Commit | Line | Data |
---|---|---|
86530b38 AT |
1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> |
2 | <html> | |
3 | <head> | |
4 | <link rel="STYLESHEET" href="ext.css" type='text/css' /> | |
5 | <link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /> | |
6 | <link rel='start' href='../index.html' title='Python Documentation Index' /> | |
7 | <link rel="first" href="ext.html" title='Extending and Embedding the Python Interpreter' /> | |
8 | <link rel='contents' href='contents.html' title="Contents" /> | |
9 | <link rel='last' href='about.html' title='About this document...' /> | |
10 | <link rel='help' href='about.html' title='About this document...' /> | |
11 | <link rel="prev" href="thinIce.html" /> | |
12 | <link rel="parent" href="refcounts.html" /> | |
13 | <link rel="next" href="cplusplus.html" /> | |
14 | <meta name='aesop' content='information' /> | |
15 | <title>1.10.4 NULL Pointers | |
16 | </title> | |
17 | </head> | |
18 | <body> | |
19 | <DIV CLASS="navigation"> | |
20 | <div id='top-navigation-panel' xml:id='top-navigation-panel'> | |
21 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
22 | <tr> | |
23 | <td class='online-navigation'><a rel="prev" title="1.10.3 Thin Ice" | |
24 | href="thinIce.html"><img src='../icons/previous.png' | |
25 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
26 | <td class='online-navigation'><a rel="parent" title="1.10 Reference Counts" | |
27 | href="refcounts.html"><img src='../icons/up.png' | |
28 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
29 | <td class='online-navigation'><a rel="next" title="1.11 Writing Extensions in" | |
30 | href="cplusplus.html"><img src='../icons/next.png' | |
31 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
32 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> | |
33 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
34 | href="contents.html"><img src='../icons/contents.png' | |
35 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
36 | <td class='online-navigation'><img src='../icons/blank.png' | |
37 | border='0' height='32' alt='' width='32' /></td> | |
38 | <td class='online-navigation'><img src='../icons/blank.png' | |
39 | border='0' height='32' alt='' width='32' /></td> | |
40 | </tr></table> | |
41 | <div class='online-navigation'> | |
42 | <b class="navlabel">Previous:</b> | |
43 | <a class="sectref" rel="prev" href="thinIce.html">1.10.3 Thin Ice</A> | |
44 | <b class="navlabel">Up:</b> | |
45 | <a class="sectref" rel="parent" href="refcounts.html">1.10 Reference Counts</A> | |
46 | <b class="navlabel">Next:</b> | |
47 | <a class="sectref" rel="next" href="cplusplus.html">1.11 Writing Extensions in</A> | |
48 | </div> | |
49 | <hr /></div> | |
50 | </DIV> | |
51 | <!--End of Navigation Panel--> | |
52 | ||
53 | <H2><A NAME="SECTION0031040000000000000000"></A><A NAME="nullPointers"></A> | |
54 | <BR> | |
55 | 1.10.4 NULL Pointers | |
56 | ||
57 | </H2> | |
58 | ||
59 | <P> | |
60 | In general, functions that take object references as arguments do not | |
61 | expect you to pass them <tt class="constant">NULL</tt> pointers, and will dump core (or | |
62 | cause later core dumps) if you do so. Functions that return object | |
63 | references generally return <tt class="constant">NULL</tt> only to indicate that an | |
64 | exception occurred. The reason for not testing for <tt class="constant">NULL</tt> | |
65 | arguments is that functions often pass the objects they receive on to | |
66 | other function -- if each function were to test for <tt class="constant">NULL</tt>, | |
67 | there would be a lot of redundant tests and the code would run more | |
68 | slowly. | |
69 | ||
70 | <P> | |
71 | It is better to test for <tt class="constant">NULL</tt> only at the ``source:'' when a | |
72 | pointer that may be <tt class="constant">NULL</tt> is received, for example, from | |
73 | <tt class="cfunction">malloc()</tt> or from a function that may raise an exception. | |
74 | ||
75 | <P> | |
76 | The macros <tt class="cfunction">Py_INCREF()</tt> and <tt class="cfunction">Py_DECREF()</tt> | |
77 | do not check for <tt class="constant">NULL</tt> pointers -- however, their variants | |
78 | <tt class="cfunction">Py_XINCREF()</tt> and <tt class="cfunction">Py_XDECREF()</tt> do. | |
79 | ||
80 | <P> | |
81 | The macros for checking for a particular object type | |
82 | (<code>Py<var>type</var>_Check()</code>) don't check for <tt class="constant">NULL</tt> pointers -- | |
83 | again, there is much code that calls several of these in a row to test | |
84 | an object against various different expected types, and this would | |
85 | generate redundant tests. There are no variants with <tt class="constant">NULL</tt> | |
86 | checking. | |
87 | ||
88 | <P> | |
89 | The C function calling mechanism guarantees that the argument list | |
90 | passed to C functions (<code>args</code> in the examples) is never | |
91 | <tt class="constant">NULL</tt> -- in fact it guarantees that it is always a tuple.<A NAME="tex2html4" | |
92 | HREF="#foot464"><SUP>1.4</SUP></A> | |
93 | <P> | |
94 | It is a severe error to ever let a <tt class="constant">NULL</tt> pointer ``escape'' to | |
95 | the Python user. | |
96 | ||
97 | <P> | |
98 | <BR><HR><H4>Footnotes</H4> | |
99 | <DL> | |
100 | <DT><A NAME="foot464">... tuple.</A><A | |
101 | href="nullPointers.html#tex2html4"><SUP>1.4</SUP></A></DT> | |
102 | <DD> | |
103 | These guarantees don't hold when you use the ``old'' style | |
104 | calling convention -- this is still found in much existing code. | |
105 | ||
106 | </DD> | |
107 | </DL> | |
108 | <DIV CLASS="navigation"> | |
109 | <div class='online-navigation'> | |
110 | <p></p><hr /> | |
111 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> | |
112 | <tr> | |
113 | <td class='online-navigation'><a rel="prev" title="1.10.3 Thin Ice" | |
114 | href="thinIce.html"><img src='../icons/previous.png' | |
115 | border='0' height='32' alt='Previous Page' width='32' /></A></td> | |
116 | <td class='online-navigation'><a rel="parent" title="1.10 Reference Counts" | |
117 | href="refcounts.html"><img src='../icons/up.png' | |
118 | border='0' height='32' alt='Up One Level' width='32' /></A></td> | |
119 | <td class='online-navigation'><a rel="next" title="1.11 Writing Extensions in" | |
120 | href="cplusplus.html"><img src='../icons/next.png' | |
121 | border='0' height='32' alt='Next Page' width='32' /></A></td> | |
122 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> | |
123 | <td class='online-navigation'><a rel="contents" title="Table of Contents" | |
124 | href="contents.html"><img src='../icons/contents.png' | |
125 | border='0' height='32' alt='Contents' width='32' /></A></td> | |
126 | <td class='online-navigation'><img src='../icons/blank.png' | |
127 | border='0' height='32' alt='' width='32' /></td> | |
128 | <td class='online-navigation'><img src='../icons/blank.png' | |
129 | border='0' height='32' alt='' width='32' /></td> | |
130 | </tr></table> | |
131 | <div class='online-navigation'> | |
132 | <b class="navlabel">Previous:</b> | |
133 | <a class="sectref" rel="prev" href="thinIce.html">1.10.3 Thin Ice</A> | |
134 | <b class="navlabel">Up:</b> | |
135 | <a class="sectref" rel="parent" href="refcounts.html">1.10 Reference Counts</A> | |
136 | <b class="navlabel">Next:</b> | |
137 | <a class="sectref" rel="next" href="cplusplus.html">1.11 Writing Extensions in</A> | |
138 | </div> | |
139 | </div> | |
140 | <hr /> | |
141 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
142 | </DIV> | |
143 | <!--End of Navigation Panel--> | |
144 | <ADDRESS> | |
145 | See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. | |
146 | </ADDRESS> | |
147 | </BODY> | |
148 | </HTML> |