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/Misc/README.valgrind

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 1This document describes some caveats about the use of Valgrind with
 2Python.  Valgrind is used periodically by Python developers to try
 3to ensure there are no memory leaks or invalid memory reads/writes.
 4
 5If you don't want to read about the details of using Valgrind, there
 6are still two things you must do to suppress the warnings.  First,
 7you must use a suppressions file.  One is supplied in
 8Misc/valgrind-python.supp.  Second, you must do one of the following:
 9
10  * Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c,
11    then rebuild Python
12  * Uncomment the lines in Misc/valgrind-python.supp that
13    suppress the warnings for PyObject_Free and PyObject_Realloc
14
15If you want to use Valgrind more effectively and catch even more
16memory leaks, you will need to configure python --without-pymalloc.
17PyMalloc allocates a few blocks in big chunks and most object
18allocations don't call malloc, they use chunks doled about by PyMalloc
19from the big blocks.  This means Valgrind can't detect
20many allocations (and frees), except for those that are forwarded
21to the system malloc.  Note: configuring python --without-pymalloc
22makes Python run much slower, especially when running under Valgrind.
23You may need to run the tests in batches under Valgrind to keep
24the memory usage down to allow the tests to complete.  It seems to take
25about 5 times longer to run --without-pymalloc.
26
27Apr 15, 2006:
28  test_ctypes causes Valgrind 3.1.1 to fail (crash).
29  test_socket_ssl should be skipped when running valgrind.
30	The reason is that it purposely uses uninitialized memory.
31	This causes many spurious warnings, so it's easier to just skip it.
32
33
34Details:
35--------
36Python uses its own small-object allocation scheme on top of malloc,
37called PyMalloc.
38
39Valgrind may show some unexpected results when PyMalloc is used.
40Starting with Python 2.3, PyMalloc is used by default.  You can disable
41PyMalloc when configuring python by adding the --without-pymalloc option.
42If you disable PyMalloc, most of the information in this document and
43the supplied suppressions file will not be useful.  As discussed above,
44disabling PyMalloc can catch more problems.
45
46If you use valgrind on a default build of Python,  you will see
47many errors like:
48
49        ==6399== Use of uninitialised value of size 4
50        ==6399== at 0x4A9BDE7E: PyObject_Free (obmalloc.c:711)
51        ==6399== by 0x4A9B8198: dictresize (dictobject.c:477)
52
53These are expected and not a problem.  Tim Peters explains
54the situation:
55
56        PyMalloc needs to know whether an arbitrary address is one
57	that's managed by it, or is managed by the system malloc.
58	The current scheme allows this to be determined in constant
59	time, regardless of how many memory areas are under pymalloc's
60	control.
61
62        The memory pymalloc manages itself is in one or more "arenas",
63	each a large contiguous memory area obtained from malloc.
64	The base address of each arena is saved by pymalloc
65	in a vector.  Each arena is carved into "pools", and a field at
66	the start of each pool contains the index of that pool's arena's
67	base address in that vector.
68
69        Given an arbitrary address, pymalloc computes the pool base
70	address corresponding to it, then looks at "the index" stored
71	near there.  If the index read up is out of bounds for the
72	vector of arena base addresses pymalloc maintains, then
73	pymalloc knows for certain that this address is not under
74	pymalloc's control.  Otherwise the index is in bounds, and
75	pymalloc compares
76
77            the arena base address stored at that index in the vector
78
79        to
80
81            the arbitrary address pymalloc is investigating
82
83        pymalloc controls this arbitrary address if and only if it lies
84        in the arena the address's pool's index claims it lies in.
85
86        It doesn't matter whether the memory pymalloc reads up ("the
87	index") is initialized.  If it's not initialized, then
88	whatever trash gets read up will lead pymalloc to conclude
89	(correctly) that the address isn't controlled by it, either
90	because the index is out of bounds, or the index is in bounds
91	but the arena it represents doesn't contain the address.
92
93        This determination has to be made on every call to one of
94	pymalloc's free/realloc entry points, so its speed is critical
95	(Python allocates and frees dynamic memory at a ferocious rate
96	-- everything in Python, from integers to "stack frames",
97	lives in the heap).