cpython/Misc/SpecialBuilds.txt

This file describes some special Python build types enabled via compile-time
preprocessor directives.

IMPORTANT: if you want to build a debug-enabled Python, it is recommended that
you use ``./configure --with-pydebug``, rather than the options listed here.

However, if you wish to define some of these options individually, it is best
to define them in the EXTRA_CFLAGS make variable;
``make EXTRA_CFLAGS="-DPy_REF_DEBUG"``.


Py_REF_DEBUG
------------

Turn on aggregate reference counting.  This arranges that extern _Py_RefTotal
hold a count of all references, the sum of ob_refcnt across all objects.
Passing ``-X showrefcount`` on the command line causes the interactive
interpreter to print the reference count total as well the number of memory
blocks allocated after each statement:

    >>> 23
    23
    [8288 refs, 14332 blocks]
    >>>

Note that if this count increases when you're not storing away new objects,
there's probably a leak.  Remember, though, that in interactive mode the special
name "_" holds a reference to the last result displayed!

Py_REF_DEBUG also checks after every decref to verify that the refcount hasn't
gone negative, and causes an immediate fatal error if it has.

Py_DEBUG implies Py_REF_DEBUG.

Special gimmicks:

sys.gettotalrefcount()
    Return current total of all refcounts.


Py_TRACE_REFS
-------------

Build option: ``./configure --with-trace-refs``.

Turn on heavy reference debugging.  This is major surgery.  Every PyObject grows
two more pointers, to maintain a doubly-linked list of all live heap-allocated
objects.  Most built-in type objects are not in this list, as they're statically
allocated.

Note that because the fundamental PyObject layout changes, Python modules
compiled with Py_TRACE_REFS are incompatible with modules compiled without it.

Special gimmicks:

sys.getobjects(max[, type])
    Return list of the (no more than) max most-recently allocated objects, most
    recently allocated first in the list, least-recently allocated last in the
    list.  max=0 means no limit on list length.  If an optional type object is
    passed, the list is also restricted to objects of that type.  The return
    list itself, and some temp objects created just to call sys.getobjects(),
    are excluded from the return list.  Note that the list returned is just
    another object, though, so may appear in the return list the next time you
    call getobjects(); note that every object in the list is kept alive too,
    simply by virtue of being in the list.

envvar PYTHONDUMPREFS
    If this envvar exists, Py_FinalizeEx() arranges to print a list of all
    still-live heap objects.  This is printed twice, in different formats,
    before and after Py_FinalizeEx has cleaned up everything it can clean up.  The
    first output block produces the repr() of each object so is more
    informative; however, a lot of stuff destined to die is still alive then.
    The second output block is much harder to work with (repr() can't be invoked
    anymore -- the interpreter has been torn down too far), but doesn't list any
    objects that will die.  The tool script combinerefs.py can be run over this
    to combine the info from both output blocks.  The second output block, and
    combinerefs.py, were new in Python 2.3b1.


Py_DEBUG
--------

This is what is generally meant by "a debug build" of Python.

Py_DEBUG implies LLTRACE and Py_REF_DEBUG. In addition, C assert()s are enabled
(via the C way: by not defining NDEBUG), and some routines do additional sanity
checks inside "#ifdef Py_DEBUG" blocks.


LLTRACE
-------

Compile in support for Low Level TRACE-ing of the main interpreter loop.

When this preprocessor symbol is defined, before PyEval_EvalFrame executes a
frame's code it checks the frame's global namespace for a variable
"__ltrace__".  If such a variable is found, mounds of information about what
the interpreter is doing are sprayed to stdout, such as every opcode and opcode
argument and values pushed onto and popped off the value stack.

Not useful very often, but very useful when needed.

Py_DEBUG implies LLTRACE.