mirror of https://github.com/python/cpython
Made the explanation more accurate; trimmed trailing whitespace; fixed
a typo.
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This document describes some caveats about the use of Valgrind with
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This document describes some caveats about the use of Valgrind with
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Python. Valgrind is used periodically by Python developers to try
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Python. Valgrind is used periodically by Python developers to try
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to ensure there are no memory leaks or invalid memory reads/writes.
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to ensure there are no memory leaks or invalid memory reads/writes.
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If you don't want to read about the details of using Valgrind, there
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If you don't want to read about the details of using Valgrind, there
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are still two things you must do to suppress the warnings. First,
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are still two things you must do to suppress the warnings. First,
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you must use a suppressions file. One is supplied in
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you must use a suppressions file. One is supplied in
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Misc/valgrind-python.supp. Second, you must do one of the following:
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Misc/valgrind-python.supp. Second, you must do one of the following:
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* Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c,
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* Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c,
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then rebuild Python
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then rebuild Python
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* Uncomment the lines in Misc/valgrind-python.supp that
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* Uncomment the lines in Misc/valgrind-python.supp that
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suppress the warnings for PyObject_Free and PyObject_Realloc
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suppress the warnings for PyObject_Free and PyObject_Realloc
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Details:
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Details:
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--------
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--------
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Python uses its own allocation scheme on top of malloc called PyMalloc.
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Python uses its own small-object allocation scheme on top of malloc,
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Valgrind my show some unexpected results when PyMalloc is used.
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called PyMalloc.
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Valgrind may show some unexpected results when PyMalloc is used.
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Starting with Python 2.3, PyMalloc is used by default. You can disable
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Starting with Python 2.3, PyMalloc is used by default. You can disable
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PyMalloc when configuring python by adding the --without-pymalloc option.
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PyMalloc when configuring python by adding the --without-pymalloc option.
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If you disable PyMalloc, most of the information in this document and
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If you disable PyMalloc, most of the information in this document and
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the supplied suppressions file will not be useful.
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the supplied suppressions file will not be useful.
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If you use valgrind on a default build of Python, you will see
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If you use valgrind on a default build of Python, you will see
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@ -32,18 +34,19 @@ These are expected and not a problem. Tim Peters explains
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the situation:
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the situation:
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PyMalloc needs to know whether an arbitrary address is one
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PyMalloc needs to know whether an arbitrary address is one
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that's managed by it, or is managed by the system malloc.
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that's managed by it, or is managed by the system malloc.
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The current scheme allows this to be determined in constant
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The current scheme allows this to be determined in constant
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time, regardless of how many memory areas are under pymalloc's
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time, regardless of how many memory areas are under pymalloc's
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control.
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control.
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The memory pymalloc manages itself is in one or more "arenas",
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The memory pymalloc manages itself is in one or more "arenas",
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each a large contiguous memory area obtained from malloc.
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each a large contiguous memory area obtained from malloc.
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The base address of each arena is saved by pymalloc
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The base address of each arena is saved by pymalloc
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in a vector, and a field at the start of each arena contains
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in a vector. Each arena is carved into "pools", and a field at
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the index of that arena's base address in that vector.
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the start of each pool contains the index of that pool's arena's
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base address in that vector.
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Given an arbitrary address, pymalloc computes the arena base
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Given an arbitrary address, pymalloc computes the pool base
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address corresponding to it, then looks at "the index" stored
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address corresponding to it, then looks at "the index" stored
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near there. If the index read up is out of bounds for the
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near there. If the index read up is out of bounds for the
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vector of arena base addresses pymalloc maintains, then
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vector of arena base addresses pymalloc maintains, then
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@ -55,14 +58,17 @@ the situation:
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to
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to
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the computed arena address
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the arbitrary address pymalloc is investigating
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pymalloc controls this arena if and only if they're equal.
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pymalloc controls this arbitrary address if and only if it lies
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in the arena the address's pool's index claims it lies in.
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It doesn't matter whether the memory pymalloc reads up ("the
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It doesn't matter whether the memory pymalloc reads up ("the
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index") is initialized. If it's not initialized, then
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index") is initialized. If it's not initialized, then
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whatever trash gets read up will lead pymalloc to conclude
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whatever trash gets read up will lead pymalloc to conclude
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(correctly) that the address isn't controlled by it.
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(correctly) that the address isn't controlled by it, either
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because the index is out of bounds, or the index is in bounds
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but the arena it represents doesn't contain the address.
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This determination has to be made on every call to one of
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This determination has to be made on every call to one of
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pymalloc's free/realloc entry points, so its speed is critical
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pymalloc's free/realloc entry points, so its speed is critical
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