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authorTobias Klauser <tklauser@distanz.ch>2008-01-27 11:37:44 +0100
committerTobias Klauser <tklauser@xenon.tklauser.home>2008-01-27 11:37:44 +0100
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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>
+
+<hr>
+<p>
+<center>
+<table border=2 width="80%" bgcolor="ivory">
+<tr align=center>
+<td width="25%">
+<a href="../cref.html"> Top</a>
+</td><td width="25%">
+<a href="../master_index.html"> Master Index</a>
+</td><td width="25%">
+<a href="keywords.html"> Keywords</a>
+</td><td width="25%">
+<a href="../FUNCTIONS/funcref.htm"> Functions</a>
+</td>
+</tr>
+</table>
+</center>
+<p>
+<hr>
+<address>Martin Leslie
+</address><p>
+</body>
+</html>