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<h3 class="section">A.4 Important Data Types</h3>

<p>The result of subtracting two pointers in C is always an integer, but the
precise data type varies from C compiler to C compiler.  Likewise, the
data type of the result of <code>sizeof</code> also varies between compilers. 
ISO defines standard aliases for these two types, so you can refer to
them in a portable fashion.  They are defined in the header file
<samp><span class="file">stddef.h</span></samp>. 
<a name="index-stddef_002eh-3738"></a>
<!-- stddef.h -->
<!-- ISO -->

<div class="defun">
&mdash; Data Type: <b>ptrdiff_t</b><var><a name="index-ptrdiff_005ft-3739"></a></var><br>
<blockquote><p>This is the signed integer type of the result of subtracting two
pointers.  For example, with the declaration <code>char *p1, *p2;</code>, the
expression <code>p2 - p1</code> is of type <code>ptrdiff_t</code>.  This will
probably be one of the standard signed integer types (<code>short&nbsp;int</code><!-- /@w -->, <code>int</code> or <code>long&nbsp;int</code><!-- /@w -->), but might be a nonstandard
type that exists only for this purpose. 
</p></blockquote></div>

<!-- stddef.h -->
<!-- ISO -->
<div class="defun">
&mdash; Data Type: <b>size_t</b><var><a name="index-size_005ft-3740"></a></var><br>
<blockquote><p>This is an unsigned integer type used to represent the sizes of objects. 
The result of the <code>sizeof</code> operator is of this type, and functions
such as <code>malloc</code> (see <a href="Unconstrained-Allocation.html#Unconstrained-Allocation">Unconstrained Allocation</a>) and
<code>memcpy</code> (see <a href="Copying-and-Concatenation.html#Copying-and-Concatenation">Copying and Concatenation</a>) accept arguments of
this type to specify object sizes.

        <p><strong>Usage Note:</strong> <code>size_t</code> is the preferred way to declare any
arguments or variables that hold the size of an object. 
</p></blockquote></div>

   <p>In the GNU system <code>size_t</code> is equivalent to either
<code>unsigned&nbsp;int</code><!-- /@w --> or <code>unsigned&nbsp;long&nbsp;int</code><!-- /@w -->.  These types
have identical properties on the GNU system and, for most purposes, you
can use them interchangeably.  However, they are distinct as data types,
which makes a difference in certain contexts.

   <p>For example, when you specify the type of a function argument in a
function prototype, it makes a difference which one you use.  If the
system header files declare <code>malloc</code> with an argument of type
<code>size_t</code> and you declare <code>malloc</code> with an argument of type
<code>unsigned int</code>, you will get a compilation error if <code>size_t</code>
happens to be <code>unsigned long int</code> on your system.  To avoid any
possibility of error, when a function argument or value is supposed to
have type <code>size_t</code>, never declare its type in any other way.

   <p><strong>Compatibility Note:</strong> Implementations of C before the advent of
ISO&nbsp;C<!-- /@w --> generally used <code>unsigned int</code> for representing object sizes
and <code>int</code> for pointer subtraction results.  They did not
necessarily define either <code>size_t</code> or <code>ptrdiff_t</code>.  Unix
systems did define <code>size_t</code>, in <samp><span class="file">sys/types.h</span></samp>, but the
definition was usually a signed type.

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