[Python-Dev] Python under valgrind (original) (raw)

Gustavo Carneiro gjcarneiro at gmail.com
Fri Nov 28 13:54:38 CET 2008

2008/11/28 Hrvoje Niksic <hrvoje.niksic at avl.com>

A friend pointed out that running python under valgrind (simply "valgrind python") produces a lot of "invalid read" errors. Reading up on Misc/README.valgrind only seems to describe why "uninitialized reads" should occur, not invalid ones. For example:

$ valgrind python [... lots of output ...] ==31428== Invalid read of size 4 ==31428== at 0x808EBDF: PyObjectFree (in /usr/bin/python2.5) ==31428== by 0x810DD0A: (within /usr/bin/python2.5) ==31428== by 0x810DD34: PyNodeFree (in /usr/bin/python2.5) ==31428== by 0x80EDAD9: PyRunInteractiveOneFlags (in /usr/bin/python2.5) ==31428== by 0x80EDDB7: PyRunInteractiveLoopFlags (in /usr/bin/python2.5) ==31428== by 0x80EE515: PyRunAnyFileExFlags (in /usr/bin/python2.5) ==31428== by 0x80595E6: PyMain (in /usr/bin/python2.5) ==31428== by 0x8058961: main (in /usr/bin/python2.5) ==31428== Address 0x43bf010 is 3,112 bytes inside a block of size 6,016 free'd ==31428== at 0x4024B4A: free (vgreplacemalloc.c:323) ==31428== by 0x8059C07: (within /usr/bin/python2.5) ==31428== by 0x80EDAA5: PyRunInteractiveOneFlags (in /usr/bin/python2.5) ... valgrind claims that Python reads 4 bytes inside a block on which free() has already been called. Is valgrind wrong, or is Python really doing that? Googling revealed previous reports of this, normally answered by a reference to README.valgrind. But README.valgrind justifies reading from ununitialized memory, which doesn't help me understand how reading from the middle of a block of freed memory (more precisely, memory on which the libc free() has already been called) would be okay. I suppose valgrind could be confused by PyFree's pool address validation that intentionally reads the memory just before the allocated block, and incorrectly attributes it to a previously allocated (and hence freed) block, but I can't prove that. Has anyone investigated this kind of valgrind report?

I can't answer your question directly, but I can tell you that whenever I have to debug memory problems with python extensions is usually use my own python compiled with --with-pydebug --without-pymalloc. It really helps with valgrind.

-- Gustavo J. A. M. Carneiro INESC Porto, Telecommunications and Multimedia Unit "The universe is always one step beyond logic." -- Frank Herbert -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20081128/13408172/attachment.htm>

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