| |
| :mod:`mmap` --- Memory-mapped file support |
| ========================================== |
| |
| .. module:: mmap |
| :synopsis: Interface to memory-mapped files for Unix and Windows. |
| |
| |
| Memory-mapped file objects behave like both strings and like file objects. |
| Unlike normal string objects, however, these are mutable. You can use mmap |
| objects in most places where strings are expected; for example, you can use the |
| :mod:`re` module to search through a memory-mapped file. Since they're mutable, |
| you can change a single character by doing ``obj[index] = 'a'``, or change a |
| substring by assigning to a slice: ``obj[i1:i2] = '...'``. You can also read |
| and write data starting at the current file position, and :meth:`seek` through |
| the file to different positions. |
| |
| A memory-mapped file is created by the :func:`mmap` function, which is different |
| on Unix and on Windows. In either case you must provide a file descriptor for a |
| file opened for update. If you wish to map an existing Python file object, use |
| its :meth:`fileno` method to obtain the correct value for the *fileno* |
| parameter. Otherwise, you can open the file using the :func:`os.open` function, |
| which returns a file descriptor directly (the file still needs to be closed when |
| done). |
| |
| For both the Unix and Windows versions of the function, *access* may be |
| specified as an optional keyword parameter. *access* accepts one of three |
| values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY` to |
| specify readonly, write-through or copy-on-write memory respectively. *access* |
| can be used on both Unix and Windows. If *access* is not specified, Windows |
| mmap returns a write-through mapping. The initial memory values for all three |
| access types are taken from the specified file. Assignment to an |
| :const:`ACCESS_READ` memory map raises a :exc:`TypeError` exception. Assignment |
| to an :const:`ACCESS_WRITE` memory map affects both memory and the underlying |
| file. Assignment to an :const:`ACCESS_COPY` memory map affects memory but does |
| not update the underlying file. |
| |
| To map anonymous memory, -1 should be passed as the fileno along with the length. |
| |
| |
| .. function:: mmap(fileno, length[, tagname[, access[, offset]]]) |
| |
| **(Windows version)** Maps *length* bytes from the file specified by the file |
| handle *fileno*, and returns a mmap object. If *length* is larger than the |
| current size of the file, the file is extended to contain *length* bytes. If |
| *length* is ``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). |
| |
| *tagname*, if specified and not ``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 ``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. |
| |
| *offset* may be specified as a non-negative integer offset. mmap references will |
| be relative to the offset from the beginning of the file. *offset* defaults to 0. |
| *offset* must be a multiple of the ALLOCATIONGRANULARITY. |
| |
| |
| .. function:: mmap(fileno, length[, flags[, prot[, access[, offset]]]]) |
| :noindex: |
| |
| **(Unix version)** Maps *length* bytes from the file specified by the file |
| descriptor *fileno*, and returns a mmap object. If *length* is ``0``, the |
| maximum length of the map will be the current size of the file when :func:`mmap` |
| is called. |
| |
| *flags* specifies the nature of the mapping. :const:`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 :const:`MAP_SHARED` creates a mapping |
| that's shared with all other processes mapping the same areas of the file. The |
| default value is :const:`MAP_SHARED`. |
| |
| *prot*, if specified, gives the desired memory protection; the two most useful |
| values are :const:`PROT_READ` and :const:`PROT_WRITE`, to specify that the pages |
| may be read or written. *prot* defaults to :const:`PROT_READ \| PROT_WRITE`. |
| |
| *access* may be specified in lieu of *flags* and *prot* as an optional keyword |
| parameter. It is an error to specify both *flags*, *prot* and *access*. See |
| the description of *access* above for information on how to use this parameter. |
| |
| *offset* may be specified as a non-negative integer offset. mmap references will |
| be relative to the offset from the beginning of the file. *offset* defaults to 0. |
| *offset* must be a multiple of the PAGESIZE or ALLOCATIONGRANULARITY. |
| |
| This example shows a simple way of using :func:`mmap`:: |
| |
| import mmap |
| |
| # write a simple example file |
| with open("hello.txt", "w") as f: |
| f.write("Hello Python!\n") |
| |
| with open("hello.txt", "r+") as f: |
| # memory-map the file, size 0 means whole file |
| map = mmap.mmap(f.fileno(), 0) |
| # read content via standard file methods |
| print map.readline() # prints "Hello Python!" |
| # read content via slice notation |
| print map[:5] # prints "Hello" |
| # update content using slice notation; |
| # note that new content must have same size |
| map[6:] = " world!\n" |
| # ... and read again using standard file methods |
| map.seek(0) |
| print map.readline() # prints "Hello world!" |
| # close the map |
| map.close() |
| |
| |
| The next example demonstrates how to create an anonymous map and exchange |
| data between the parent and child processes:: |
| |
| import mmap |
| import os |
| |
| map = mmap.mmap(-1, 13) |
| map.write("Hello world!") |
| |
| pid = os.fork() |
| |
| if pid == 0: # In a child process |
| map.seek(0) |
| print map.readline() |
| |
| map.close() |
| |
| |
| Memory-mapped file objects support the following methods: |
| |
| |
| .. method:: mmap.close() |
| |
| Close the file. Subsequent calls to other methods of the object will result in |
| an exception being raised. |
| |
| |
| .. method:: mmap.find(string[, start]) |
| |
| Returns the lowest index in the object where the substring *string* is found. |
| Returns ``-1`` on failure. *start* is the index at which the search begins, and |
| defaults to zero. |
| |
| |
| .. method:: mmap.flush([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 *offset* and *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. |
| |
| |
| .. method:: mmap.move(dest, src, count) |
| |
| Copy the *count* bytes starting at offset *src* to the destination index *dest*. |
| If the mmap was created with :const:`ACCESS_READ`, then calls to move will throw |
| a :exc:`TypeError` exception. |
| |
| |
| .. method:: mmap.read(num) |
| |
| Return a string containing up to *num* bytes starting from the current file |
| position; the file position is updated to point after the bytes that were |
| returned. |
| |
| |
| .. method:: mmap.read_byte() |
| |
| Returns a string of length 1 containing the character at the current file |
| position, and advances the file position by 1. |
| |
| |
| .. method:: mmap.readline() |
| |
| Returns a single line, starting at the current file position and up to the next |
| newline. |
| |
| |
| .. method:: mmap.resize(newsize) |
| |
| Resizes the map and the underlying file, if any. If the mmap was created with |
| :const:`ACCESS_READ` or :const:`ACCESS_COPY`, resizing the map will throw a |
| :exc:`TypeError` exception. |
| |
| |
| .. method:: mmap.seek(pos[, whence]) |
| |
| Set the file's current position. *whence* argument is optional and defaults to |
| ``os.SEEK_SET`` or ``0`` (absolute file positioning); other values are |
| ``os.SEEK_CUR`` or ``1`` (seek relative to the current position) and |
| ``os.SEEK_END`` or ``2`` (seek relative to the file's end). |
| |
| |
| .. method:: mmap.size() |
| |
| Return the length of the file, which can be larger than the size of the |
| memory-mapped area. |
| |
| |
| .. method:: mmap.tell() |
| |
| Returns the current position of the file pointer. |
| |
| |
| .. method:: mmap.write(string) |
| |
| Write the bytes in *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 :const:`ACCESS_READ`, then writing to it |
| will throw a :exc:`TypeError` exception. |
| |
| |
| .. method:: mmap.write_byte(byte) |
| |
| Write the single-character string *byte* into memory at the current position of |
| the file pointer; the file position is advanced by ``1``. If the mmap was |
| created with :const:`ACCESS_READ`, then writing to it will throw a |
| :exc:`TypeError` exception. |
| |
| |