# `ctypes` — Uma biblioteca de funções externas para Python

Código-fonte: Lib/ctypes

`ctypes` é uma biblioteca de funções externas para Python. Ela fornece tipos de dados compatíveis com C e permite funções de chamada em DLLs ou bibliotecas compartilhadas. Ela pode ser usada para agrupar essas bibliotecas em Python puro.

## Tutorial do ctypes

Nota: Os exemplos de código neste tutorial usam `doctest` para garantir que realmente funcionem. Como algumas amostras de código se comportam de maneira diferente no Linux, Windows ou macOS, elas contêm diretrizes de doctest nos comentários.

Nota: Alguns exemplos de código fazem referência ao tipo ctypes `c_int`. Em plataformas em que `sizeof(long) == sizeof(int)` é um apelido para `c_long`. Então, você não deve ficar confuso se `c_long` for impresso se você esperaria `c_int` — eles são, na verdade, o mesmo tipo.

### Carregando bibliotecas de ligação dinâmica

`ctypes` exporta o cdll e, no Windows, os objetos windll e oledll para carregar bibliotecas de vínculo dinâmico.

You load libraries by accessing them as attributes of these objects. cdll loads libraries which export functions using the standard `cdecl` calling convention, while windll libraries call functions using the `stdcall` calling convention. oledll also uses the `stdcall` calling convention, and assumes the functions return a Windows `HRESULT` error code. The error code is used to automatically raise an `OSError` exception when the function call fails.

Alterado na versão 3.3: Erros do Windows costumavam levantar `WindowsError`, que agora é um apelido de `OSError`.

Here are some examples for Windows. Note that `msvcrt` is the MS standard C library containing most standard C functions, and uses the cdecl calling convention:

    >>> from ctypes import *
    >>> print(windll.kernel32)
    <WinDLL 'kernel32', handle ... at ...>
    >>> print(cdll.msvcrt)
    <CDLL 'msvcrt', handle ... at ...>
    >>> libc = cdll.msvcrt
    >>>

O Windows acrescenta automaticamente o sufixo de arquivo `.dll` usual.

Nota

Acessar a biblioteca padrão C por meio de `cdll.msvcrt` usará uma versão desatualizada da biblioteca, que pode ser incompatível com a usada pelo Python. Onde possível, use a funcionalidade nativa do Python; caso contrário, importe e use o módulo `msvcrt`.

On Linux, it is required to specify the filename including the extension to load a library, so attribute access can not be used to load libraries. Either the `LoadLibrary()` method of the dll loaders should be used, or you should load the library by creating an instance of CDLL by calling the constructor:

    >>> cdll.LoadLibrary("libc.so.6")
    <CDLL 'libc.so.6', handle ... at ...>
    >>> libc = CDLL("libc.so.6")
    >>> libc
    <CDLL 'libc.so.6', handle ... at ...>
    >>>

### Acessando funções de dlls carregadas

Funções são acessadas como atributos de objetos dll:

    >>> from ctypes import *
    >>> libc.printf
    <_FuncPtr object at 0x...>
    >>> print(windll.kernel32.GetModuleHandleA)
    <_FuncPtr object at 0x...>
    >>> print(windll.kernel32.MyOwnFunction)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "ctypes.py", line 239, in __getattr__
        func = _StdcallFuncPtr(name, self)
    AttributeError: function 'MyOwnFunction' not found
    >>>

Note that win32 system dlls like `kernel32` and `user32` often export ANSI as well as UNICODE versions of a function. The UNICODE version is exported with an `W` appended to the name, while the ANSI version is exported with an `A` appended to the name. The win32 `GetModuleHandle` function, which returns a module handle for a given module name, has the following C prototype, and a macro is used to expose one of them as `GetModuleHandle` depending on whether UNICODE is defined or not:

    /* ANSI version */
    HMODULE GetModuleHandleA(LPCSTR lpModuleName);
    /* UNICODE version */
    HMODULE GetModuleHandleW(LPCWSTR lpModuleName);

windll não tenta selecionar um deles magicamente, você deve acessar a versão necessária especificando `GetModuleHandleA` ou `GetModuleHandleW` explicitamente e então chamá-lo com objetos bytes ou string, respectivamente.

Às vezes, DLLs exportam funções com nomes que não são identificadores Python válidos, como `"??2@YAPAXI@Z"`. Nesse caso, você precisa usar `getattr()` para recuperar a função:

    >>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")
    <_FuncPtr object at 0x...>
    >>>

No Windows, algumas dlls exportam funções não por nome, mas por ordinal. Essas funções podem ser acessadas indexando o objeto dll com o número ordinal:

    >>> cdll.kernel32[1]
    <_FuncPtr object at 0x...>
    >>> cdll.kernel32[0]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "ctypes.py", line 310, in __getitem__
        func = _StdcallFuncPtr(name, self)
    AttributeError: function ordinal 0 not found
    >>>

### Chamando funções

You can call these functions like any other Python callable. This example uses the `time()` function, which returns system time in seconds since the Unix epoch, and the `GetModuleHandleA()` function, which returns a win32 module handle.

This example calls both functions with a `NULL` pointer (`None` should be used as the `NULL` pointer):

    >>> print(libc.time(None))
    1150640792
    >>> print(hex(windll.kernel32.GetModuleHandleA(None)))
    0x1d000000
    >>>

`ValueError` é gerado quando você chama uma função `stdcall` com a convenção de chamada `cdecl`, ou vice-versa:

    >>> cdll.kernel32.GetModuleHandleA(None)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: Procedure probably called with not enough arguments (4 bytes missing)
    >>>

    >>> windll.msvcrt.printf(b"spam")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: Procedure probably called with too many arguments (4 bytes in excess)
    >>>

Para descobrir a convenção de chamada correta, você precisa consultar o arquivo de cabeçalho C ou a documentação da função que deseja chamar.

No Windows, `ctypes` usa o tratamento de exceções estruturado do win32 para evitar travamentos devido a falhas gerais de proteção quando funções são chamadas com valores de argumentos inválidos:

    >>> windll.kernel32.GetModuleHandleA(32)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: exception: access violation reading 0x00000020
    >>>

No entanto, existem maneiras suficientes de travar o Python com `ctypes`, então você deve ter cuidado de qualquer maneira. O módulo `faulthandler` pode ser útil na depuração de travamentos (por exemplo, de falhas de segmentação produzidas por chamadas errôneas da biblioteca C).

`None`, integers, bytes objects and (unicode) strings are the only native Python objects that can directly be used as parameters in these function calls. `None` is passed as a C `NULL` pointer, bytes objects and strings are passed as pointer to the memory block that contains their data (`char*` or `wchar_t*`). Python integers are passed as the platforms default C `int` type, their value is masked to fit into the C type.

Antes de prosseguirmos com a chamada de funções com outros tipos de parâmetros, precisamos aprender mais sobre os tipos de dados `ctypes`.

### Tipos de dados fundamentais

`ctypes` define uma série de tipos de dados primitivos compatíveis com C:

Tipo ctypes | Tipo em C | Tipo em Python
--- | --- | ---
`c_bool` | `_Bool` | bool (1)
`c_char` | `char` | objeto bytes de 1 caractere
`c_wchar` | `wchar_t` | string de 1 caractere
`c_byte` | `char` | int
`c_ubyte` | `unsigned char` | int
`c_short` | `short` | int
`c_ushort` | `unsigned short` | int
`c_int` | `int` | int
`c_uint` | `unsigned int` | int
`c_long` | `long` | int
`c_ulong` | `unsigned long` | int
`c_longlong` | `__int64` ou `long long` | int
`c_ulonglong` | `unsigned __int64` ou `unsigned long long` | int
`c_size_t` | `size_t` | int
`c_ssize_t` | `ssize_t` or `Py_ssize_t` | int
`c_float` | `float` | ponto flutuante
`c_double` | `double` | ponto flutuante
`c_longdouble` | `long double` | ponto flutuante
`c_char_p` | `char*` (finalizado com NUL) | objeto bytes ou `None`
`c_wchar_p` | `wchar_t*` (finalizado com NUL) | String ou `None`
`c_void_p` | `void*` | int ou `None`

1. O construtor aceita qualquer objeto com um valor verdade.

Todos esses tipos podem ser criados chamando-os com um inicializador opcional do tipo e valor corretos:

    >>> c_int()
    c_long(0)
    >>> c_wchar_p("Hello, World")
    c_wchar_p(140018365411392)
    >>> c_ushort(-3)
    c_ushort(65533)
    >>>

Como esses tipos são mutáveis, seus valores também podem ser alterados posteriormente:

    >>> i = c_int(42)
    >>> print(i)
    c_long(42)
    >>> print(i.value)
    42
    >>> i.value = -99
    >>> print(i.value)
    -99
    >>>

Assigning a new value to instances of the pointer types `c_char_p`, `c_wchar_p`, and `c_void_p` changes the memory location they point to, not the contents of the memory block (of course not, because Python bytes objects are immutable):

    >>> s = "Hello, World"
    >>> c_s = c_wchar_p(s)
    >>> print(c_s)
    c_wchar_p(139966785747344)
    >>> print(c_s.value)
    Hello World
    >>> c_s.value = "Hi, there"
    >>> print(c_s)              # the memory location has changed
    c_wchar_p(139966783348904)
    >>> print(c_s.value)
    Hi, there
    >>> print(s)                # first object is unchanged
    Hello, World
    >>>

No entanto, tome cuidado para não passá-los para funções que esperam ponteiros para memória mutável. Se precisar de blocos de memória mutáveis, o ctypes possui uma função `create_string_buffer()` que os cria de várias maneiras. O conteúdo do bloco de memória atual pode ser acessado (ou alterado) com a propriedade `raw`; se quiser acessá-lo como uma string terminada em NUL, use a propriedade `value`:

    >>> from ctypes import *
    >>> p = create_string_buffer(3)            # create a 3 byte buffer, initialized to NUL bytes
    >>> print(sizeof(p), repr(p.raw))
    3 b'\x00\x00\x00'
    >>> p = create_string_buffer(b"Hello")     # create a buffer containing a NUL terminated string
    >>> print(sizeof(p), repr(p.raw))
    6 b'Hello\x00'
    >>> print(repr(p.value))
    b'Hello'
    >>> p = create_string_buffer(b"Hello", 10) # create a 10 byte buffer
    >>> print(sizeof(p), repr(p.raw))
    10 b'Hello\x00\x00\x00\x00\x00'
    >>> p.value = b"Hi"
    >>> print(sizeof(p), repr(p.raw))
    10 b'Hi\x00lo\x00\x00\x00\x00\x00'
    >>>

The `create_string_buffer()` function replaces the `c_buffer()` function (which is still available as an alias), as well as the `c_string()` function from earlier ctypes releases. To create a mutable memory block containing unicode characters of the C type `wchar_t` use the `create_unicode_buffer()` function.

### Chamando funções, continuação

Observe que printf imprime no canal de saída padrão real, não em `sys.stdout`, então esses exemplos só funcionarão no prompt do console, não de dentro do IDLE ou PythonWin:

    >>> printf = libc.printf
    >>> printf(b"Hello, %s\n", b"World!")
    Hello, World!
    14
    >>> printf(b"Hello, %S\n", "World!")
    Hello, World!
    14
    >>> printf(b"%d bottles of beer\n", 42)
    42 bottles of beer
    19
    >>> printf(b"%f bottles of beer\n", 42.5)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ArgumentError: argument 2: TypeError: Don't know how to convert parameter 2
    >>>

Como mencionado anteriormente, todos os tipos do Python, exceto inteiros, strings e objetos bytes, precisam ser encapsulados em seu tipo correspondente de `ctypes`, para que possam ser convertidos para o tipo de dado C necessário:

    >>> printf(b"An int %d, a double %f\n", 1234, c_double(3.14))
    An int 1234, a double 3.140000
    31
    >>>

### Chamando funções variadas

Em muitas plataformas, chamar funções variádicas por meio do ctypes é exatamente o mesmo que chamar funções com um número fixo de parâmetros. Em algumas plataformas, em particular no ARM64 para plataformas Apple, a convenção de chamada para funções variádicas difere da usada para funções regulares.

On those platforms it is required to specify the argtypes attribute for the regular, non-variadic, function arguments:

    libc.printf.argtypes = [ctypes.c_char_p]

Because specifying the attribute does inhibit portability it is advised to always specify `argtypes` for all variadic functions.

### Chamando funções com seus próprios tipos de dados personalizados

You can also customize `ctypes` argument conversion to allow instances of your own classes be used as function arguments. `ctypes` looks for an `_as_parameter_` attribute and uses this as the function argument. Of course, it must be one of integer, string, or bytes:

    >>> class Bottles:
    ...     def __init__(self, number):
    ...         self._as_parameter_ = number
    ...
    >>> bottles = Bottles(42)
    >>> printf(b"%d bottles of beer\n", bottles)
    42 bottles of beer
    19
    >>>

If you don’t want to store the instance’s data in the `_as_parameter_` instance variable, you could define a `property` which makes the attribute available on request.

### Especificando os tipos de argumentos necessários (protótipos de função)

It is possible to specify the required argument types of functions exported from DLLs by setting the `argtypes` attribute.

`argtypes` must be a sequence of C data types (the `printf` function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):

    >>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
    >>> printf(b"String '%s', Int %d, Double %f\n", b"Hi", 10, 2.2)
    String 'Hi', Int 10, Double 2.200000
    37
    >>>

Especificar um formato protege contra tipos de argumentos incompatíveis (assim como um protótipo para uma função em C), e tenta converter os argumentos para tipos válidos:

    >>> printf(b"%d %d %d", 1, 2, 3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ArgumentError: argument 2: TypeError: wrong type
    >>> printf(b"%s %d %f\n", b"X", 2, 3)
    X 2 3.000000
    13
    >>>

If you have defined your own classes which you pass to function calls, you have to implement a `from_param()` class method for them to be able to use them in the `argtypes` sequence. The `from_param()` class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, its `_as_parameter_` attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, bytes, a `ctypes` instance, or an object with an `_as_parameter_` attribute.

### Tipos de Retorno

By default functions are assumed to return the C `int` type. Other return types can be specified by setting the `restype` attribute of the function object.

Here is a more advanced example, it uses the `strchr` function, which expects a string pointer and a char, and returns a pointer to a string:

    >>> strchr = libc.strchr
    >>> strchr(b"abcdef", ord("d"))
    8059983
    >>> strchr.restype = c_char_p    # c_char_p is a pointer to a string
    >>> strchr(b"abcdef", ord("d"))
    b'def'
    >>> print(strchr(b"abcdef", ord("x")))
    None
    >>>

If you want to avoid the `ord("x")` calls above, you can set the `argtypes` attribute, and the second argument will be converted from a single character Python bytes object into a C char:

    >>> strchr.restype = c_char_p
    >>> strchr.argtypes = [c_char_p, c_char]
    >>> strchr(b"abcdef", b"d")
    'def'
    >>> strchr(b"abcdef", b"def")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ArgumentError: argument 2: TypeError: one character string expected
    >>> print(strchr(b"abcdef", b"x"))
    None
    >>> strchr(b"abcdef", b"d")
    'def'
    >>>

You can also use a callable Python object (a function or a class for example) as the `restype` attribute, if the foreign function returns an integer. The callable will be called with the integer the C function returns, and the result of this call will be used as the result of your function call. This is useful to check for error return values and automatically raise an exception:

    >>> GetModuleHandle = windll.kernel32.GetModuleHandleA
    >>> def ValidHandle(value):
    ...     if value == 0:
    ...         raise WinError()
    ...     return value
    ...
    >>>
    >>> GetModuleHandle.restype = ValidHandle
    >>> GetModuleHandle(None)
    486539264
    >>> GetModuleHandle("something silly")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 3, in ValidHandle
    OSError: [Errno 126] The specified module could not be found.
    >>>

`WinError` é uma função que chamará a API `FormatMessage()` do Windows para obter a representação em string de um código de erro, e retorna uma exceção. `WinError` aceita um parâmetro de código de erro opcional, se nenhum for usado, ela chama `GetLastError()` para recuperá-lo.

Please note that a much more powerful error checking mechanism is available through the `errcheck` attribute; see the reference manual for details.

### Passando ponteiros (ou: passando parâmetros por referência)

Às vezes, uma função da API C espera um ponteiro para um tipo de dado como parâmetro, provavelmente para escrever no local correspondente, ou se os dados forem muito grandes para serem passados por valor. Isso também é conhecido como passar parâmetros por referência.

O `ctypes` exporta a função `byref()` que é usada para passar parâmetros por referência. O mesmo efeito pode ser alcançado com a função `pointer()`, embora `pointer()` faça muito mais trabalho, já que ela constrói um objeto ponteiro real, então é mais rápido usar `byref()` se você não precisar do objeto ponteiro no próprio Python:

    >>> i = c_int()
    >>> f = c_float()
    >>> s = create_string_buffer(b'\000' * 32)
    >>> print(i.value, f.value, repr(s.value))
    0 0.0 b''
    >>> libc.sscanf(b"1 3.14 Hello", b"%d %f %s",
    ...             byref(i), byref(f), s)
    3
    >>> print(i.value, f.value, repr(s.value))
    1 3.1400001049 b'Hello'
    >>>

### Estruturas e uniões

Structures and unions must derive from the `Structure` and `Union` base classes which are defined in the `ctypes` module. Each subclass must define a `_fields_` attribute. `_fields_` must be a list of 2-tuples, containing a field name and a field type.

O tipo do campo deve ser um tipo `ctypes` como `c_int`, ou qualquer outro tipo `ctypes` derivado: estrutura, união, array, ponteiro.

Aqui está um exemplo simples de uma estrutura POINT, que contém dois inteiros nomeados x e y, e também mostra como inicializar uma estrutura no construtor:

    >>> from ctypes import *
    >>> class POINT(Structure):
    ...     _fields_ = [("x", c_int),
    ...                 ("y", c_int)]
    ...
    >>> point = POINT(10, 20)
    >>> print(point.x, point.y)
    10 20
    >>> point = POINT(y=5)
    >>> print(point.x, point.y)
    0 5
    >>> POINT(1, 2, 3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: too many initializers
    >>>

Você pode, no entanto, construir estruturas muito mais complicadas. Uma estrutura pode conter outras estruturas, usando uma estrutura como um tipo de campo.

Aqui está uma estrutura RECT que contém dois POINTs nomeados upperleft e lowerright:

    >>> class RECT(Structure):
    ...     _fields_ = [("upperleft", POINT),
    ...                 ("lowerright", POINT)]
    ...
    >>> rc = RECT(point)
    >>> print(rc.upperleft.x, rc.upperleft.y)
    0 5
    >>> print(rc.lowerright.x, rc.lowerright.y)
    0 0
    >>>

Estruturas aninhadas também podem ser inicializadas no construtor de várias maneiras:

    >>> r = RECT(POINT(1, 2), POINT(3, 4))
    >>> r = RECT((1, 2), (3, 4))

Field descriptors can be retrieved from the class, they are useful for debugging because they can provide useful information:

    >>> print(POINT.x)
    <Field type=c_long, ofs=0, size=4>
    >>> print(POINT.y)
    <Field type=c_long, ofs=4, size=4>
    >>>

Aviso

O `ctypes` não suporta passar uniões ou estruturas com campos de bits (bit-fields) para funções por valor. Embora isso possa funcionar em x86 de 32 bits, não é garantido pela biblioteca que funcione no caso geral. Uniões e estruturas com campos de bits devem sempre ser passadas para funções por ponteiro.

### Structure/union alignment and byte order

By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior by specifying a `_pack_` class attribute in the subclass definition. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what `#pragma pack(n)` also does in MSVC.

`ctypes` usa a ordem de bytes nativa para Estruturas e Uniões. Para construir estruturas com ordem de bytes não nativa, você pode usar classes base `BigEndianStructure`, `LittleEndianStructure`, `BigEndianUnion` e `LittleEndianUnion`. Essas classes não podem conter campos de ponteiros.

### Campos de bit em estruturas e uniões

It is possible to create structures and unions containing bit fields. Bit fields are only possible for integer fields, the bit width is specified as the third item in the `_fields_` tuples:

    >>> class Int(Structure):
    ...     _fields_ = [("first_16", c_int, 16),
    ...                 ("second_16", c_int, 16)]
    ...
    >>> print(Int.first_16)
    <Field type=c_long, ofs=0:0, bits=16>
    >>> print(Int.second_16)
    <Field type=c_long, ofs=0:16, bits=16>
    >>>

### Vetores

Vetores são sequências que contêm um número fixo de instâncias do mesmo tipo.

A maneira recomendada de criar tipos vetor é multiplicando um tipo de dado por um inteiro positivo:

    TenPointsArrayType = POINT * 10

Aqui está um exemplo de um tipo de dado um tanto artificial, uma estrutura contendo 4 POINTs entre outras coisas:

    >>> from ctypes import *
    >>> class POINT(Structure):
    ...     _fields_ = ("x", c_int), ("y", c_int)
    ...
    >>> class MyStruct(Structure):
    ...     _fields_ = [("a", c_int),
    ...                 ("b", c_float),
    ...                 ("point_array", POINT * 4)]
    >>>
    >>> print(len(MyStruct().point_array))
    4
    >>>

Instâncias são criadas da maneira usual, chamando a classe:

    arr = TenPointsArrayType()
    for pt in arr:
        print(pt.x, pt.y)

O código acima exibe uma série de linhas `0 0`, pois o conteúdo do vetor é inicializado com zeros.

Inicializadores do tipo correto também podem ser especificados:

    >>> from ctypes import *
    >>> TenIntegers = c_int * 10
    >>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    >>> print(ii)
    <c_long_Array_10 object at 0x...>
    >>> for i in ii: print(i, end=" ")
    ...
    1 2 3 4 5 6 7 8 9 10
    >>>

### Ponteiros

Instâncias de ponteiro são criadas chamando a função `pointer()` em um tipo `ctypes`:

    >>> from ctypes import *
    >>> i = c_int(42)
    >>> pi = pointer(i)
    >>>

Instâncias de ponteiros têm um atributo `contents` que retorna o objeto para qual o ponteiro aponta, o objeto `i` acima:

    >>> pi.contents
    c_long(42)
    >>>

Note que o `ctypes` não possui OOR (retorno de objeto original), ele constrói um objeto novo e equivalente cada vez que você recupera um atributo:

    >>> pi.contents is i
    False
    >>> pi.contents is pi.contents
    False
    >>>

Atribuir outra instância `c_int` ao atributo do conteúdo do ponteiro faria com que o ponteiro apontasse para o local de memória onde ela está armazenada:

    >>> i = c_int(99)
    >>> pi.contents = i
    >>> pi.contents
    c_long(99)
    >>>

Instâncias de ponteiro também podem ser indexadas com inteiros:

    >>> pi[0]
    99
    >>>

Atribuir a um índice inteiro altera o valor apontado:

    >>> print(i)
    c_long(99)
    >>> pi[0] = 22
    >>> print(i)
    c_long(22)
    >>>

Também é possível usar índices diferentes de 0, mas você deve saber o que está fazendo, assim como em C: Você pode acessar ou alterar locais arbitrários da memória. Geralmente, você só usa este recurso se receber um ponteiro de uma função C, e você sabe que o ponteiro na verdade aponta para um vetor em vez de um único item.

Nos bastidores, a função `pointer()` faz mais do que simplesmente criar instâncias de ponteiro, ela precisa criar tipos de ponteiro primeiro. Isso é feito com a função `POINTER()`, que aceita qualquer tipo `ctypes`, e retorna um novo tipo:

    >>> PI = POINTER(c_int)
    >>> PI
    <class 'ctypes.LP_c_long'>
    >>> PI(42)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: expected c_long instead of int
    >>> PI(c_int(42))
    <ctypes.LP_c_long object at 0x...>
    >>>

Chamar o tipo ponteiro sem um argumento cria um ponteiro `NULL`. Ponteiros `NULL` possuem o valor booleano `False`:

    >>> null_ptr = POINTER(c_int)()
    >>> print(bool(null_ptr))
    False
    >>>

O `ctypes` verifica por `NULL` ao desreferenciar ponteiros (mas desreferenciar ponteiros inválidos que não sejam `NULL` travaria o Python):

    >>> null_ptr[0]
    Traceback (most recent call last):
        ....
    ValueError: NULL pointer access
    >>>

    >>> null_ptr[0] = 1234
    Traceback (most recent call last):
        ....
    ValueError: NULL pointer access
    >>>

### Conversão de Tipos

Usually, ctypes does strict type checking. This means, if you have `POINTER(c_int)` in the `argtypes` list of a function or as the type of a member field in a structure definition, only instances of exactly the same type are accepted. There are some exceptions to this rule, where ctypes accepts other objects. For example, you can pass compatible array instances instead of pointer types. So, for `POINTER(c_int)`, ctypes accepts an array of c_int:

    >>> class Bar(Structure):
    ...     _fields_ = [("count", c_int), ("values", POINTER(c_int))]
    ...
    >>> bar = Bar()
    >>> bar.values = (c_int * 3)(1, 2, 3)
    >>> bar.count = 3
    >>> for i in range(bar.count):
    ...     print(bar.values[i])
    ...
    1
    2
    3
    >>>

In addition, if a function argument is explicitly declared to be a pointer type (such as `POINTER(c_int)`) in `argtypes`, an object of the pointed type (`c_int` in this case) can be passed to the function. ctypes will apply the required `byref()` conversion in this case automatically.

Para definir um campo do tipo PONTEIRO como `NULL`, você pode atribuir `None`:

    >>> bar.values = None
    >>>

Às vezes, você tem instâncias de tipos incompatíveis. Em C, você pode converter um tipo em outro tipo. O `ctypes` fornece uma função `cast()` que pode ser usada da mesma maneira. A estrutura `Bar` definida acima aceita ponteiros `POINTER(c_int)` ou vetores `c_int` para seu campo `values`, mas não instâncias de outros tipos:

    >>> bar.values = (c_byte * 4)()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
    >>>

Para esses casos, a função `cast()` é útil.

A função `cast()` pode ser usada para converter uma instância ctypes em um ponteiro para um diferente tipo de dado ctypes. `cast()` recebe dois parâmetros, um objeto ctypes que é ou pode ser convertido para um ponteiro de algum tipo, e um tipo ponteiro ctypes. Ela retorna uma instância do segundo argumento, que referencia o mesmo bloco de memória que o primeiro argumento:

    >>> a = (c_byte * 4)()
    >>> cast(a, POINTER(c_int))
    <ctypes.LP_c_long object at ...>
    >>>

Então, `cast()` pode ser usada para atribuir ao campo `values` da estrutura `Bar`:

    >>> bar = Bar()
    >>> bar.values = cast((c_byte * 4)(), POINTER(c_int))
    >>> print(bar.values[0])
    0
    >>>

### Tipos Incompletos

Tipos Incompletos são estruturas, uniões ou vetores, cujos membros ainda não foram especificados. Em C, eles são especificados por declarações antecipadas, que são definidas posteriormente:

    struct cell; /* forward declaration */

    struct cell {
        char *name;
        struct cell *next;
    };

A tradução direta para código ctypes seria esta, mas não funciona:

    >>> class cell(Structure):
    ...     _fields_ = [("name", c_char_p),
    ...                 ("next", POINTER(cell))]
    ...
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 2, in cell
    NameError: name 'cell' is not defined
    >>>

because the new `class cell` is not available in the class statement itself. In `ctypes`, we can define the `cell` class and set the `_fields_` attribute later, after the class statement:

    >>> from ctypes import *
    >>> class cell(Structure):
    ...     pass
    ...
    >>> cell._fields_ = [("name", c_char_p),
    ...                  ("next", POINTER(cell))]
    >>>

Vamos tentar. Criamos duas instâncias de `cell`, e deixamos que elas apontem uma para a outra, e finalmente seguimos a cadeia de ponteiros algumas vezes:

    >>> c1 = cell()
    >>> c1.name = b"foo"
    >>> c2 = cell()
    >>> c2.name = b"bar"
    >>> c1.next = pointer(c2)
    >>> c2.next = pointer(c1)
    >>> p = c1
    >>> for i in range(8):
    ...     print(p.name, end=" ")
    ...     p = p.next[0]
    ...
    foo bar foo bar foo bar foo bar
    >>>

### Funções Callbacks

`ctypes` permite criar ponteiros de função C chamáveis a partir de chamáveis Python. Estas são às vezes chamadas de funções de callback

Primeiro, você deve criar uma classe para função de retorno. A classe sabe a convenção de chamada , o tipo de retorno, e o número e tipos de argumentos que essa função irá receber.

A função de fábrica `CFUNCTYPE()` cria tipos para funções de retorno usando a convenção de chamada `cdecl`. No Windows, a função de fábrica `WINFUNCTYPE()` cria tipos para funções de retorno usando a convenção de chamada `stdcall`.

Ambas estas funções de fábrica são chamadas com o tipo de resultado como primeiro argumento, e os tipos de argumento esperados da função de retorno como os argumentos restantes.

I will present an example here which uses the standard C library’s `qsort()` function, that is used to sort items with the help of a callback function. `qsort()` will be used to sort an array of integers:

    >>> IntArray5 = c_int * 5
    >>> ia = IntArray5(5, 1, 7, 33, 99)
    >>> qsort = libc.qsort
    >>> qsort.restype = None
    >>>

`qsort()` must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer otherwise.

Então, nossa função de retorno (callback function) recebe ponteiros para inteiros, e deve retornar um inteiro. Primeiro criamos o `type` para a função de retorno:

    >>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
    >>>

Para começar, aqui está uma função de retorno (callback) simples que mostra os valores que lhe são passados:

    >>> def py_cmp_func(a, b):
    ...     print("py_cmp_func", a[0], b[0])
    ...     return 0
    ...
    >>> cmp_func = CMPFUNC(py_cmp_func)
    >>>

O resultado:

    >>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
    py_cmp_func 5 1
    py_cmp_func 33 99
    py_cmp_func 7 33
    py_cmp_func 5 7
    py_cmp_func 1 7
    >>>

Agora podemos realmente comparar os dois itens e retornar um resultado útil:

    >>> def py_cmp_func(a, b):
    ...     print("py_cmp_func", a[0], b[0])
    ...     return a[0] - b[0]
    ...
    >>>
    >>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
    py_cmp_func 5 1
    py_cmp_func 33 99
    py_cmp_func 7 33
    py_cmp_func 1 7
    py_cmp_func 5 7
    >>>

Como podemos verificar facilmente, nosso vetor está ordenado agora:

    >>> for i in ia: print(i, end=" ")
    ...
    1 5 7 33 99
    >>>

As fábricas de funções podem ser usadas como fábricas de decoradores, então também podemos escrever:

    >>> @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
    ... def py_cmp_func(a, b):
    ...     print("py_cmp_func", a[0], b[0])
    ...     return a[0] - b[0]
    ...
    >>> qsort(ia, len(ia), sizeof(c_int), py_cmp_func)
    py_cmp_func 5 1
    py_cmp_func 33 99
    py_cmp_func 7 33
    py_cmp_func 1 7
    py_cmp_func 5 7
    >>>

Nota

Certifique-se de manter referências aos objetos `CFUNCTYPE()` enquanto eles forem usados a partir do código C. O `ctypes` não faz isso, e se você não o fizer, eles podem ser coletados pelo coletor de lixo, travando seu programa quando uma função de retorno é chamada.

Além disso, note que se a função de retorno for chamada em uma thread criada fora do controle do Python (por exemplo, pelo código externo que chama a função de retorno), o ctypes cria uma nova thread Python “dummy” (fictícia) em cada invocação. Este comportamento é correto para a maioria dos propósitos, mas significa que os valores armazenados com `threading.local` não sobreviverão entre diferentes funções de retorno, mesmo quando essas chamadas são feitas a partir da mesma thread C.

### Acessando valores exportados de dlls

Some shared libraries not only export functions, they also export variables. An example in the Python library itself is the `Py_OptimizeFlag`, an integer set to 0, 1, or 2, depending on the `-O` or `-OO` flag given on startup.

`ctypes` can access values like this with the `in_dll()` class methods of the type. pythonapi is a predefined symbol giving access to the Python C api:

    >>> opt_flag = c_int.in_dll(pythonapi, "Py_OptimizeFlag")
    >>> print(opt_flag)
    c_long(0)
    >>>

If the interpreter would have been started with `-O`, the sample would have printed `c_long(1)`, or `c_long(2)` if `-OO` would have been specified.

Um exemplo estendido que também demonstra o uso de ponteiros acessa o ponteiro `PyImport_FrozenModules` exportado pelo Python.

Citando a documentação para esse valor:

> Este ponteiro é inicializado para apontar para um vetor de registros de `_frozen`, terminado por um cujos membros são todos `NULL` ou zero. Quando um módulo congelado é importado, ele é pesquisado nesta tabela. O código de terceiros pode fazer truques com isso para fornecer uma coleção criada dinamicamente de módulos congelados.

Então, manipular este ponteiro pode até ser útil. Para restringir o tamanho do exemplo, mostramos apenas como esta tabela pode ser lida com `ctypes`:

    >>> from ctypes import *
    >>>
    >>> class struct_frozen(Structure):
    ...     _fields_ = [("name", c_char_p),
    ...                 ("code", POINTER(c_ubyte)),
    ...                 ("size", c_int)]
    ...
    >>>

Nós definimos tipo de dado `_frozen`, para que possamos obter o ponteiro para a tabela:

    >>> FrozenTable = POINTER(struct_frozen)
    >>> table = FrozenTable.in_dll(pythonapi, "PyImport_FrozenModules")
    >>>

Como `table` é um `pointer` para o vetor de registros `struct_frozen`, nós podemos iterar sobre ele, mas só temos que nos certificar de que nosso loop termine, porque ponteiros não têm tamanho. Cedo ou tarde, ele provavelmente travaria com uma violação de acesso ou algo assim, então é melhor sair do loop quando atingirmos a entrada `NULL`:

    >>> for item in table:
    ...     if item.name is None:
    ...         break
    ...     print(item.name.decode("ascii"), item.size)
    ...
    _frozen_importlib 31764
    _frozen_importlib_external 41499
    __hello__ 161
    __phello__ -161
    __phello__.spam 161
    >>>

O fato de que o Python padrão tem um módulo congelado e um pacote congelado (indicado pelo membro `size` negativo) não é bem conhecido, é usado apenas para testes. Experimente com `import __hello__` por exemplo.

### Surpresas

Existem algumas pontos limites no `ctypes` onde você pode esperar algo diferente do que realmente ocorre.

Considere o exemplo a seguir:

    >>> from ctypes import *
    >>> class POINT(Structure):
    ...     _fields_ = ("x", c_int), ("y", c_int)
    ...
    >>> class RECT(Structure):
    ...     _fields_ = ("a", POINT), ("b", POINT)
    ...
    >>> p1 = POINT(1, 2)
    >>> p2 = POINT(3, 4)
    >>> rc = RECT(p1, p2)
    >>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
    1 2 3 4
    >>> # now swap the two points
    >>> rc.a, rc.b = rc.b, rc.a
    >>> print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
    3 4 3 4
    >>>

Hm. Nós com certeza esperávamos que a última instrução exibisse `3 4 1 2`. O que aconteceu? Aqui estão os passos da linha `rc.a, rc.b = rc.b, rc.a` acima:

    >>> temp0, temp1 = rc.b, rc.a
    >>> rc.a = temp0
    >>> rc.b = temp1
    >>>

Observe que `temp0` e `temp1` ainda são objetos que utilizam o buffer interno do objeto `rc` acima. Portanto, executar `rc.a = temp0` copia o conteúdo do buffer de `temp0` para o buffer de `rc`. Isso, por sua vez, altera o conteúdo de `temp1`. Assim, a última atribuição `rc.b = temp1` não produz o efeito esperado.

Tenha em mente que obter sub-objetos de estruturas, uniões e vetores não copia o sub-objeto; em vez disso, recupera um objeto de encapsulamento (wrapper) que acessa o buffer subjacente do objeto raiz.

Outro exemplo que pode se comportar de maneira diferente do que alguém poderia esperar é o seguinte:

    >>> s = c_char_p()
    >>> s.value = b"abc def ghi"
    >>> s.value
    b'abc def ghi'
    >>> s.value is s.value
    False
    >>>

Nota

Objetos instanciados a partir de `c_char_p` só podem ter seu valor definido para bytes ou inteiros.

Por que está imprimindo `False`? Instâncias de ctypes são objetos que contêm um bloco de memória, além de alguns descriptors que acessam o conteúdo dessa memória. Armazenar um objeto Python no bloco de memória não armazena o próprio objeto; em vez disso, armazena-se o seu atributo `contents`. Acessar o contents novamente constrói um novo objeto Python a cada vez!

### Tipos de dados de tamanho variável

O `ctypes` fornece algum suporte para vetores e estruturas de tamanhos variável.

A função `resize()` pode ser usada para redimensionar o buffer de memória de um objeto ctypes existente. A função recebe o objeto como primeiro argumento, e o tamanho solicitado em bytes como o segundo argumento. O bloco de memória não pode ser tornado menor do que o bloco de memória natural especificado pelo tipo do objeto, um `ValueError` é levantado se isso for tentado:

    >>> short_array = (c_short * 4)()
    >>> print(sizeof(short_array))
    8
    >>> resize(short_array, 4)
    Traceback (most recent call last):
        ...
    ValueError: minimum size is 8
    >>> resize(short_array, 32)
    >>> sizeof(short_array)
    32
    >>> sizeof(type(short_array))
    8
    >>>

Isso é bom e funcional, mas como alguém acessaria os elementos contidos neste vetor? Já que o tipo ainda sabe apenas sobre 4 elementos, obtemos erros ao acessar outros elementos:

    >>> short_array[:]
    [0, 0, 0, 0]
    >>> short_array[7]
    Traceback (most recent call last):
        ...
    IndexError: invalid index
    >>>

Outra maneira de usar tipos de dados de tamanho variável com o `ctypes` é usar a natureza dinâmica do Python, e (re)definir o tipo de dados depois que o tamanho necessário já for conhecido, caso a caso.

## Referência ctypes

### Encontrando bibliotecas compartilhadas

Ao programar em uma linguagem compilada, bibliotecas compartilhadas são acessadas ao compilar/linkar (compiling/linking) um programa, e quando o programa é executado.

The purpose of the `find_library()` function is to locate a library in a way similar to what the compiler or runtime loader does (on platforms with several versions of a shared library the most recent should be loaded), while the ctypes library loaders act like when a program is run, and call the runtime loader directly.

The `ctypes.util` module provides a function which can help to determine the library to load.

`ctypes.util.``find_library`(name)

  Try to find a library and return a pathname. name is the library name without any prefix like lib, suffix like `.so`, `.dylib` or version number (this is the form used for the posix linker option `-l`). If no library can be found, returns `None`.

The exact functionality is system dependent.

On Linux, `find_library()` tries to run external programs (`/sbin/ldconfig`, `gcc`, `objdump` and `ld`) to find the library file. It returns the filename of the library file.

Alterado na versão 3.6: On Linux, the value of the environment variable `LD_LIBRARY_PATH` is used when searching for libraries, if a library cannot be found by any other means.

Veja alguns exemplos:

    >>> from ctypes.util import find_library
    >>> find_library("m")
    'libm.so.6'
    >>> find_library("c")
    'libc.so.6'
    >>> find_library("bz2")
    'libbz2.so.1.0'
    >>>

On macOS, `find_library()` tries several predefined naming schemes and paths to locate the library, and returns a full pathname if successful:

    >>> from ctypes.util import find_library
    >>> find_library("c")
    '/usr/lib/libc.dylib'
    >>> find_library("m")
    '/usr/lib/libm.dylib'
    >>> find_library("bz2")
    '/usr/lib/libbz2.dylib'
    >>> find_library("AGL")
    '/System/Library/Frameworks/AGL.framework/AGL'
    >>>

On Windows, `find_library()` searches along the system search path, and returns the full pathname, but since there is no predefined naming scheme a call like `find_library("c")` will fail and return `None`.

If wrapping a shared library with `ctypes`, it may be better to determine the shared library name at development time, and hardcode that into the wrapper module instead of using `find_library()` to locate the library at runtime.

### Loading shared libraries

There are several ways to load shared libraries into the Python process. One way is to instantiate one of the following classes:

class `ctypes.``CDLL`(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=None)

  Instances of this class represent loaded shared libraries. Functions in these libraries use the standard C calling convention, and are assumed to return `int`.

  On Windows creating a `CDLL` instance may fail even if the DLL name exists. When a dependent DLL of the loaded DLL is not found, a `OSError` error is raised with the message “[WinError 126] The specified module could not be found”. This error message does not contain the name of the missing DLL because the Windows API does not return this information making this error hard to diagnose. To resolve this error and determine which DLL is not found, you need to find the list of dependent DLLs and determine which one is not found using Windows debugging and tracing tools.

Ver também

Microsoft DUMPBIN tool – A tool to find DLL dependents.

class `ctypes.``OleDLL`(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=None)

  Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the `stdcall` calling convention, and are assumed to return the windows specific `HRESULT` code. `HRESULT` values contain information specifying whether the function call failed or succeeded, together with additional error code. If the return value signals a failure, an `OSError` is automatically raised.

  Alterado na versão 3.3: `WindowsError` used to be raised.

class `ctypes.``WinDLL`(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=None)

  Windows only: Instances of this class represent loaded shared libraries, functions in these libraries use the `stdcall` calling convention, and are assumed to return `int` by default.

The Python global interpreter lock is released before calling any function exported by these libraries, and reacquired afterwards.

class `ctypes.``PyDLL`(name, mode=DEFAULT_MODE, handle=None)

  Instâncias desta classe se comportam como instâncias `CDLL`, exceto que o GIL do Python não é liberado durante a chamada da função e, após a execução da função, o sinalizador de erro do Python é verificado. Se o sinalizador de erro estiver definido, uma exceção Python é levantada.

  Portanto, isso só é útil para chamar funções da api C do Python diretamente.

All these classes can be instantiated by calling them with at least one argument, the pathname of the shared library. If you have an existing handle to an already loaded shared library, it can be passed as the `handle` named parameter, otherwise the underlying platforms `dlopen` or `LoadLibrary` function is used to load the library into the process, and to get a handle to it.

O parâmetro mode pode ser utilizado para especificar como a biblioteca é carregada. Para detalhes, consulte a página de manual dlopen(3). No Windows, mode é ignorado. Em sistemas posix, RTLD_NOW é sempre adicionado e, não é configurável.

The use_errno parameter, when set to true, enables a ctypes mechanism that allows accessing the system `errno` error number in a safe way. `ctypes` maintains a thread-local copy of the systems `errno` variable; if you call foreign functions created with `use_errno=True` then the `errno` value before the function call is swapped with the ctypes private copy, the same happens immediately after the function call.

The function `ctypes.get_errno()` returns the value of the ctypes private copy, and the function `ctypes.set_errno()` changes the ctypes private copy to a new value and returns the former value.

The use_last_error parameter, when set to true, enables the same mechanism for the Windows error code which is managed by the `GetLastError()` and `SetLastError()` Windows API functions; `ctypes.get_last_error()` and `ctypes.set_last_error()` are used to request and change the ctypes private copy of the windows error code.

The winmode parameter is used on Windows to specify how the library is loaded (since mode is ignored). It takes any value that is valid for the Win32 API `LoadLibraryEx` flags parameter. When omitted, the default is to use the flags that result in the most secure DLL load to avoiding issues such as DLL hijacking. Passing the full path to the DLL is the safest way to ensure the correct library and dependencies are loaded.

Alterado na versão 3.8: Added winmode parameter.

`ctypes.``RTLD_GLOBAL`

  Flag to use as mode parameter. On platforms where this flag is not available, it is defined as the integer zero.

`ctypes.``RTLD_LOCAL`

  Flag to use as mode parameter. On platforms where this is not available, it is the same as RTLD_GLOBAL.

`ctypes.``DEFAULT_MODE`

  The default mode which is used to load shared libraries. On OSX 10.3, this is RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.

Instances of these classes have no public methods. Functions exported by the shared library can be accessed as attributes or by index. Please note that accessing the function through an attribute caches the result and therefore accessing it repeatedly returns the same object each time. On the other hand, accessing it through an index returns a new object each time:

    >>> from ctypes import CDLL
    >>> libc = CDLL("libc.so.6")  # On Linux
    >>> libc.time == libc.time
    True
    >>> libc['time'] == libc['time']
    False

The following public attributes are available, their name starts with an underscore to not clash with exported function names:

`PyDLL.``_handle`

  The system handle used to access the library.

`PyDLL.``_name`

  The name of the library passed in the constructor.

Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the `LibraryLoader` class, either by calling the `LoadLibrary()` method, or by retrieving the library as attribute of the loader instance.

class `ctypes.``LibraryLoader`(dlltype)

  Class which loads shared libraries. dlltype should be one of the `CDLL`, `PyDLL`, `WinDLL`, or `OleDLL` types.

  `__getattr__()` has special behavior: It allows loading a shared library by accessing it as attribute of a library loader instance. The result is cached, so repeated attribute accesses return the same library each time.

  `LoadLibrary`(name)

    Load a shared library into the process and return it. This method always returns a new instance of the library.

These prefabricated library loaders are available:

`ctypes.``cdll`

  Creates `CDLL` instances.

`ctypes.``windll`

  Windows only: Creates `WinDLL` instances.

`ctypes.``oledll`

  Windows only: Creates `OleDLL` instances.

`ctypes.``pydll`

  Creates `PyDLL` instances.

For accessing the C Python api directly, a ready-to-use Python shared library object is available:

`ctypes.``pythonapi`

  An instance of `PyDLL` that exposes Python C API functions as attributes. Note that all these functions are assumed to return C `int`, which is of course not always the truth, so you have to assign the correct `restype` attribute to use these functions.

Levanta um evento de auditoria `ctypes.dlopen` com o argumento `name`.

Accessing a function on a loaded library raises an auditing event `ctypes.dlsym` with arguments `library` (the library object) and `name` (the symbol’s name as a string or integer).

In cases when only the library handle is available rather than the object, accessing a function raises an auditing event `ctypes.dlsym/handle` with arguments `handle` (the raw library handle) and `name`.

### Foreign functions

As explained in the previous section, foreign functions can be accessed as attributes of loaded shared libraries. The function objects created in this way by default accept any number of arguments, accept any ctypes data instances as arguments, and return the default result type specified by the library loader. They are instances of a private class:

class `ctypes.``_FuncPtr`

  Base class for C callable foreign functions.

  Instances of foreign functions are also C compatible data types; they represent C function pointers.

  This behavior can be customized by assigning to special attributes of the foreign function object.

  `restype`

    Assign a ctypes type to specify the result type of the foreign function. Use `None` for `void`, a function not returning anything.

    It is possible to assign a callable Python object that is not a ctypes type, in this case the function is assumed to return a C `int`, and the callable will be called with this integer, allowing further processing or error checking. Using this is deprecated, for more flexible post processing or error checking use a ctypes data type as `restype` and assign a callable to the `errcheck` attribute.

  `argtypes`

    Assign a tuple of ctypes types to specify the argument types that the function accepts. Functions using the `stdcall` calling convention can only be called with the same number of arguments as the length of this tuple; functions using the C calling convention accept additional, unspecified arguments as well.

    When a foreign function is called, each actual argument is passed to the `from_param()` class method of the items in the `argtypes` tuple, this method allows adapting the actual argument to an object that the foreign function accepts. For example, a `c_char_p` item in the `argtypes` tuple will convert a string passed as argument into a bytes object using ctypes conversion rules.

    New: It is now possible to put items in argtypes which are not ctypes types, but each item must have a `from_param()` method which returns a value usable as argument (integer, string, ctypes instance). This allows defining adapters that can adapt custom objects as function parameters.

  `errcheck`

    Assign a Python function or another callable to this attribute. The callable will be called with three or more arguments:

    `callable`(result, func, arguments)

      result is what the foreign function returns, as specified by the `restype` attribute.

      func is the foreign function object itself, this allows reusing the same callable object to check or post process the results of several functions.

      arguments is a tuple containing the parameters originally passed to the function call, this allows specializing the behavior on the arguments used.

    The object that this function returns will be returned from the foreign function call, but it can also check the result value and raise an exception if the foreign function call failed.

exception `ctypes.``ArgumentError`

  Esta exceção é levantada quando uma chamada de função externa não consegue converter um dos argumentos passados.

On Windows, when a foreign function call raises a system exception (for example, due to an access violation), it will be captured and replaced with a suitable Python exception. Further, an auditing event `ctypes.seh_exception` with argument `code` will be raised, allowing an audit hook to replace the exception with its own.

Some ways to invoke foreign function calls may raise an auditing event `ctypes.call_function` with arguments `function pointer` and `arguments`.

### Function prototypes

Foreign functions can also be created by instantiating function prototypes. Function prototypes are similar to function prototypes in C; they describe a function (return type, argument types, calling convention) without defining an implementation. The factory functions must be called with the desired result type and the argument types of the function, and can be used as decorator factories, and as such, be applied to functions through the `@wrapper` syntax. See Funções Callbacks for examples.

`ctypes.``CFUNCTYPE`(restype, *argtypes, use_errno=False, use_last_error=False)

  The returned function prototype creates functions that use the standard C calling convention. The function will release the GIL during the call. If use_errno is set to true, the ctypes private copy of the system `errno` variable is exchanged with the real `errno` value before and after the call; use_last_error does the same for the Windows error code.

`ctypes.``WINFUNCTYPE`(restype, *argtypes, use_errno=False, use_last_error=False)

  Windows only: The returned function prototype creates functions that use the `stdcall` calling convention. The function will release the GIL during the call. use_errno and use_last_error have the same meaning as above.

`ctypes.``PYFUNCTYPE`(restype, *argtypes)

  The returned function prototype creates functions that use the Python calling convention. The function will not release the GIL during the call.

Function prototypes created by these factory functions can be instantiated in different ways, depending on the type and number of the parameters in the call:

> `prototype`(address)

>   Returns a foreign function at the specified address which must be an integer.

> `prototype`(callable)

>   Create a C callable function (a callback function) from a Python callable.

> `prototype`(func_spec[, paramflags])

>   Returns a foreign function exported by a shared library. func_spec must be a 2-tuple `(name_or_ordinal, library)`. The first item is the name of the exported function as string, or the ordinal of the exported function as small integer. The second item is the shared library instance.

> `prototype`(vtbl_index, name[, paramflags[, iid]])

>   Returns a foreign function that will call a COM method. vtbl_index is the index into the virtual function table, a small non-negative integer. name is name of the COM method. iid is an optional pointer to the interface identifier which is used in extended error reporting.

>   COM methods use a special calling convention: They require a pointer to the COM interface as first argument, in addition to those parameters that are specified in the `argtypes` tuple.

> The optional paramflags parameter creates foreign function wrappers with much more functionality than the features described above.

> paramflags must be a tuple of the same length as `argtypes`.

> Each item in this tuple contains further information about a parameter, it must be a tuple containing one, two, or three items.

> The first item is an integer containing a combination of direction flags for the parameter:

> > 1

> >   Specifies an input parameter to the function.

> > 2

> >   Output parameter. The foreign function fills in a value.

> > 4

> >   Input parameter which defaults to the integer zero.

> The optional second item is the parameter name as string. If this is specified, the foreign function can be called with named parameters.

> The optional third item is the default value for this parameter.

This example demonstrates how to wrap the Windows `MessageBoxW` function so that it supports default parameters and named arguments. The C declaration from the windows header file is this:

    WINUSERAPI int WINAPI
    MessageBoxW(
        HWND hWnd,
        LPCWSTR lpText,
        LPCWSTR lpCaption,
        UINT uType);

Here is the wrapping with `ctypes`:

    >>> from ctypes import c_int, WINFUNCTYPE, windll
    >>> from ctypes.wintypes import HWND, LPCWSTR, UINT
    >>> prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
    >>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1, "caption", "Hello from ctypes"), (1, "flags", 0)
    >>> MessageBox = prototype(("MessageBoxW", windll.user32), paramflags)

The `MessageBox` foreign function can now be called in these ways:

    >>> MessageBox()
    >>> MessageBox(text="Spam, spam, spam")
    >>> MessageBox(flags=2, text="foo bar")

A second example demonstrates output parameters. The win32 `GetWindowRect` function retrieves the dimensions of a specified window by copying them into `RECT` structure that the caller has to supply. Here is the C declaration:

    WINUSERAPI BOOL WINAPI
    GetWindowRect(
         HWND hWnd,
         LPRECT lpRect);

Here is the wrapping with `ctypes`:

    >>> from ctypes import POINTER, WINFUNCTYPE, windll, WinError
    >>> from ctypes.wintypes import BOOL, HWND, RECT
    >>> prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
    >>> paramflags = (1, "hwnd"), (2, "lprect")
    >>> GetWindowRect = prototype(("GetWindowRect", windll.user32), paramflags)
    >>>

Functions with output parameters will automatically return the output parameter value if there is a single one, or a tuple containing the output parameter values when there are more than one, so the GetWindowRect function now returns a RECT instance, when called.

Output parameters can be combined with the `errcheck` protocol to do further output processing and error checking. The win32 `GetWindowRect` api function returns a `BOOL` to signal success or failure, so this function could do the error checking, and raises an exception when the api call failed:

    >>> def errcheck(result, func, args):
    ...     if not result:
    ...         raise WinError()
    ...     return args
    ...
    >>> GetWindowRect.errcheck = errcheck
    >>>

If the `errcheck` function returns the argument tuple it receives unchanged, `ctypes` continues the normal processing it does on the output parameters. If you want to return a tuple of window coordinates instead of a `RECT` instance, you can retrieve the fields in the function and return them instead, the normal processing will no longer take place:

    >>> def errcheck(result, func, args):
    ...     if not result:
    ...         raise WinError()
    ...     rc = args[1]
    ...     return rc.left, rc.top, rc.bottom, rc.right
    ...
    >>> GetWindowRect.errcheck = errcheck
    >>>

### Funções utilitárias

`ctypes.``addressof`(obj)

  Returns the address of the memory buffer as integer. obj must be an instance of a ctypes type.

  Levanta um evento de auditoria `ctypes.addressof` com o argumento `obj`.

`ctypes.``alignment`(obj_or_type)

  Returns the alignment requirements of a ctypes type. obj_or_type must be a ctypes type or instance.

`ctypes.``byref`(obj[, offset])

  Returns a light-weight pointer to obj, which must be an instance of a ctypes type. offset defaults to zero, and must be an integer that will be added to the internal pointer value.

  `byref(obj, offset)` corresponds to this C code:

      (((char *)&obj) + offset)

  The returned object can only be used as a foreign function call parameter. It behaves similar to `pointer(obj)`, but the construction is a lot faster.

`ctypes.``cast`(obj, type)

  Esta função é semelhante ao operador cast em C. Retorna uma nova instância de type que aponta para o mesmo bloco de memória que obj. type deve ser um tipo de ponteiro e obj deve ser um objeto que pode ser interpretado como um ponteiro.

`ctypes.``create_string_buffer`(init_or_size, size=None)

  This function creates a mutable character buffer. The returned object is a ctypes array of `c_char`.

  init_or_size must be an integer which specifies the size of the array, or a bytes object which will be used to initialize the array items.

  If a bytes object is specified as first argument, the buffer is made one item larger than its length so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the bytes should not be used.

  Levanta um evento de auditoria `ctypes.create_string_buffer` com os argumentos `init`, `size`.

`ctypes.``create_unicode_buffer`(init_or_size, size=None)

  This function creates a mutable unicode character buffer. The returned object is a ctypes array of `c_wchar`.

  init_or_size must be an integer which specifies the size of the array, or a string which will be used to initialize the array items.

  If a string is specified as first argument, the buffer is made one item larger than the length of the string so that the last element in the array is a NUL termination character. An integer can be passed as second argument which allows specifying the size of the array if the length of the string should not be used.

  Levanta um evento de auditoria `ctypes.create_unicode_buffer` com os argumentos `init`, `size`.

`ctypes.``DllCanUnloadNow`()

  Windows only: This function is a hook which allows implementing in-process COM servers with ctypes. It is called from the DllCanUnloadNow function that the _ctypes extension dll exports.

`ctypes.``DllGetClassObject`()

  Windows only: This function is a hook which allows implementing in-process COM servers with ctypes. It is called from the DllGetClassObject function that the `_ctypes` extension dll exports.

`ctypes.util.``find_library`(name)

  Try to find a library and return a pathname. name is the library name without any prefix like `lib`, suffix like `.so`, `.dylib` or version number (this is the form used for the posix linker option `-l`). If no library can be found, returns `None`.

  The exact functionality is system dependent.

`ctypes.util.``find_msvcrt`()

  Windows only: return the filename of the VC runtime library used by Python, and by the extension modules. If the name of the library cannot be determined, `None` is returned.

  If you need to free memory, for example, allocated by an extension module with a call to the `free(void *)`, it is important that you use the function in the same library that allocated the memory.

`ctypes.``FormatError`([code])

  Windows only: Returns a textual description of the error code code. If no error code is specified, the last error code is used by calling the Windows api function GetLastError.

`ctypes.``GetLastError`()

  Windows only: Returns the last error code set by Windows in the calling thread. This function calls the Windows `GetLastError()` function directly, it does not return the ctypes-private copy of the error code.

`ctypes.``get_errno`()

  Returns the current value of the ctypes-private copy of the system `errno` variable in the calling thread.

  Levanta um evento de auditoria `ctypes.get_errno` sem argumentos.

`ctypes.``get_last_error`()

  Windows only: returns the current value of the ctypes-private copy of the system `LastError` variable in the calling thread.

  Levanta um evento de auditoria `ctypes.get_last_error` sem argumentos.

`ctypes.``memmove`(dst, src, count)

  Same as the standard C memmove library function: copies count bytes from src to dst. dst and src must be integers or ctypes instances that can be converted to pointers.

`ctypes.``memset`(dst, c, count)

  Same as the standard C memset library function: fills the memory block at address dst with count bytes of value c. dst must be an integer specifying an address, or a ctypes instance.

`ctypes.``POINTER`(type)

  This factory function creates and returns a new ctypes pointer type. Pointer types are cached and reused internally, so calling this function repeatedly is cheap. type must be a ctypes type.

`ctypes.``pointer`(obj)

  This function creates a new pointer instance, pointing to obj. The returned object is of the type `POINTER(type(obj))`.

  Nota: se você pretende apenas passar um ponteiro para um objeto em uma chamada de função externa, deve usar `byref(obj)`, que é muito mais rápido.

`ctypes.``resize`(obj, size)

  Esta função redimensiona o buffer de memória interno de obj, que deve ser uma instância de um tipo ctypes. Não é possível reduzir o buffer para menos do que o tamanho nativo do tipo do objeto, conforme dado por `sizeof(type(obj))`, mas é possível aumentá-lo.

`ctypes.``set_errno`(value)

  Define o valor atual da cópia privada do ctypes da variável do sistema `errno` na thread de chamada para value e devolve o valor anterior.

  Levanta um evento de auditoria `ctypes.set_errno` com o argumento `errno`.

`ctypes.``set_last_error`(value)

  Windows only: set the current value of the ctypes-private copy of the system `LastError` variable in the calling thread to value and return the previous value.

  Levanta um evento de auditoria `ctypes.set_last_error` com o argumento `error`.

`ctypes.``sizeof`(obj_or_type)

  Retornar o tamanho em bytes de um tipo ctypes ou buffer de memória de instância. Faz o mesmo que o operador C `sizeof`.

`ctypes.``string_at`(address, size=- 1)

  This function returns the C string starting at memory address address as a bytes object. If size is specified, it is used as size, otherwise the string is assumed to be zero-terminated.

  Raises an auditing event `ctypes.string_at` with arguments `address`, `size`.

`ctypes.``WinError`(code=None, descr=None)

  Windows only: this function is probably the worst-named thing in ctypes. It creates an instance of OSError. If code is not specified, `GetLastError` is called to determine the error code. If descr is not specified, `FormatError()` is called to get a textual description of the error.

  Alterado na versão 3.3: An instance of `WindowsError` used to be created.

`ctypes.``wstring_at`(address, size=- 1)

  This function returns the wide character string starting at memory address address as a string. If size is specified, it is used as the number of characters of the string, otherwise the string is assumed to be zero-terminated.

  Raises an auditing event `ctypes.wstring_at` with arguments `address`, `size`.

### Data types

class `ctypes.``_CData`

  This non-public class is the common base class of all ctypes data types. Among other things, all ctypes type instances contain a memory block that hold C compatible data; the address of the memory block is returned by the `addressof()` helper function. Another instance variable is exposed as `_objects`; this contains other Python objects that need to be kept alive in case the memory block contains pointers.

  Common methods of ctypes data types, these are all class methods (to be exact, they are methods of the metaclass):

  `from_buffer`(source[, offset])

    This method returns a ctypes instance that shares the buffer of the source object. The source object must support the writeable buffer interface. The optional offset parameter specifies an offset into the source buffer in bytes; the default is zero. If the source buffer is not large enough a `ValueError` is raised.

    Levanta um evento de auditoria `ctypes.cdata/buffer` com os argumentos `pointer`, `size`, `offset`.

  `from_buffer_copy`(source[, offset])

    This method creates a ctypes instance, copying the buffer from the source object buffer which must be readable. The optional offset parameter specifies an offset into the source buffer in bytes; the default is zero. If the source buffer is not large enough a `ValueError` is raised.

    Levanta um evento de auditoria `ctypes.cdata/buffer` com os argumentos `pointer`, `size`, `offset`.

  `from_address`(address)

    This method returns a ctypes type instance using the memory specified by address which must be an integer.

    Este método, e outros que indiretamente chamam este método, levantam um evento de auditoria `ctypes.cdata` com o argumento `address`.

  `from_param`(obj)

    This method adapts obj to a ctypes type. It is called with the actual object used in a foreign function call when the type is present in the foreign function’s `argtypes` tuple; it must return an object that can be used as a function call parameter.

    All ctypes data types have a default implementation of this classmethod that normally returns obj if that is an instance of the type. Some types accept other objects as well.

  `in_dll`(library, name)

    This method returns a ctypes type instance exported by a shared library. name is the name of the symbol that exports the data, library is the loaded shared library.

  Variáveis de instância comuns dos tipos de dados ctypes:

  `_b_base_`

    Sometimes ctypes data instances do not own the memory block they contain, instead they share part of the memory block of a base object. The `_b_base_` read-only member is the root ctypes object that owns the memory block.

  `_b_needsfree_`

    This read-only variable is true when the ctypes data instance has allocated the memory block itself, false otherwise.

  `_objects`

    This member is either `None` or a dictionary containing Python objects that need to be kept alive so that the memory block contents is kept valid. This object is only exposed for debugging; never modify the contents of this dictionary.

### Tipos de dados fundamentais

class `ctypes.``_SimpleCData`

  This non-public class is the base class of all fundamental ctypes data types. It is mentioned here because it contains the common attributes of the fundamental ctypes data types. `_SimpleCData` is a subclass of `_CData`, so it inherits their methods and attributes. ctypes data types that are not and do not contain pointers can now be pickled.

  Instances have a single attribute:

  `value`

    This attribute contains the actual value of the instance. For integer and pointer types, it is an integer, for character types, it is a single character bytes object or string, for character pointer types it is a Python bytes object or string.

    When the `value` attribute is retrieved from a ctypes instance, usually a new object is returned each time. `ctypes` does not implement original object return, always a new object is constructed. The same is true for all other ctypes object instances.

Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a `restype` of `c_char_p`, you will always receive a Python bytes object, not a `c_char_p` instance.

Subclasses of fundamental data types do not inherit this behavior. So, if a foreign functions `restype` is a subclass of `c_void_p`, you will receive an instance of this subclass from the function call. Of course, you can get the value of the pointer by accessing the `value` attribute.

These are the fundamental ctypes data types:

class `ctypes.``c_byte`

  Representa o tipo de dados C `signed char` e interpreta o valor como um inteiro pequeno. O construtor aceita um inicializador inteiro opcional; não há verificação de overflow.

class `ctypes.``c_char`

  Representa o tipo de dados C `char` e interpreta o valor como um único caractere. O construtor aceita um inicializador de string opcional; o comprimento da string deve ser exatamente um caractere.

class `ctypes.``c_char_p`

  Representa o tipo de dados C `char*` quando aponta para uma string terminada em zero. Para um ponteiro de caractere geral que também pode apontar para dados binários, deve-se usar `POINTER(c_char)`. O construtor aceita um endereço inteiro ou um objeto bytes.

class `ctypes.``c_double`

  Representa o tipo de dados C `double`. O construtor aceita um inicializador opcional de tipo float.

class `ctypes.``c_longdouble`

  Representa o tipo de dados C `long double`. O construtor aceita um inicializador opcional de tipo float. Em plataformas onde `sizeof(long double) == sizeof(double)`, é um alias para `c_double`.

class `ctypes.``c_float`

  Representa o tipo de dados C `float`. O construtor aceita um inicializador opcional de tipo float.

class `ctypes.``c_int`

  Represents the C `signed int` datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where `sizeof(int) == sizeof(long)` it is an alias to `c_long`.

class `ctypes.``c_int8`

  Represents the C 8-bit `signed int` datatype. Usually an alias for `c_byte`.

class `ctypes.``c_int16`

  Represents the C 16-bit `signed int` datatype. Usually an alias for `c_short`.

class `ctypes.``c_int32`

  Represents the C 32-bit `signed int` datatype. Usually an alias for `c_int`.

class `ctypes.``c_int64`

  Represents the C 64-bit `signed int` datatype. Usually an alias for `c_longlong`.

class `ctypes.``c_long`

  Represents the C `signed long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_longlong`

  Represents the C `signed long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_short`

  Represents the C `signed short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_size_t`

  Represents the C `size_t` datatype.

class `ctypes.``c_ssize_t`

  Represents the C `ssize_t` datatype.

  Novo na versão 3.2.

class `ctypes.``c_ubyte`

  Represents the C `unsigned char` datatype, it interprets the value as small integer. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_uint`

  Represents the C `unsigned int` datatype. The constructor accepts an optional integer initializer; no overflow checking is done. On platforms where `sizeof(int) == sizeof(long)` it is an alias for `c_ulong`.

class `ctypes.``c_uint8`

  Represents the C 8-bit `unsigned int` datatype. Usually an alias for `c_ubyte`.

class `ctypes.``c_uint16`

  Represents the C 16-bit `unsigned int` datatype. Usually an alias for `c_ushort`.

class `ctypes.``c_uint32`

  Represents the C 32-bit `unsigned int` datatype. Usually an alias for `c_uint`.

class `ctypes.``c_uint64`

  Represents the C 64-bit `unsigned int` datatype. Usually an alias for `c_ulonglong`.

class `ctypes.``c_ulong`

  Represents the C `unsigned long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_ulonglong`

  Represents the C `unsigned long long` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_ushort`

  Represents the C `unsigned short` datatype. The constructor accepts an optional integer initializer; no overflow checking is done.

class `ctypes.``c_void_p`

  Represents the C `void*` type. The value is represented as integer. The constructor accepts an optional integer initializer.

class `ctypes.``c_wchar`

  Represents the C `wchar_t` datatype, and interprets the value as a single character unicode string. The constructor accepts an optional string initializer, the length of the string must be exactly one character.

class `ctypes.``c_wchar_p`

  Represents the C `wchar_t*` datatype, which must be a pointer to a zero-terminated wide character string. The constructor accepts an integer address, or a string.

class `ctypes.``c_bool`

  Represent the C `bool` datatype (more accurately, `_Bool` from C99). Its value can be `True` or `False`, and the constructor accepts any object that has a truth value.

class `ctypes.``HRESULT`

  Windows only: Represents a `HRESULT` value, which contains success or error information for a function or method call.

class `ctypes.``py_object`

  Representa o tipo de dados C `PyObject*`. Chamar isto sem um argumento cria um ponteiro `NULL` `PyObject*`.

The `ctypes.wintypes` module provides quite some other Windows specific data types, for example `HWND`, `WPARAM`, or `DWORD`. Some useful structures like `MSG` or `RECT` are also defined.

### Tipos de dados estruturados

class `ctypes.``Union`(*args, **kw)

  Classe base abstrata para uniões em ordem de bytes nativa.

class `ctypes.``BigEndianStructure`(*args, **kw)

  Classe base abstrata para estruturas em ordem de bytes big endian.

class `ctypes.``LittleEndianStructure`(*args, **kw)

  Classe base abstrata para estruturas em ordem de bytes little endian.

Structures with non-native byte order cannot contain pointer type fields, or any other data types containing pointer type fields.

class `ctypes.``Structure`(*args, **kw)

  Classe base abstrata para estruturas em ordem de bytes nativa.

  Os tipos concretos de estruturas e uniões devem ser criados por subclasse de um destes tipos e, pelo menos, definir uma variável de classe `_fields_`. O `ctypes` criará descritores que permitem ler e escrever os campos por meio de acessos diretos aos respectivos atributos. Estes são eles

  `_fields_`

    Uma sequência que define os campos da estrutura. Os itens devem ser tuplas com 2 ou 3 elementos. O primeiro item é o nome do campo; o segundo item especifica o tipo do campo, que pode ser qualquer tipo de dados ctypes.

    Para campos de tipo inteiro como `c_int`, pode ser fornecido um terceiro item opcional. Deve ser um inteiro positivo pequeno que define a largura em bits do campo.

    Os nomes dos campos devem ser únicos em uma estrutura ou união. Isto não é verificado, apenas um campo pode ser acessado quando os nomes são repetidos.

    É possível definir a variável de classe `_fields_` depois da instrução de classe que define a subclasse de Structure, o que permite criar tipos de dados que referenciam diretamente ou indiretamente a si mesmos:

        class List(Structure):
            pass
        List._fields_ = [("pnext", POINTER(List)),
                         ...
                        ]

    The `_fields_` class variable must, however, be defined before the type is first used (an instance is created, `sizeof()` is called on it, and so on). Later assignments to the `_fields_` class variable will raise an AttributeError.

    It is possible to define sub-subclasses of structure types, they inherit the fields of the base class plus the `_fields_` defined in the sub-subclass, if any.

  `_pack_`

    An optional small integer that allows overriding the alignment of structure fields in the instance. `_pack_` must already be defined when `_fields_` is assigned, otherwise it will have no effect.

  `_anonymous_`

    An optional sequence that lists the names of unnamed (anonymous) fields. `_anonymous_` must be already defined when `_fields_` is assigned, otherwise it will have no effect.

    The fields listed in this variable must be structure or union type fields. `ctypes` will create descriptors in the structure type that allows accessing the nested fields directly, without the need to create the structure or union field.

    Here is an example type (Windows):

        class _U(Union):
            _fields_ = [("lptdesc", POINTER(TYPEDESC)),
                        ("lpadesc", POINTER(ARRAYDESC)),
                        ("hreftype", HREFTYPE)]

        class TYPEDESC(Structure):
            _anonymous_ = ("u",)
            _fields_ = [("u", _U),
                        ("vt", VARTYPE)]

    The `TYPEDESC` structure describes a COM data type, the `vt` field specifies which one of the union fields is valid. Since the `u` field is defined as anonymous field, it is now possible to access the members directly off the TYPEDESC instance. `td.lptdesc` and `td.u.lptdesc` are equivalent, but the former is faster since it does not need to create a temporary union instance:

        td = TYPEDESC()
        td.vt = VT_PTR
        td.lptdesc = POINTER(some_type)
        td.u.lptdesc = POINTER(some_type)

  It is possible to define sub-subclasses of structures, they inherit the fields of the base class. If the subclass definition has a separate `_fields_` variable, the fields specified in this are appended to the fields of the base class.

  Structure and union constructors accept both positional and keyword arguments. Positional arguments are used to initialize member fields in the same order as they are appear in `_fields_`. Keyword arguments in the constructor are interpreted as attribute assignments, so they will initialize `_fields_` with the same name, or create new attributes for names not present in `_fields_`.

### Arrays and pointers

class `ctypes.``Array`(*args)

  Abstract base class for arrays.

  The recommended way to create concrete array types is by multiplying any `ctypes` data type with a non-negative integer. Alternatively, you can subclass this type and define `_length_` and `_type_` class variables. Array elements can be read and written using standard subscript and slice accesses; for slice reads, the resulting object is not itself an `Array`.

  `_length_`

    A positive integer specifying the number of elements in the array. Out-of-range subscripts result in an `IndexError`. Will be returned by `len()`.

  `_type_`

    Specifies the type of each element in the array.

  Array subclass constructors accept positional arguments, used to initialize the elements in order.

class `ctypes.``_Pointer`

  Private, abstract base class for pointers.

  Concrete pointer types are created by calling `POINTER()` with the type that will be pointed to; this is done automatically by `pointer()`.

  If a pointer points to an array, its elements can be read and written using standard subscript and slice accesses. Pointer objects have no size, so `len()` will raise `TypeError`. Negative subscripts will read from the memory before the pointer (as in C), and out-of-range subscripts will probably crash with an access violation (if you’re lucky).

  `_type_`

    Specifies the type pointed to.

  `contents`

    Returns the object to which to pointer points. Assigning to this attribute changes the pointer to point to the assigned object.