Variables

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html21" HREF="c08.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html27" HREF="c08.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html29" HREF="node2.shtml"><img height=36 width=36 alt="NEXT" src="http://www.cs.utah.edu/icons/latex2html/next.gif"></A></td> <td><a href="contents-node1.shtml"><img height=36 width=36 alt="CONTENTS" src="http://www.cs.utah.edu/icons/latex2html/toc.gif"></a></td> <td><a href="reset.hamlet"><img height=36 width=36 alt="RESET" src="http://www.cs.utah.edu/icons/latex2html/stop.gif"></a></td> <td><a href="copyright.html"><img height=36 width=36 alt="COPYRIGHT" src="http://www.cs.utah.edu/icons/latex2html/copyright.gif"></a></td> <td><a href="mailto:hamlet@cs.utah.edu"><img height=36 width=36 alt="FEEDBACK" src="http://www.cs.utah.edu/icons/latex2html/feedback.gif"></a></td> <td>     <a href="http://www.cs.utah.edu/%7Ehamlet/icons.html"><img height=36 width=36 alt=ABOUT src=http://www.cs.utah.edu/icons/latex2html/about.gif></a>  </td> </table> </center> <HR><H1><A NAME="SECTION00010000000000000000">Variables</A></H1> <P> C provides more access to low-level implementation details that is typically afforded by high-level languages. This contributes to C's flexibility and its power as a systems programming language, but it also makes it possible to write obscure non-portable programs. It can also lead to confusion on the part of beginning C programmers. In this section we'll see a few examples of this low-level access. <P> C will allow a program to determine how many bytes are occupied by a variable of a particular type. This capability is useful in one situation--when a program needs to allocate space for a variable on the heap. Since we not discuss heap allocation until later in the course, this capability will not be any more of a curiosity to you until then. But it is kind of fun to play with. <P> Take a look at the program ``sizes.c'' in your ``examples'' directory (or <A HREF="../examples/sizes.c#sizesc">view it directly</A>). All it does is to declare several variables of different types and the calculate and display their sizes. If you run it, you can see how much space is taken up by various kinds of variables. <P> Seven kinds of variables are represented: characters, three different lengths of integers, and three different lengths of float point numbers. Does running this program reveal anything of interest? <P> <P><A NAME="tex2html4" HREF="answer22.html">Click here for the answer</A><P> <P> I want to emphasize that being able to determine the size of a variable has absolutely no practical value at the time being. Eventually, as mentioned above, we will find an application. <P> C will also let you determine the location of a variable in the computer's memory. That is, at any point in a program's execution a program can ask where a variable is currently located. Although we will not have much use for this capability until we begin talking about pointers, we have already used the capability several times as you will see shortly. <P> Take a look at the program ``addresses.c'' in your ``examples'' directory (or <A HREF="../examples/addresses.c#addressesc">view it directly</A>). This program simply declares the same set of variables as the last program, and then prints out their addresses (instead of their sizes). Run the program. Do you notice anything interesting? <P> <P><A NAME="tex2html5" HREF="answer32.html">Click here for the answer</A><P> <P> Remember: this may be somewhat uninteresting, but this is because the ability to calculate the address of a variable is not very useful at the moment. As we mentioned above, however, we have already used the address operator in some of our programs. 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