# Suporte a Coleta Cíclica de Lixo

O suporte do Python para detectar e coletar o lixo, que envolve referencias circulares, requer suporte dos tipos de objetos que são “contêiners” para outros objetos que também podem ser contêiners. Tipos que não armazenam referências a outros tipos de objetos, ou que apenas armazenam referências a tipos atômicos (como números ou strings), não precisam fornecer nenhum suporte explicito para coleta de lixo.

To create a container type, the `tp_flags` field of the type object must include the `Py_TPFLAGS_HAVE_GC` and provide an implementation of the `tp_traverse` handler. If instances of the type are mutable, a `tp_clear` implementation must also be provided.

`Py_TPFLAGS_HAVE_GC`

  Objetos com esse tipo de sinalizador definido devem estar em conformidade com regras documentadas aqui. Por conveniência esses objetos serão referenciados como objetos de contêiner.

Construtores para tipos de contêiner devem obedecer a duas regras:

1. The memory for the object must be allocated using `PyObject_GC_New` or `PyObject_GC_NewVar`.

2. Uma vez que todos os campos que podem conter referências a outros containers foram inicializados, deve-se chamar `PyObject_GC_Track()`.

Da mesma forma, o desalocador para o objeto deve estar em conformidade com regras semelhantes:

1. Antes que os campos que fazer referência a outros containers sejam invalidados, `PyObject_GC_UnTrack()` deve ser chamado.

2. A memória destinada ao objeto deve ser desalocada usando `PyObject_GC_Del()`.

   Aviso

   If a type adds the Py_TPFLAGS_HAVE_GC, then it must implement at least a `tp_traverse` handler or explicitly use one from its subclass or subclasses.

   When calling `PyType_Ready()` or some of the APIs that indirectly call it like `PyType_FromSpecWithBases()` or `PyType_FromSpec()` the interpreter will automatically populate the `tp_flags`, `tp_traverse` and `tp_clear` fields if the type inherits from a class that implements the garbage collector protocol and the child class does not include the `Py_TPFLAGS_HAVE_GC` flag.

PyObject_GC_New(TYPE, typeobj)

  Analogous to `PyObject_New` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

PyObject_GC_NewVar(TYPE, typeobj, size)

  Analogous to `PyObject_NewVar` but for container objects with the `Py_TPFLAGS_HAVE_GC` flag set.

PyObject_GC_Resize(TYPE, op, newsize)

  Resize an object allocated by `PyObject_NewVar`. Returns the resized object of type `TYPE*` (refers to any C type) or `NULL` on failure.

  op must be of type PyVarObject* and must not be tracked by the collector yet. newsize must be of type `Py_ssize_t`.

void PyObject_GC_Track(PyObject *op)
  Parte da ABI Estável.

  Adds the object op to the set of container objects tracked by the collector. The collector can run at unexpected times so objects must be valid while being tracked. This should be called once all the fields followed by the `tp_traverse` handler become valid, usually near the end of the constructor.

int PyObject_IS_GC(PyObject *obj)

  Returns non-zero if the object implements the garbage collector protocol, otherwise returns 0.

  The object cannot be tracked by the garbage collector if this function returns 0.

int PyObject_GC_IsTracked(PyObject *op)
  Parte da ABI Estável desde a versão 3.9.

  Returns 1 if the object type of op implements the GC protocol and op is being currently tracked by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_tracked()`.

  Novo na versão 3.9.

int PyObject_GC_IsFinalized(PyObject *op)
  Parte da ABI Estável desde a versão 3.9.

  Returns 1 if the object type of op implements the GC protocol and op has been already finalized by the garbage collector and 0 otherwise.

  This is analogous to the Python function `gc.is_finalized()`.

  Novo na versão 3.9.

void PyObject_GC_Del(void *op)
  Parte da ABI Estável.

  Releases memory allocated to an object using `PyObject_GC_New` or `PyObject_GC_NewVar`.

void PyObject_GC_UnTrack(void *op)
  Parte da ABI Estável.

  Remove the object op from the set of container objects tracked by the collector. Note that `PyObject_GC_Track()` can be called again on this object to add it back to the set of tracked objects. The deallocator (`tp_dealloc` handler) should call this for the object before any of the fields used by the `tp_traverse` handler become invalid.

Alterado na versão 3.8: The `_PyObject_GC_TRACK()` and `_PyObject_GC_UNTRACK()` macros have been removed from the public C API.

The `tp_traverse` handler accepts a function parameter of this type:

typedef int (*visitproc)(PyObject *object, void *arg)
  Parte da ABI Estável.

  Type of the visitor function passed to the `tp_traverse` handler. The function should be called with an object to traverse as object and the third parameter to the `tp_traverse` handler as arg. The Python core uses several visitor functions to implement cyclic garbage detection; it’s not expected that users will need to write their own visitor functions.

The `tp_traverse` handler must have the following type:

typedef int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
  Parte da ABI Estável.

  Traversal function for a container object. Implementations must call the visit function for each object directly contained by self, with the parameters to visit being the contained object and the arg value passed to the handler. The visit function must not be called with a `NULL` object argument. If visit returns a non-zero value that value should be returned immediately.

To simplify writing `tp_traverse` handlers, a `Py_VISIT()` macro is provided. In order to use this macro, the `tp_traverse` implementation must name its arguments exactly visit and arg:

void Py_VISIT(PyObject *o)

  If o is not `NULL`, call the visit callback, with arguments o and arg. If visit returns a non-zero value, then return it. Using this macro, `tp_traverse` handlers look like:

      static int
      my_traverse(Noddy *self, visitproc visit, void *arg)
      {
          Py_VISIT(self->foo);
          Py_VISIT(self->bar);
          return 0;
      }

The `tp_clear` handler must be of the `inquiry` type, or `NULL` if the object is immutable.

typedef int (*inquiry)(PyObject *self)
  Parte da ABI Estável.

  Drop references that may have created reference cycles. Immutable objects do not have to define this method since they can never directly create reference cycles. Note that the object must still be valid after calling this method (don’t just call `Py_DECREF()` on a reference). The collector will call this method if it detects that this object is involved in a reference cycle.

## Controlando o estado do coletor de lixo

The C-API provides the following functions for controlling garbage collection runs.

Py_ssize_t PyGC_Collect(void)
  Parte da ABI Estável.

  Perform a full garbage collection, if the garbage collector is enabled. (Note that `gc.collect()` runs it unconditionally.)

  Returns the number of collected + unreachable objects which cannot be collected. If the garbage collector is disabled or already collecting, returns `0` immediately. Errors during garbage collection are passed to `sys.unraisablehook`. This function does not raise exceptions.

int PyGC_Enable(void)
  Parte da ABI Estável desde a versão 3.10.

  Enable the garbage collector: similar to `gc.enable()`. Returns the previous state, 0 for disabled and 1 for enabled.

  Novo na versão 3.10.

int PyGC_Disable(void)
  Parte da ABI Estável desde a versão 3.10.

  Disable the garbage collector: similar to `gc.disable()`. Returns the previous state, 0 for disabled and 1 for enabled.

  Novo na versão 3.10.

int PyGC_IsEnabled(void)
  Parte da ABI Estável desde a versão 3.10.

  Query the state of the garbage collector: similar to `gc.isenabled()`. Returns the current state, 0 for disabled and 1 for enabled.

  Novo na versão 3.10.