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| 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> | |
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| 141 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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