[Python-Dev] tp_dealloc (original) (raw)

Amaury Forgeot d'Arc amauryfa at gmail.com
Tue Jun 1 15:00:21 CEST 2010

2010/6/1 <smarv at gmx.net>:

This said, there may be a bug somewhere, but what do you want us to look at? Do you have a case that we could reproduce and investigate?

-- Amaury Forgeot d'Arc Thank you, I'm not a C-Developer, but still I have one more detail: I call pydecRef( pyObj) of dll (version 3.1.1), ( which calls tpdealloc, which calls my freeMem() method)) No problem is reported here. Now, the freed memory should not be accessed anymore by python31.dll. You may fill the freed pyObjectHead with invalid values, in my case it's:  obrefcnt= 7851148, obtype = $80808080 But later, when I call PyFinalize, there inside is some access to the same freed memory; this causes an AV, more precisely, when the value $80808080 is checked. My Delphi-Debugger shows the following byte-sequence inside python31.dll: 5EC3568B7424088B4604F74054004000007504 5E                  - pop esi C3                  - ret 56                  - push esi 8B742408            - mov esi, [esp+$08] 8B4604              - mov eax, [esi+$04] // eax = $80808080 // F7405400400000      - test [eax+$54], $00004000 // AV exception by read of address $808080D4 // 7504                - jnz $1e03681b Maybe this can help someone, thank you!

I'm sorry but this kind of issue is difficult to investigate without the source code. Normally I would compile everything (python & your program) in debug mode, and try to see why the object is used after tp_dealloc.

For example, it's possible that your code does not handle reference counts correctly A call to Py_INCREF() may be missing somewhere, for example. This is a common error. tp_dealloc() is called when the reference count falls to zero, but if the object is still referenced elsewhere, memory will be accessed again!

Without further information, I cannot consider this as a problem in Python. I know other extension modules that manage memory in their own way, and work. It's more probably an issue in the code of your type.

-- Amaury Forgeot d'Arc

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