horribly slimy segault c-extension

Carl Banks imbosol at aerojockey.com
Sun Jun 1 00:16:29 CEST 2003

John Does wrote:
> There should be no need for me to show all my code for this issue.
> First off: i run a program and it sefaults normally, but when i run it
> through gdb it does not.
> Here are the relevant lines of code:
>        printf("q:%i \n",Q->ob_refcnt);
>        Py_DECREF(Q);
>        printf("qa:%i \n",Q->ob_refcnt);
>        Q = newQ ;
>        printf("qaa:%i \n",Q->ob_refcnt);
> And the output:
> q:1 
> Segmentation fault
> Without asking me for any more code, can someone give me some
> precise steps that I could take to begin resolving/exposing this
> issue.

First thing to try:

    printf("q:%i \n",Q->ob_refcnt);
    /* printf("qa:%i \n",Q->ob_refcnt); */
    Q = newQ ;
    printf("qaa:%i \n",Q->ob_refcnt);

You're trying to access Q after its refcount fell to zero.  Py_DECREF
frees the memory pointed at by Q when that happens.  Trying to access
freed memory (as your printf does) is an undefined operation, and
could very well segfault.

Just a guess, does the DECREFed object have a large memory footprint
(say, more than 8K)?


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