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 want to enable valgrind support in Python, you will need to 6configure Python --with-valgrind option or an older option 7--without-pymalloc. 8 9UPDATE: Python 3.6 now supports PYTHONMALLOC=malloc environment variable which 10can be used to force the usage of the malloc() allocator of the C library. 11 12If you don't want to read about the details of using Valgrind, there 13are still two things you must do to suppress the warnings. First, 14you must use a suppressions file. One is supplied in 15Misc/valgrind-python.supp. Second, you must do one of the following: 16 17 * Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c, 18 then rebuild Python 19 * Uncomment the lines in Misc/valgrind-python.supp that 20 suppress the warnings for PyObject_Free and PyObject_Realloc 21 22If you want to use Valgrind more effectively and catch even more 23memory leaks, you will need to configure python --without-pymalloc. 24PyMalloc allocates a few blocks in big chunks and most object 25allocations don't call malloc, they use chunks doled about by PyMalloc 26from the big blocks. This means Valgrind can't detect 27many allocations (and frees), except for those that are forwarded 28to the system malloc. Note: configuring python --without-pymalloc 29makes Python run much slower, especially when running under Valgrind. 30You may need to run the tests in batches under Valgrind to keep 31the memory usage down to allow the tests to complete. It seems to take 32about 5 times longer to run --without-pymalloc. 33 34Apr 15, 2006: 35 test_ctypes causes Valgrind 3.1.1 to fail (crash). 36 test_socket_ssl should be skipped when running valgrind. 37 The reason is that it purposely uses uninitialized memory. 38 This causes many spurious warnings, so it's easier to just skip it. 39 40 41Details: 42-------- 43Python uses its own small-object allocation scheme on top of malloc, 44called PyMalloc. 45 46Valgrind may show some unexpected results when PyMalloc is used. 47Starting with Python 2.3, PyMalloc is used by default. You can disable 48PyMalloc when configuring python by adding the --without-pymalloc option. 49If you disable PyMalloc, most of the information in this document and 50the supplied suppressions file will not be useful. As discussed above, 51disabling PyMalloc can catch more problems. 52 53PyMalloc uses 256KB chunks of memory, so it can't detect anything 54wrong within these blocks. For that reason, compiling Python 55--without-pymalloc usually increases the usefulness of other tools. 56 57If you use valgrind on a default build of Python, you will see 58many errors like: 59 60 ==6399== Use of uninitialised value of size 4 61 ==6399== at 0x4A9BDE7E: PyObject_Free (obmalloc.c:711) 62 ==6399== by 0x4A9B8198: dictresize (dictobject.c:477) 63 64These are expected and not a problem. Tim Peters explains 65the situation: 66 67 PyMalloc needs to know whether an arbitrary address is one 68 that's managed by it, or is managed by the system malloc. 69 The current scheme allows this to be determined in constant 70 time, regardless of how many memory areas are under pymalloc's 71 control. 72 73 The memory pymalloc manages itself is in one or more "arenas", 74 each a large contiguous memory area obtained from malloc. 75 The base address of each arena is saved by pymalloc 76 in a vector. Each arena is carved into "pools", and a field at 77 the start of each pool contains the index of that pool's arena's 78 base address in that vector. 79 80 Given an arbitrary address, pymalloc computes the pool base 81 address corresponding to it, then looks at "the index" stored 82 near there. If the index read up is out of bounds for the 83 vector of arena base addresses pymalloc maintains, then 84 pymalloc knows for certain that this address is not under 85 pymalloc's control. Otherwise the index is in bounds, and 86 pymalloc compares 87 88 the arena base address stored at that index in the vector 89 90 to 91 92 the arbitrary address pymalloc is investigating 93 94 pymalloc controls this arbitrary address if and only if it lies 95 in the arena the address's pool's index claims it lies in. 96 97 It doesn't matter whether the memory pymalloc reads up ("the 98 index") is initialized. If it's not initialized, then 99 whatever trash gets read up will lead pymalloc to conclude 100 (correctly) that the address isn't controlled by it, either 101 because the index is out of bounds, or the index is in bounds 102 but the arena it represents doesn't contain the address. 103 104 This determination has to be made on every call to one of 105 pymalloc's free/realloc entry points, so its speed is critical 106 (Python allocates and frees dynamic memory at a ferocious rate 107 -- everything in Python, from integers to "stack frames", 108 lives in the heap). 109