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<H2><A NAME="SECTION0031040000000000000000"></A><A NAME="nullPointers"></A>
<BR>
1.10.4 NULL Pointers

</H2>

<P>
In general, functions that take object references as arguments do not
expect you to pass them <tt class="constant">NULL</tt> pointers, and will dump core (or
cause later core dumps) if you do so.  Functions that return object
references generally return <tt class="constant">NULL</tt> only to indicate that an
exception occurred.  The reason for not testing for <tt class="constant">NULL</tt>
arguments is that functions often pass the objects they receive on to
other function -- if each function were to test for <tt class="constant">NULL</tt>,
there would be a lot of redundant tests and the code would run more
slowly.

<P>
It is better to test for <tt class="constant">NULL</tt> only at the ``source:'' when a
pointer that may be <tt class="constant">NULL</tt> is received, for example, from
<tt class="cfunction">malloc()</tt> or from a function that may raise an exception.

<P>
The macros <tt class="cfunction">Py_INCREF()</tt> and <tt class="cfunction">Py_DECREF()</tt>
do not check for <tt class="constant">NULL</tt> pointers -- however, their variants
<tt class="cfunction">Py_XINCREF()</tt> and <tt class="cfunction">Py_XDECREF()</tt> do.

<P>
The macros for checking for a particular object type
(<code>Py<var>type</var>_Check()</code>) don't check for <tt class="constant">NULL</tt> pointers --
again, there is much code that calls several of these in a row to test
an object against various different expected types, and this would
generate redundant tests.  There are no variants with <tt class="constant">NULL</tt>
checking.

<P>
The C function calling mechanism guarantees that the argument list
passed to C functions (<code>args</code> in the examples) is never
<tt class="constant">NULL</tt> -- in fact it guarantees that it is always a tuple.<A NAME="tex2html4"
  HREF="#foot464"><SUP>1.4</SUP></A>
<P>
It is a severe error to ever let a <tt class="constant">NULL</tt> pointer ``escape'' to
the Python user.

<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot464">... tuple.</A><A
 href="nullPointers.html#tex2html4"><SUP>1.4</SUP></A></DT>
<DD>
These guarantees don't hold when you use the ``old'' style
calling convention -- this is still found in much existing code.

</DD>
</DL>
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