1 This document describes some caveats about the use of Valgrind with 2 Python. Valgrind is used periodically by Python developers to try 3 to ensure there are no memory leaks or invalid memory reads/writes. 4 5 UPDATE: Python 3.6 now supports PYTHONMALLOC=malloc environment variable which 6 can be used to force the usage of the malloc() allocator of the C library. 7 8 If you don't want to read about the details of using Valgrind, there 9 are still two things you must do to suppress the warnings. First, 10 you must use a suppressions file. One is supplied in 11 Misc/valgrind-python.supp. Second, you must do one of the following: 12 13 * Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c, 14 then rebuild Python 15 * Uncomment the lines in Misc/valgrind-python.supp that 16 suppress the warnings for PyObject_Free and PyObject_Realloc 17 18 If you want to use Valgrind more effectively and catch even more 19 memory leaks, you will need to configure python --without-pymalloc. 20 PyMalloc allocates a few blocks in big chunks and most object 21 allocations don't call malloc, they use chunks doled about by PyMalloc 22 from the big blocks. This means Valgrind can't detect 23 many allocations (and frees), except for those that are forwarded 24 to the system malloc. Note: configuring python --without-pymalloc 25 makes Python run much slower, especially when running under Valgrind. 26 You may need to run the tests in batches under Valgrind to keep 27 the memory usage down to allow the tests to complete. It seems to take 28 about 5 times longer to run --without-pymalloc. 29 30 Apr 15, 2006: 31 test_ctypes causes Valgrind 3.1.1 to fail (crash). 32 test_socket_ssl should be skipped when running valgrind. 33 The reason is that it purposely uses uninitialized memory. 34 This causes many spurious warnings, so it's easier to just skip it. 35 36 37 Details: 38 -------- 39 Python uses its own small-object allocation scheme on top of malloc, 40 called PyMalloc. 41 42 Valgrind may show some unexpected results when PyMalloc is used. 43 Starting with Python 2.3, PyMalloc is used by default. You can disable 44 PyMalloc when configuring python by adding the --without-pymalloc option. 45 If you disable PyMalloc, most of the information in this document and 46 the supplied suppressions file will not be useful. As discussed above, 47 disabling PyMalloc can catch more problems. 48 49 If you use valgrind on a default build of Python, you will see 50 many errors like: 51 52 ==6399== Use of uninitialised value of size 4 53 ==6399== at 0x4A9BDE7E: PyObject_Free (obmalloc.c:711) 54 ==6399== by 0x4A9B8198: dictresize (dictobject.c:477) 55 56 These are expected and not a problem. Tim Peters explains 57 the situation: 58 59 PyMalloc needs to know whether an arbitrary address is one 60 that's managed by it, or is managed by the system malloc. 61 The current scheme allows this to be determined in constant 62 time, regardless of how many memory areas are under pymalloc's 63 control. 64 65 The memory pymalloc manages itself is in one or more "arenas", 66 each a large contiguous memory area obtained from malloc. 67 The base address of each arena is saved by pymalloc 68 in a vector. Each arena is carved into "pools", and a field at 69 the start of each pool contains the index of that pool's arena's 70 base address in that vector. 71 72 Given an arbitrary address, pymalloc computes the pool base 73 address corresponding to it, then looks at "the index" stored 74 near there. If the index read up is out of bounds for the 75 vector of arena base addresses pymalloc maintains, then 76 pymalloc knows for certain that this address is not under 77 pymalloc's control. Otherwise the index is in bounds, and 78 pymalloc compares 79 80 the arena base address stored at that index in the vector 81 82 to 83 84 the arbitrary address pymalloc is investigating 85 86 pymalloc controls this arbitrary address if and only if it lies 87 in the arena the address's pool's index claims it lies in. 88 89 It doesn't matter whether the memory pymalloc reads up ("the 90 index") is initialized. If it's not initialized, then 91 whatever trash gets read up will lead pymalloc to conclude 92 (correctly) that the address isn't controlled by it, either 93 because the index is out of bounds, or the index is in bounds 94 but the arena it represents doesn't contain the address. 95 96 This determination has to be made on every call to one of 97 pymalloc's free/realloc entry points, so its speed is critical 98 (Python allocates and frees dynamic memory at a ferocious rate 99 -- everything in Python, from integers to "stack frames", 100 lives in the heap). 101
