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<H2><A NAME="SECTION0031030000000000000000"></A><A NAME="thinIce"></A>
<BR>
1.10.3 Thin Ice

</H2>

<P>
There are a few situations where seemingly harmless use of a borrowed
reference can lead to problems.  These all have to do with implicit
invocations of the interpreter, which can cause the owner of a
reference to dispose of it.

<P>
The first and most important case to know about is using
<tt class="cfunction">Py_DECREF()</tt> on an unrelated object while borrowing a
reference to a list item.  For instance:

<P>
<div class="verbatim"><pre>
void
bug(PyObject *list)
{
    PyObject *item = PyList_GetItem(list, 0);

    PyList_SetItem(list, 1, PyInt_FromLong(0L));
    PyObject_Print(item, stdout, 0); /* BUG! */
}
</pre></div>

<P>
This function first borrows a reference to <code>list[0]</code>, then
replaces <code>list[1]</code> with the value <code>0</code>, and finally prints
the borrowed reference.  Looks harmless, right?  But it's not!

<P>
Let's follow the control flow into <tt class="cfunction">PyList_SetItem()</tt>.  The list
owns references to all its items, so when item 1 is replaced, it has
to dispose of the original item 1.  Now let's suppose the original
item 1 was an instance of a user-defined class, and let's further
suppose that the class defined a <tt class="method">__del__()</tt> method.  If this
class instance has a reference count of 1, disposing of it will call
its <tt class="method">__del__()</tt> method.

<P>
Since it is written in Python, the <tt class="method">__del__()</tt> method can execute
arbitrary Python code.  Could it perhaps do something to invalidate
the reference to <code>item</code> in <tt class="cfunction">bug()</tt>?  You bet!  Assuming
that the list passed into <tt class="cfunction">bug()</tt> is accessible to the
<tt class="method">__del__()</tt> method, it could execute a statement to the effect of
"<tt class="samp">del list[0]</tt>", and assuming this was the last reference to that
object, it would free the memory associated with it, thereby
invalidating <code>item</code>.

<P>
The solution, once you know the source of the problem, is easy:
temporarily increment the reference count.  The correct version of the
function reads:

<P>
<div class="verbatim"><pre>
void
no_bug(PyObject *list)
{
    PyObject *item = PyList_GetItem(list, 0);

    Py_INCREF(item);
    PyList_SetItem(list, 1, PyInt_FromLong(0L));
    PyObject_Print(item, stdout, 0);
    Py_DECREF(item);
}
</pre></div>

<P>
This is a true story.  An older version of Python contained variants
of this bug and someone spent a considerable amount of time in a C
debugger to figure out why his <tt class="method">__del__()</tt> methods would fail...

<P>
The second case of problems with a borrowed reference is a variant
involving threads.  Normally, multiple threads in the Python
interpreter can't get in each other's way, because there is a global
lock protecting Python's entire object space.  However, it is possible
to temporarily release this lock using the macro
Py_BEGIN_ALLOW_THREADS, and to re-acquire it using
Py_END_ALLOW_THREADS.  This is common around blocking
I/O calls, to let other threads use the processor while waiting for
the I/O to complete.  Obviously, the following function has the same
problem as the previous one:

<P>
<div class="verbatim"><pre>
void
bug(PyObject *list)
{
    PyObject *item = PyList_GetItem(list, 0);
    Py_BEGIN_ALLOW_THREADS
    ...some blocking I/O call...
    Py_END_ALLOW_THREADS
    PyObject_Print(item, stdout, 0); /* BUG! */
}
</pre></div>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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