\section{\module{mmap} ---Memory-mapped file support} \declaremodule{builtin}{mmap}\modulesynopsis{Interface to memory-mapped files for \UNIX\ and Windows.} Memory-mapped file objects behave like both strings and likefile objects.  Unlike normal string objects, however, these aremutable.  You can use mmap objects in most places where stringsare expected; for example, you can use the \module{re} module tosearch through a memory-mapped file.  Since they're mutable, you canchange a single character by doing \code{obj[\var{index}] = 'a'}, orchange a substring by assigning to a slice:\code{obj[\var{i1}:\var{i2}] = '...'}.  You can also read and writedata starting at the current file position, and \method{seek()}through the file to different positions. A memory-mapped file is created by the \function{mmap()} function,which is different on \UNIX{} and on Windows.  In either case you mustprovide a file descriptor for a file opened for update.If you wish to map an existing Python file object, use its\method{fileno()} method to obtain the correct value for the\var{fileno} parameter.  Otherwise, you can open the file using the\function{os.open()} function, which returns a file descriptordirectly (the file still needs to be closed when done). For both the \UNIX{} and Windows versions of the function,\var{access} may be specified as an optional keyword parameter.\var{access} accepts one of three values: \constant{ACCESS_READ},\constant{ACCESS_WRITE}, or \constant{ACCESS_COPY} to specifyreadonly, write-through or copy-on-write memory respectively.\var{access} can be used on both \UNIX{} and Windows.  If\var{access} is not specified, Windows mmap returns a write-throughmapping.  The initial memory values for all three access types aretaken from the specified file.  Assignment to an\constant{ACCESS_READ} memory map raises a \exception{TypeError}exception.  Assignment to an \constant{ACCESS_WRITE} memory mapaffects both memory and the underlying file.  Assignment to an\constant{ACCESS_COPY} memory map affects memory but does not updatethe underlying file.  \versionchanged[To map anonymous memory,-1 should be passed as the fileno along with the length]{2.5} \begin{funcdesc}{mmap}{fileno, length\optional{, tagname\optional{, access}}}  \strong{(Windows version)} Maps \var{length} bytes from the file  specified by the file handle \var{fileno}, and returns a mmap  object.  If \var{length} is larger than the current size of the file,  the file is extended to contain \var{length} bytes.  If \var{length}  is \code{0}, the maximum length of the map is the current size  of the file, except that if the file is empty Windows raises an  exception (you cannot create an empty mapping on Windows).   \var{tagname}, if specified and not \code{None}, is a string giving  a tag name for the mapping.  Windows allows you to have many  different mappings against the same file.  If you specify the name  of an existing tag, that tag is opened, otherwise a new tag of this  name is created.  If this parameter is omitted or \code{None}, the  mapping is created without a name.  Avoiding the use of the tag  parameter will assist in keeping your code portable between \UNIX{}  and Windows.\end{funcdesc} \begin{funcdescni}{mmap}{fileno, length\optional{, flags\optional{,                         prot\optional{, access}}}}  \strong{(\UNIX{} version)} Maps \var{length} bytes from the file  specified by the file descriptor \var{fileno}, and returns a mmap  object.  If \var{length} is \code{0}, the maximum length of the map  will be the current size of the file when \function{mmap()} is  called.    \var{flags} specifies the nature of the mapping.  \constant{MAP_PRIVATE} creates a private copy-on-write mapping, so  changes to the contents of the mmap object will be private to this  process, and \constant{MAP_SHARED} creates a mapping that's shared  with all other processes mapping the same areas of the file.  The  default value is \constant{MAP_SHARED}.   \var{prot}, if specified, gives the desired memory protection; the  two most useful values are \constant{PROT_READ} and  \constant{PROT_WRITE}, to specify that the pages may be read or  written.  \var{prot} defaults to \constant{PROT_READ | PROT_WRITE}.   \var{access} may be specified in lieu of \var{flags} and \var{prot}  as an optional keyword parameter.  It is an error to specify both  \var{flags}, \var{prot} and \var{access}.  See the description of  \var{access} above for information on how to use this parameter.\end{funcdescni}  Memory-mapped file objects support the following methods:  \begin{methoddesc}{close}{}  Close the file.  Subsequent calls to other methods of the object  will result in an exception being raised.\end{methoddesc} \begin{methoddesc}{find}{string\optional{, start}}  Returns the lowest index in the object where the substring  \var{string} is found.  Returns \code{-1} on failure.  \var{start}  is the index at which the search begins, and defaults to zero.\end{methoddesc} \begin{methoddesc}{flush}{\optional{offset, size}}  Flushes changes made to the in-memory copy of a file back to disk.  Without use of this call there is no guarantee that changes are  written back before the object is destroyed.  If \var{offset} and  \var{size} are specified, only changes to the given range of bytes  will be flushed to disk; otherwise, the whole extent of the mapping  is flushed.\end{methoddesc} \begin{methoddesc}{move}{\var{dest}, \var{src}, \var{count}}  Copy the \var{count} bytes starting at offset \var{src} to the  destination index \var{dest}.  If the mmap was created with  \constant{ACCESS_READ}, then calls to move will throw a  \exception{TypeError} exception.\end{methoddesc} \begin{methoddesc}{read}{\var{num}}  Return a string containing up to \var{num} bytes starting from the  current file position; the file position is updated to point after the  bytes that were returned.\end{methoddesc} \begin{methoddesc}{read_byte}{}  Returns a string of length 1 containing the character at the current  file position, and advances the file position by 1.\end{methoddesc} \begin{methoddesc}{readline}{}  Returns a single line, starting at the current file position and up to  the next newline.\end{methoddesc} \begin{methoddesc}{resize}{\var{newsize}}  Resizes the map and the underlying file, if any.  If the mmap was created with \constant{ACCESS_READ} or  \constant{ACCESS_COPY}, resizing the map will throw a \exception{TypeError} exception.\end{methoddesc} \begin{methoddesc}{seek}{pos\optional{, whence}}  Set the file's current position.  \var{whence} argument is optional  and defaults to \code{os.SEEK_SET} or \code{0} (absolute file  positioning); other values are \code{os.SEEK_CUR} or \code{1} (seek  relative to the current position) and \code{os.SEEK_END} or \code{2}  (seek relative to the file's end).\end{methoddesc} \begin{methoddesc}{size}{}  Return the length of the file, which can be larger than the size of  the memory-mapped area.\end{methoddesc} \begin{methoddesc}{tell}{}  Returns the current position of the file pointer.\end{methoddesc} \begin{methoddesc}{write}{\var{string}}  Write the bytes in \var{string} into memory at the current position  of the file pointer; the file position is updated to point after the  bytes that were written. If the mmap was created with  \constant{ACCESS_READ}, then writing to it will throw a  \exception{TypeError} exception.\end{methoddesc} \begin{methoddesc}{write_byte}{\var{byte}}  Write the single-character string \var{byte} into memory at the  current position of the file pointer; the file position is advanced  by \code{1}. If the mmap was created with \constant{ACCESS_READ},  then writing to it will throw a \exception{TypeError} exception.\end{methoddesc}