This file describes some special Python build types enabled via

compile-time preprocessor defines.

It is best to define these options in the EXTRA_CFLAGS make variable;

``make EXTRA_CFLAGS="-DPy_REF_DEBUG"``.

---------------------------------------------------------------------------

Py_REF_DEBUG                                              introduced in 1.4

                                                 named REF_DEBUG before 1.4

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.  In a debug-mode build, this is where the "8288" comes from

in

    >>> 23

    23

    [8288 refs]

    >>>

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.

Special gimmicks:

sys.gettotalrefcount()

    Return current total of all refcounts.

    Available under Py_REF_DEBUG in Python 2.3.

    Before 2.3, Py_TRACE_REFS was required to enable this function.

---------------------------------------------------------------------------

Py_TRACE_REFS                                             introduced in 1.4

                                                named TRACE_REFS before 1.4

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 builtin type objects are not in this list,

as they're statically allocated.  Starting in Python 2.3, if COUNT_ALLOCS

(see below) is also defined, a static type object T does appear in this

list if at least one object of type T has been created.

Note that because the fundamental PyObject layout changes, Python modules

compiled with Py_TRACE_REFS are incompatible with modules compiled without

it.

Py_TRACE_REFS implies Py_REF_DEBUG.

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.

envar PYTHONDUMPREFS

    If this envar exists, Py_Finalize() arranges to print a list of

    all still-live heap objects.  This is printed twice, in different

    formats, before and after Py_Finalize 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.

---------------------------------------------------------------------------

PYMALLOC_DEBUG                                            introduced in 2.3

When pymalloc is enabled (WITH_PYMALLOC is defined), calls to the PyObject_

memory routines are handled by Python's own small-object allocator, while

calls to the PyMem_ memory routines are directed to the system malloc/

realloc/free.  If PYMALLOC_DEBUG is also defined, calls to both PyObject_

and PyMem_ memory routines are directed to a special debugging mode of

Python's small-object allocator.

This mode fills dynamically allocated memory blocks with special,

recognizable bit patterns, and adds debugging info on each end of

dynamically allocated memory blocks.  The special bit patterns are:

#define CLEANBYTE     0xCB   /* clean (newly allocated) memory */

#define DEADBYTE      0xDB   /* dead (newly freed) memory */

#define FORBIDDENBYTE 0xFB   /* forbidden -- untouchable bytes */

Strings of these bytes are unlikely to be valid addresses, floats, or 7-bit

ASCII strings.

Let S = sizeof(size_t). 2*S bytes are added at each end of each block of N

bytes requested.  The memory layout is like so, where p represents the

address returned by a malloc-like or realloc-like function (p[i:j] means

the slice of bytes from *(p+i) inclusive up to *(p+j) exclusive; note that

the treatment of negative indices differs from a Python slice):

p[-2*S:-S]

    Number of bytes originally asked for.  This is a size_t, big-endian

    (easier to read in a memory dump).

p[-S:0]

    Copies of FORBIDDENBYTE.  Used to catch under- writes and reads.

p[0:N]

    The requested memory, filled with copies of CLEANBYTE, used to catch

    reference to uninitialized memory.

    When a realloc-like function is called requesting a larger memory

    block, the new excess bytes are also filled with CLEANBYTE.

    When a free-like function is called, these are overwritten with

    DEADBYTE, to catch reference to freed memory.  When a realloc-

    like function is called requesting a smaller memory block, the excess

    old bytes are also filled with DEADBYTE.

p[N:N+S]

    Copies of FORBIDDENBYTE.  Used to catch over- writes and reads.

p[N+S:N+2*S]

    A serial number, incremented by 1 on each call to a malloc-like or

    realloc-like function.

    Big-endian size_t.

    If "bad memory" is detected later, the serial number gives an

    excellent way to set a breakpoint on the next run, to capture the

    instant at which this block was passed out.  The static function

    bumpserialno() in obmalloc.c is the only place the serial number

    is incremented, and exists so you can set such a breakpoint easily.

A realloc-like or free-like function first checks that the FORBIDDENBYTEs

at each end are intact.  If they've been altered, diagnostic output is

written to stderr, and the program is aborted via Py_FatalError().  The

other main failure mode is provoking a memory error when a program

reads up one of the special bit patterns and tries to use it as an address.

If you get in a debugger then and look at the object, you're likely

to see that it's entirely filled with 0xDB (meaning freed memory is

getting used) or 0xCB (meaning uninitialized memory is getting used).

Note that PYMALLOC_DEBUG requires WITH_PYMALLOC.

Special gimmicks:

envar PYTHONMALLOCSTATS

    If this envar exists, a report of pymalloc summary statistics is

    printed to stderr whenever a new arena is allocated, and also

    by Py_Finalize().

Changed in 2.5:  The number of extra bytes allocated is 4*sizeof(size_t).

Before it was 16 on all boxes, reflecting that Python couldn't make use of

allocations >= 2**32 bytes even on 64-bit boxes before 2.5.

---------------------------------------------------------------------------

Py_DEBUG                                                  introduced in 1.5

                                                     named DEBUG before 1.5

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

Py_DEBUG implies LLTRACE, Py_REF_DEBUG, Py_TRACE_REFS, and

PYMALLOC_DEBUG (if WITH_PYMALLOC is enabled).  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.

---------------------------------------------------------------------------

COUNT_ALLOCS                                            introduced in 0.9.9

                                             partly broken in 2.2 and 2.2.1

Each type object grows three new members:

    /* Number of times an object of this type was allocated. */

    int tp_allocs;

    /* Number of times an object of this type was deallocated. */

    int tp_frees;

    /* Highwater mark:  the maximum value of tp_allocs - tp_frees so

     * far; or, IOW, the largest number of objects of this type alive at

     * the same time.

     */

    int tp_maxalloc;

Allocation and deallocation code keeps these counts up to date.

Py_Finalize() displays a summary of the info returned by sys.getcounts()

(see below), along with assorted other special allocation counts (like

the number of tuple allocations satisfied by a tuple free-list, the number

of 1-character strings allocated, etc).

Before Python 2.2, type objects were immortal, and the COUNT_ALLOCS

implementation relies on that.  As of Python 2.2, heap-allocated type/

class objects can go away.  COUNT_ALLOCS can blow up in 2.2 and 2.2.1

because of this; this was fixed in 2.2.2.  Use of COUNT_ALLOCS makes

all heap-allocated type objects immortal, except for those for which no

object of that type is ever allocated.

Starting with Python 2.3, If Py_TRACE_REFS is also defined, COUNT_ALLOCS

arranges to ensure that the type object for each allocated object

appears in the doubly-linked list of all objects maintained by

Py_TRACE_REFS.

Special gimmicks:

sys.getcounts()

    Return a list of 4-tuples, one entry for each type object for which

    at least one object of that type was allocated.  Each tuple is of

    the form:

        (tp_name, tp_allocs, tp_frees, tp_maxalloc)

    Each distinct type object gets a distinct entry in this list, even

    if two or more type objects have the same tp_name (in which case

    there's no way to distinguish them by looking at this list).  The

    list is ordered by time of first object allocation:  the type object

    for which the first allocation of an object of that type occurred

    most recently is at the front of the list.

---------------------------------------------------------------------------

LLTRACE                                          introduced well before 1.0

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

When this preprocessor symbol is defined, before PyEval_EvalFrame

(eval_frame in 2.3 and 2.2, eval_code2 before that) executes a frame's code

it checks the frame's global namespace for a variable "__lltrace__".  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.

---------------------------------------------------------------------------

CALL_PROFILE                                      introduced for Python 2.3

Count the number of function calls executed.

When this symbol is defined, the ceval mainloop and helper functions

count the number of function calls made.  It keeps detailed statistics

about what kind of object was called and whether the call hit any of

the special fast paths in the code.

---------------------------------------------------------------------------

WITH_TSC                                          introduced for Python 2.4

Super-lowlevel profiling of the interpreter.  When enabled, the sys

module grows a new function:

settscdump(bool)

    If true, tell the Python interpreter to dump VM measurements to

    stderr.  If false, turn off dump.  The measurements are based on the

    processor's time-stamp counter.

This build option requires a small amount of platform specific code.

Currently this code is present for linux/x86 and any PowerPC platform

that uses GCC (i.e. OS X and linux/ppc).

On the PowerPC the rate at which the time base register is incremented

is not defined by the architecture specification, so you'll need to

find the manual for your specific processor.  For the 750CX, 750CXe

and 750FX (all sold as the G3) we find:

    The time base counter is clocked at a frequency that is

    one-fourth that of the bus clock.

This build is enabled by the --with-tsc flag to configure.