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<title>Null</title>
<head>
<script language="JavaScript">
</script>
</head>
<body bgcolor="#ffffcc">
<hr>
<center><h1>NULL</h1></center>
<hr>
<p>
NULL is used in several ways.
<ul>
<li>As a pointer to address zero. NULL is defined in several ANSI headers
as the
<a href="define_preprocessor.html">symbolic constant</a>
<a href="../CONCEPT/pointers.html#void">(void *)0</a>. Zero is often returned from
functions that normaly return a pointer to signal an error. It is therefore
conveniant to compare a function return code with NULL to catch errors.
<pre>
if ((fp=<a href="../FUNCTIONS/fopen.html">fopen</a>("/etc/hosts","r") == NULL)
{
exit(0);
}
</pre>
<p>
<li>To mark the end of a character string. A null character is used to
terminate the string. For example if you coded:
<pre>
char * text="abc";
</pre>
You will actually reserve <b>FOUR</b> bytes containing in ASCII hex.
<pre>
61 62 63 00
a b c \0
</pre>
The null at the end can be coded by using the
<a href="../FUNCTIONS/escape.html">escape sequence</a> '\0'.
</ul>
<p>
\0 is actually an octal <a href="../FUNCTIONS/escape.html">escape sequence</a>,
strictly speeking it should
be written as \000
<p>
<hr>
<h2>ANSI headers</h2>
The following headers define NULL.
<ul>
<li><a href="../FUNCTIONS/funcref.htm#headers">locale.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">stddef.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">stdio.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">stdlib.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">string.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">string.h</a>
<li><a href="../FUNCTIONS/funcref.htm#headers">time.h</a>
<p>
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<p>
<hr>
<address>Martin Leslie
</address><p>
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