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vstinner merged 1 commit into from
Apr 29, 2021
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bpo-43774: Enhance debug build documentation #25712

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32 changes: 5 additions & 27 deletions Doc/c-api/intro.rst
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Expand Up @@ -749,34 +749,12 @@ It is also implied by the presence of the
not-Python-specific :c:macro:`_DEBUG` macro. When :c:macro:`Py_DEBUG` is enabled
in the Unix build, compiler optimization is disabled.

In addition to the reference count debugging described below, the following
extra checks are performed:
In addition to the reference count debugging described below, extra checks are
performed, see :ref:`Python Debug Build <debug-build>`.

* Extra checks are added to the object allocator.

* Extra checks are added to the parser and compiler.

* Downcasts from wide types to narrow types are checked for loss of information.

* A number of assertions are added to the dictionary and set implementations.
In addition, the set object acquires a :meth:`test_c_api` method.

* Sanity checks of the input arguments are added to frame creation.

* The storage for ints is initialized with a known invalid pattern to catch
reference to uninitialized digits.

* Low-level tracing and extra exception checking are added to the runtime
virtual machine.

* Extra checks are added to the memory arena implementation.

* Extra debugging is added to the thread module.

There may be additional checks not mentioned here.

Defining :c:macro:`Py_TRACE_REFS` enables reference tracing. When defined, a
circular doubly linked list of active objects is maintained by adding two extra
Defining :c:macro:`Py_TRACE_REFS` enables reference tracing
(see the :option:`configure --with-trace-refs option <--with-trace-refs>`).
When defined, a circular doubly linked list of active objects is maintained by adding two extra
fields to every :c:type:`PyObject`. Total allocations are tracked as well. Upon
exit, all existing references are printed. (In interactive mode this happens
after every statement run by the interpreter.)
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43 changes: 22 additions & 21 deletions Doc/c-api/typeobj.rst
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Expand Up @@ -482,27 +482,6 @@ metatype) initializes :c:member:`~PyTypeObject.tp_itemsize`, which means that it
type objects) *must* have the :attr:`ob_size` field.


.. c:member:: PyObject* PyObject._ob_next
PyObject* PyObject._ob_prev

These fields are only present when the macro ``Py_TRACE_REFS`` is defined.

Their initialization to ``NULL`` is taken care of by the
``PyObject_HEAD_INIT`` macro. For :ref:`statically allocated objects
<static-types>`, these fields always remain ``NULL``. For :ref:`dynamically
allocated objects <heap-types>`, these two fields are used to link the
object into a doubly-linked list of *all* live objects on the heap.

This could be used for various debugging purposes; currently the only uses
are the :func:`sys.getobjects` function and to print the objects that are
still alive at the end of a run when the environment variable
:envvar:`PYTHONDUMPREFS` is set.

**Inheritance:**

These fields are not inherited by subtypes.


.. c:member:: Py_ssize_t PyObject.ob_refcnt

This is the type object's reference count, initialized to ``1`` by the
Expand Down Expand Up @@ -540,6 +519,28 @@ type objects) *must* have the :attr:`ob_size` field.
This field is inherited by subtypes.


.. c:member:: PyObject* PyObject._ob_next
PyObject* PyObject._ob_prev

These fields are only present when the macro ``Py_TRACE_REFS`` is defined
(see the :option:`configure --with-trace-refs option <--with-trace-refs>`).

Their initialization to ``NULL`` is taken care of by the
``PyObject_HEAD_INIT`` macro. For :ref:`statically allocated objects
<static-types>`, these fields always remain ``NULL``. For :ref:`dynamically
allocated objects <heap-types>`, these two fields are used to link the
object into a doubly-linked list of *all* live objects on the heap.

This could be used for various debugging purposes; currently the only uses
are the :func:`sys.getobjects` function and to print the objects that are
still alive at the end of a run when the environment variable
:envvar:`PYTHONDUMPREFS` is set.

**Inheritance:**

These fields are not inherited by subtypes.


PyVarObject Slots
-----------------

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43 changes: 24 additions & 19 deletions Doc/using/configure.rst
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Expand Up @@ -208,43 +208,48 @@ recommended for best performance.

.. _debug-build:

Debug build
-----------
Python Debug Build
------------------

A debug build is Python built with the :option:`--with-pydebug` configure
option.

Effects of a debug build:

* Define ``Py_DEBUG`` and ``Py_REF_DEBUG`` macros.
* Display all warnings by default: the list of default warning filters is empty
in the :mod:`warnings` module.
* Add ``d`` to :data:`sys.abiflags`.
* Add :func:`sys.gettotalrefcount` function.
* Add :option:`-X showrefcount <-X>` command line option.
* Add :envvar:`PYTHONTHREADDEBUG` environment variable.
* Add support for the ``__ltrace__`` variable: enable low-level tracing in the
bytecode evaluation loop if the variable is defined.
* The list of default warning filters is empty in the :mod:`warnings` module.
* Install debug hooks on memory allocators to detect buffer overflow and other
memory errors: see :c:func:`PyMem_SetupDebugHooks`.
* Build Python with assertions (don't set ``NDEBUG`` macro):
``assert(...);`` and ``_PyObject_ASSERT(...);``.
See also the :option:`--with-assertions` configure option.
* Unicode and int objects are created with their memory filled with a pattern
to help detecting uninitialized bytes.
* Many functions ensure that are not called with an exception raised, since
they can clear or replace the current exception.
* The garbage collector (:func:`gc.collect` function) runs some basic checks on
objects consistency.
* More generally, add runtime checks, code surroundeded by ``#ifdef Py_DEBUG``
and ``#endif``.
* Install :ref:`debug hooks on memory allocators <default-memory-allocators>`
to detect buffer overflow and other memory errors.
* Define ``Py_DEBUG`` and ``Py_REF_DEBUG`` macros.
* Add runtime checks: code surroundeded by ``#ifdef Py_DEBUG`` and ``#endif``.
Enable ``assert(...)`` and ``_PyObject_ASSERT(...)`` assertions: don't set
the ``NDEBUG`` macro (see also the :option:`--with-assertions` configure
option). Main runtime checks:

* Add sanity checks on the function arguments.
* Unicode and int objects are created with their memory filled with a pattern
to detect usage of uninitialized objects.
* Ensure that functions which can clear or replace the current exception are
not called with an exception raised.
* The garbage collector (:func:`gc.collect` function) runs some basic checks
on objects consistency.
* The :c:macro:`Py_SAFE_DOWNCAST()` macro checks for integer underflow and
overflow when downcasting from wide types to narrow types.

See also the :ref:`Python Development Mode <devmode>` and the
:option:`--with-trace-refs` configure option.

.. versionchanged:: 3.8
Release builds and debug builds are now ABI compatible: defining the
``Py_DEBUG`` macro no longer implies the ``Py_TRACE_REFS`` macro, which
introduces the only ABI incompatibility.
``Py_DEBUG`` macro no longer implies the ``Py_TRACE_REFS`` macro (see the
:option:`--with-trace-refs` option), which introduces the only ABI
incompatibility.


Debug options
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4 changes: 2 additions & 2 deletions Doc/whatsnew/3.8.rst
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Expand Up @@ -215,8 +215,8 @@ Release builds and :ref:`debug builds <debug-build>` are now ABI compatible: def
``Py_DEBUG`` macro no longer implies the ``Py_TRACE_REFS`` macro, which
introduces the only ABI incompatibility. The ``Py_TRACE_REFS`` macro, which
adds the :func:`sys.getobjects` function and the :envvar:`PYTHONDUMPREFS`
environment variable, can be set using the new ``./configure --with-trace-refs``
build option.
environment variable, can be set using the new :option:`./configure
--with-trace-refs <--with-trace-refs>` build option.
(Contributed by Victor Stinner in :issue:`36465`.)

On Unix, C extensions are no longer linked to libpython except on Android
Expand Down