you to have many
  51   different mappings against the same file.  If you specify the name
  52   of an existing tag, that tag is opened, otherwise a new tag of this
  53   name is created.  If this parameter is omitted or \code{None}, the
  54   mapping is created without a name.  Avoiding the use of the tag
  55   parameter will assist in keeping your code portable between \UNIX{}
  56   and Windows.
  57 \end{funcdesc}
  58
  59 \begin{funcdescni}{mmap}{fileno, length\optional{, flags\optional{,
  60                          prot\optional{, access}}}}
  61   \strong{(\UNIX{} version)} Maps \var{length} bytes from the file
  62   specified by the file descriptor \var{fileno}, and returns a mmap
  63   object.
  64
  65   \var{flags} specifies the nature of the mapping.
  66   \constant{MAP_PRIVATE} creates a private copy-on-write mapping, so
  67   changes to the contents of the mmap object will be private to this
  68   process, and \constant{MAP_SHARED} creates a mapping that's shared
  69   with all other processes mapping the same areas of the file.  The
  70   default value is \constant{MAP_SHARED}.
  71
  72   \var{prot}, if specified, gives the desired memory protection; the
  73   two most useful values are \constant{PROT_READ} and
  74   \constant{PROT_WRITE}, to specify that the pages may be read or
  75   written.  \var{prot} defaults to \constant{PROT_READ | PROT_WRITE}.
  76
  77   \var{access} may be specified in lieu of \var{flags} and \var{prot}
  78   as an optional keyword parameter.  It is an error to specify both
  79   \var{flags}, \var{prot} and \var{access}.  See the description of
  80   \var{access} above for information on how to use this parameter.
  81 \end{funcdescni}
  82
  83
  84 Memory-mapped file objects support the following methods:
  85
  86
  87 \begin{methoddesc}{close}{}
  88   Close the file.  Subsequent calls to other methods of the object
  89   will result in an exception being raised.
  90 \end{methoddesc}
  91
  92 \begin{methoddesc}{find}{string\optional{, start}}
  93   Returns the lowest index in the object where the substring
  94   \var{string} is found.  Returns \code{-1} on failure.  \var{start}
  95   is the index at which the search begins, and defaults to zero.
  96 \end{methoddesc}
  97
  98 \begin{methoddesc}{flush}{\optional{offset, size}}
  99   Flushes changes made to the in-memory copy of a file back to disk.
 100   Without use of this call there is no guarantee that changes are
 101   written back before the object is destroyed.  If \var{offset} and
 102   \var{size} are specified, only changes to the given range of bytes
 103   will be flushed to disk; otherwise, the whole extent of the mapping
 104   is flushed.
 105 \end{methoddesc}
 106
 107 \begin{methoddesc}{move}{\var{dest}, \var{src}, \var{count}}
 108   Copy the \var{count} bytes starting at offset \var{src} to the
 109   destination index \var{dest}.  If the mmap was created with
 110   \constant{ACCESS_READ}, then calls to move will throw a
 111   \exception{TypeError} exception.
 112 \end{methoddesc}
 113
 114 \begin{methoddesc}{read}{\var{num}}
 115   Return a string containing up to \var{num} bytes starting from the
 116   current file position; the file position is updated to point after the
 117   bytes that were returned.
 118 \end{methoddesc}
 119
 120 \begin{methoddesc}{read_byte}{}
 121   Returns a string of length 1 containing the character at the current
 122   file position, and advances the file position by 1.
 123 \end{methoddesc}
 124
 125 \begin{methoddesc}{readline}{}
 126   Returns a single line, starting at the current file position and up to
 127   the next newline.
 128 \end{methoddesc}
 129
 130 \begin{methoddesc}{resize}{\var{newsize}}
 131   If the mmap was created with \constant{ACCESS_READ} or
 132   \constant{ACCESS_COPY}, resizing the map will throw a \exception{TypeError} exception.
 133 \end{methoddesc}
 134
 135 \begin{methoddesc}{seek}{pos\optional{, whence}}
 136   Set the file's current position.  \var{whence} argument is optional
 137   and defaults to \code{0} (absolute file positioning); other values
 138   are \code{1} (seek relative to the current position) and \code{2}
 139   (seek relative to the file's end).
 140 \end{methoddesc}
 141
 142 \begin{methoddesc}{size}{}
 143   Return the length of the file, which can be larger than the size of
 144   the memory-mapped area.
 145 \end{methoddesc}
 146
 147 \begin{methoddesc}{tell}{}
 148   Returns the current position of the file pointer.
 149 \end{methoddesc}
 150
 151 \begin{methoddesc}{write}{\var{string}}
 152   Write the bytes in \var{string} into memory at the current position
 153   of the file pointer; the file position is updated to point after the
 154   bytes that were written. If the mmap was created with
 155   \constant{ACCESS_READ}, then writing to it will throw a
 156   \exception{TypeError} exception.
 157 \end{methoddesc}
 158
 159 \begin{methoddesc}{write_byte}{\var{byte}}
 160   Write the single-character string \var{byte} into memory at the
 161   current position of the file pointer; the file position is advanced
 162   by \code{1}.If the mmap was created with \constant{ACCESS_READ},
 163   then writing to it will throw a \exception{TypeError} exception.
 164 \end{methoddesc}