mmap — Memory-mapped file support

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Disponibilidade

Este módulo não funciona ou não está disponível em WebAssembly. Veja Plataformas WebAssembly para mais informações.

Objetos arquivo mapeados na memória se comportam como bytearray e como objetos arquivo. Você pode usar objetos mmap na maioria dos lugares onde bytearray é esperado; por exemplo, você pode usar o módulo re para pesquisar um arquivo mapeado na memória. Você também pode alterar um único byte executando obj[index] = 97 ou alterar uma subsequência atribuindo a uma fatia: obj[i1:i2] = b'...'. Você também pode ler e gravar dados começando na posição atual do arquivo e seek() através do arquivo para diferentes posições.

A memory-mapped file is created by the mmap constructor, 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 fileno() method to obtain the correct value for the fileno parameter. Otherwise, you can open the file using the os.open() function, which returns a file descriptor directly (the file still needs to be closed when done).

Nota

Se você deseja criar um mapeamento de memória para um arquivo gravável e armazenado em buffer, deve usar flush() no arquivo primeiro. Isso é necessário para garantir que as modificações locais nos buffers estejam realmente disponíveis para o mapeamento.

Para as versões Unix e Windows do construtor, access pode ser especificado como um parâmetro opcional de palavra-chave. access aceita um dos quatro valores: ACCESS_READ, ACCESS_WRITE ou ACCESS_COPY para especificar memória somente leitura, gravação ou cópia na gravação, respectivamente ACCESS_DEFAULT para adiar para prot. access pode ser usado no Unix e no Windows. Se access não for especificado, o mmap do Windows retornará um mapeamento de gravação. Os valores iniciais da memória para todos os três tipos de acesso são obtidos do arquivo especificado. A atribuição a um mapa de memória ACCESS_READ gera uma exceção TypeError. A atribuição a um mapa de memória ACCESS_WRITE afeta a memória e o arquivo subjacente. A atribuição a um mapa de memória ACCESS_COPY afeta a memória, mas não atualiza o arquivo subjacente.

Alterado na versão 3.7: Adicionada a constante ACCESS_DEFAULT.

Para mapear a memória anônima, -1 deve ser passado como o fileno junto com o comprimento.

class mmap.mmap(fileno, length, tagname=None, access=ACCESS_DEFAULT, offset=0)

(Versão Windows) Mapeia length bytes do arquivo especificado pelo identificador de arquivo fileno e cria um objeto mmap. Se length for maior que o tamanho atual do arquivo, o arquivo será estendido para conter length bytes. Se length for 0, o tamanho máximo do mapa será o tamanho atual do arquivo, exceto que, se o arquivo estiver vazio, o Windows levantará uma exceção (você não poderá criar um mapeamento vazio no 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 tagname 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.

Levanta um evento de auditoria mmap.__new__ com os argumentos fileno, length, access, offset.

class mmap.mmap(fileno, length, flags=MAP_SHARED, prot=PROT_WRITE | PROT_READ, access=ACCESS_DEFAULT, offset=0, *, trackfd=True)

(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 mmap is called.

flags specifies the nature of the mapping. 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 MAP_SHARED creates a mapping that’s shared with all other processes mapping the same areas of the file. The default value is MAP_SHARED. Some systems have additional possible flags with the full list specified in MAP_* constants.

prot, if specified, gives the desired memory protection; the two most useful values are PROT_READ and PROT_WRITE, to specify that the pages may be read or written. prot defaults to 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 ALLOCATIONGRANULARITY which is equal to PAGESIZE on Unix systems.

If trackfd is False, the file descriptor specified by fileno will not be duplicated, and the resulting mmap object will not be associated with the map’s underlying file. This means that the size() and resize() methods will fail. This mode is useful to limit the number of open file descriptors.

To ensure validity of the created memory mapping the file specified by the descriptor fileno is internally automatically synchronized with the physical backing store on macOS.

Alterado na versão 3.13: The trackfd parameter was added.

This example shows a simple way of using mmap:

import mmap

# write a simple example file
with open("hello.txt", "wb") as f:
    f.write(b"Hello Python!\n")

with open("hello.txt", "r+b") as f:
    # memory-map the file, size 0 means whole file
    mm = mmap.mmap(f.fileno(), 0)
    # read content via standard file methods
    print(mm.readline())  # prints b"Hello Python!\n"
    # read content via slice notation
    print(mm[:5])  # prints b"Hello"
    # update content using slice notation;
    # note that new content must have same size
    mm[6:] = b" world!\n"
    # ... and read again using standard file methods
    mm.seek(0)
    print(mm.readline())  # prints b"Hello  world!\n"
    # close the map
    mm.close()

mmap can also be used as a context manager in a with statement:

import mmap

with mmap.mmap(-1, 13) as mm:
    mm.write(b"Hello world!")

Adicionado na versão 3.2: Suporte a gerenciador de contexto.

The next example demonstrates how to create an anonymous map and exchange data between the parent and child processes:

import mmap
import os

mm = mmap.mmap(-1, 13)
mm.write(b"Hello world!")

pid = os.fork()

if pid == 0:  # In a child process
    mm.seek(0)
    print(mm.readline())

    mm.close()

Levanta um evento de auditoria mmap.__new__ com os argumentos fileno, length, access, offset.

Memory-mapped file objects support the following methods:

close()

Closes the mmap. Subsequent calls to other methods of the object will result in a ValueError exception being raised. This will not close the open file.

closed

True if the file is closed.

Adicionado na versão 3.2.

find(sub[, start[, end]])

Returns the lowest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns -1 on failure.

Alterado na versão 3.5: Writable bytes-like object is now accepted.

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. offset must be a multiple of the PAGESIZE or ALLOCATIONGRANULARITY.

None is returned to indicate success. An exception is raised when the call failed.

Alterado na versão 3.8: Previously, a nonzero value was returned on success; zero was returned on error under Windows. A zero value was returned on success; an exception was raised on error under Unix.

madvise(option[, start[, length]])

Send advice option to the kernel about the memory region beginning at start and extending length bytes. option must be one of the MADV_* constants available on the system. If start and length are omitted, the entire mapping is spanned. On some systems (including Linux), start must be a multiple of the PAGESIZE.

Availability: Systems with the madvise() system call.

Adicionado na versão 3.8.

move(dest, src, count)

Copy the count bytes starting at offset src to the destination index dest. If the mmap was created with ACCESS_READ, then calls to move will raise a TypeError exception.

read([n])

Return a bytes containing up to n bytes starting from the current file position. If the argument is omitted, None or negative, return all bytes from the current file position to the end of the mapping. The file position is updated to point after the bytes that were returned.

Alterado na versão 3.3: Argument can be omitted or None.

read_byte()

Returns a byte at the current file position as an integer, and advances the file position by 1.

readline()

Returns a single line, starting at the current file position and up to the next newline. The file position is updated to point after the bytes that were returned.

resize(newsize)

Resizes the map and the underlying file, if any.

Resizing a map created with access of ACCESS_READ or ACCESS_COPY, will raise a TypeError exception. Resizing a map created with with trackfd set to False, will raise a ValueError exception.

On Windows: Resizing the map will raise an OSError if there are other maps against the same named file. Resizing an anonymous map (ie against the pagefile) will silently create a new map with the original data copied over up to the length of the new size.

Alterado na versão 3.11: Correctly fails if attempting to resize when another map is held Allows resize against an anonymous map on Windows

rfind(sub[, start[, end]])

Returns the highest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns -1 on failure.

Alterado na versão 3.5: Writable bytes-like object is now accepted.

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).

Alterado na versão 3.13: Return the new absolute position instead of None.

seekable()

Return whether the file supports seeking, and the return value is always True.

Adicionado na versão 3.13.

size()

Return the length of the file, which can be larger than the size of the memory-mapped area.

tell()

Returns the current position of the file pointer.

write(bytes)

Write the bytes in bytes into memory at the current position of the file pointer and return the number of bytes written (never less than len(bytes), since if the write fails, a ValueError will be raised). The file position is updated to point after the bytes that were written. If the mmap was created with ACCESS_READ, then writing to it will raise a TypeError exception.

Alterado na versão 3.5: Writable bytes-like object is now accepted.

Alterado na versão 3.6: The number of bytes written is now returned.

write_byte(byte)

Write the integer byte into memory at the current position of the file pointer; the file position is advanced by 1. If the mmap was created with ACCESS_READ, then writing to it will raise a TypeError exception.

Constantes MADV_*

mmap.MADV_NORMAL

mmap.MADV_RANDOM

mmap.MADV_SEQUENTIAL

mmap.MADV_WILLNEED

mmap.MADV_DONTNEED

mmap.MADV_REMOVE

mmap.MADV_DONTFORK

mmap.MADV_DOFORK

mmap.MADV_HWPOISON

mmap.MADV_MERGEABLE

mmap.MADV_UNMERGEABLE

mmap.MADV_SOFT_OFFLINE

mmap.MADV_HUGEPAGE

mmap.MADV_NOHUGEPAGE

mmap.MADV_DONTDUMP

mmap.MADV_DODUMP

mmap.MADV_FREE

mmap.MADV_NOSYNC

mmap.MADV_AUTOSYNC

mmap.MADV_NOCORE

mmap.MADV_CORE

mmap.MADV_PROTECT

mmap.MADV_FREE_REUSABLE

mmap.MADV_FREE_REUSE

These options can be passed to mmap.madvise(). Not every option will be present on every system.

Availability: Systems with the madvise() system call.

Adicionado na versão 3.8.

Constantes MAP_*

mmap.MAP_SHARED

mmap.MAP_PRIVATE

mmap.MAP_32BIT

mmap.MAP_ALIGNED_SUPER

mmap.MAP_ANON

mmap.MAP_ANONYMOUS

mmap.MAP_CONCEAL

mmap.MAP_DENYWRITE

mmap.MAP_EXECUTABLE

mmap.MAP_HASSEMAPHORE

mmap.MAP_JIT

mmap.MAP_NOCACHE

mmap.MAP_NOEXTEND

mmap.MAP_NORESERVE

mmap.MAP_POPULATE

mmap.MAP_RESILIENT_CODESIGN

mmap.MAP_RESILIENT_MEDIA

mmap.MAP_STACK

mmap.MAP_TPRO

mmap.MAP_TRANSLATED_ALLOW_EXECUTE

mmap.MAP_UNIX03

These are the various flags that can be passed to mmap.mmap(). MAP_ALIGNED_SUPER is only available at FreeBSD and MAP_CONCEAL is only available at OpenBSD. Note that some options might not be present on some systems.

Alterado na versão 3.10: Added MAP_POPULATE constant.

Adicionado na versão 3.11: Added MAP_STACK constant.

Adicionado na versão 3.12: Added MAP_ALIGNED_SUPER and MAP_CONCEAL constants.

Adicionado na versão 3.13: Added MAP_32BIT, MAP_HASSEMAPHORE, MAP_JIT, MAP_NOCACHE, MAP_NOEXTEND, MAP_NORESERVE, MAP_RESILIENT_CODESIGN, MAP_RESILIENT_MEDIA, MAP_TPRO, MAP_TRANSLATED_ALLOW_EXECUTE, and MAP_UNIX03 constants.