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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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	Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
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	</div>

	<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">
	— 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 int</code><!-- /@w -->, <code>int</code> or <code>long int</code><!-- /@w -->), but might be a nonstandard
	type that exists only for this purpose.
	</p></blockquote></div>

	<!-- stddef.h -->
	<!-- ISO -->
	<div class="defun">
	— 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 int</code><!-- /@w --> or <code>unsigned long 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 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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