mirror of
https://github.com/python/cpython
synced 2024-09-05 08:17:45 +00:00
226 lines
9.7 KiB
Plaintext
226 lines
9.7 KiB
Plaintext
This file describes some special Python build types enabled via compile-time
|
|
preprocessor defines.
|
|
|
|
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.
|
|
|
|
Special gimmicks:
|
|
|
|
sys.gettotalrefcount()
|
|
Return current total of all refcounts.
|
|
|
|
|
|
Py_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. 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.
|
|
|
|
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.
|
|
|
|
|
|
PYMALLOC_DEBUG
|
|
--------------
|
|
|
|
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]
|
|
API ID. See PEP 445. This is a character, but seems undocumented.
|
|
p[-S+1:0]
|
|
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:
|
|
|
|
envvar PYTHONMALLOCSTATS
|
|
If this envvar exists, a report of pymalloc summary statistics is printed to
|
|
stderr whenever a new arena is allocated, and also by Py_FinalizeEx().
|
|
|
|
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
|
|
--------
|
|
|
|
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
|
|
------------
|
|
|
|
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_FinalizeEx()
|
|
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
|
|
-------
|
|
|
|
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
|
|
"__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.
|