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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>

<P>

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Commit	Line	Data
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	4	<link rel="STYLESHEET" href="ext.css" type='text/css' />
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	53
	54	<H2><A NAME="SECTION0031030000000000000000"></A><A NAME="thinIce"></A>
	55	<BR>
	56	1.10.3 Thin Ice
	57
	58	</H2>
	59
	60	<P>
	61	There are a few situations where seemingly harmless use of a borrowed
	62	reference can lead to problems. These all have to do with implicit
	63	invocations of the interpreter, which can cause the owner of a
	64	reference to dispose of it.
65
66	<P>
67	The first and most important case to know about is using
68	<tt class="cfunction">Py_DECREF()</tt> on an unrelated object while borrowing a
69	reference to a list item. For instance:
70
71	<P>
72	<div class="verbatim"><pre>
73	void
74	bug(PyObject *list)
75	{
76	PyObject *item = PyList_GetItem(list, 0);
77
78	PyList_SetItem(list, 1, PyInt_FromLong(0L));
79	PyObject_Print(item, stdout, 0); /* BUG! */
80	}
81	</pre></div>
82
83	<P>
84	This function first borrows a reference to <code>list[0]</code>, then
85	replaces <code>list[1]</code> with the value <code>0</code>, and finally prints
86	the borrowed reference. Looks harmless, right? But it's not!
87
88	<P>
89	Let's follow the control flow into <tt class="cfunction">PyList_SetItem()</tt>. The list
90	owns references to all its items, so when item 1 is replaced, it has
91	to dispose of the original item 1. Now let's suppose the original
92	item 1 was an instance of a user-defined class, and let's further
93	suppose that the class defined a <tt class="method">__del__()</tt> method. If this
94	class instance has a reference count of 1, disposing of it will call
95	its <tt class="method">__del__()</tt> method.
96
97	<P>
98	Since it is written in Python, the <tt class="method">__del__()</tt> method can execute
99	arbitrary Python code. Could it perhaps do something to invalidate
100	the reference to <code>item</code> in <tt class="cfunction">bug()</tt>? You bet! Assuming
101	that the list passed into <tt class="cfunction">bug()</tt> is accessible to the
102	<tt class="method">__del__()</tt> method, it could execute a statement to the effect of
103	"<tt class="samp">del list[0]</tt>", and assuming this was the last reference to that
104	object, it would free the memory associated with it, thereby
105	invalidating <code>item</code>.
106
107	<P>
108	The solution, once you know the source of the problem, is easy:
109	temporarily increment the reference count. The correct version of the
110	function reads:
111
112	<P>
113	<div class="verbatim"><pre>
114	void
115	no_bug(PyObject *list)
116	{
117	PyObject *item = PyList_GetItem(list, 0);
118
119	Py_INCREF(item);
120	PyList_SetItem(list, 1, PyInt_FromLong(0L));
121	PyObject_Print(item, stdout, 0);
122	Py_DECREF(item);
123	}
124	</pre></div>
125
126	<P>
127	This is a true story. An older version of Python contained variants
128	of this bug and someone spent a considerable amount of time in a C
129	debugger to figure out why his <tt class="method">__del__()</tt> methods would fail...
130
131	<P>
132	The second case of problems with a borrowed reference is a variant
133	involving threads. Normally, multiple threads in the Python
134	interpreter can't get in each other's way, because there is a global
135	lock protecting Python's entire object space. However, it is possible
136	to temporarily release this lock using the macro
137	Py_BEGIN_ALLOW_THREADS, and to re-acquire it using
138	Py_END_ALLOW_THREADS. This is common around blocking
139	I/O calls, to let other threads use the processor while waiting for
140	the I/O to complete. Obviously, the following function has the same
141	problem as the previous one:
142
143	<P>
144	<div class="verbatim"><pre>
145	void
146	bug(PyObject *list)
147	{
148	PyObject *item = PyList_GetItem(list, 0);
149	Py_BEGIN_ALLOW_THREADS
150	...some blocking I/O call...
151	Py_END_ALLOW_THREADS
152	PyObject_Print(item, stdout, 0); /* BUG! */
153	}
154	</pre></div>
155
156	<P>
157
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