Structs as Arguments

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html22" HREF="c28.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html28" HREF="c28.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html30" 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">Structs as Arguments</A></H1> <P> Just like all other data types (other than arrays, which are always passed by reference), when structs are used as arguments they are passed by value. <P> This has the usual implication. Any changes made to an argument struct in a function will not be visible at the point of call. Recall that this is because, under call-by-value, a copy of each argument is made and passed along to the called function. <P> We previously introduced call-by-reference as a technique to use when you want changes made to a parameter to be reflected at the point of call. But with the introduction of structs, there is another reason to use call-by-reference. <P> Copying an int, float, char, or pointer is not a big deal, since each is relatively small (only a few bytes each). But copying an entire struct, especially a big one, is a reasonably expensive proposition. How much memory do you suppose that a struct ``person_info'' (from the examples in the last lesson) takes up? <P> <P><A NAME="tex2html4" HREF="answer15.html">Click here for the answer</A><P> <P> The solution is to pass a pointer to the struct, instead of the struct itself. Because the pointer takes up only four bytes or so, that's a big savings in copying. The called function can then access the struct indirectly through the pointer. This is a useful thing to do even if you aren't intending to modify the struct. For an example, take a look at ``struct4.c'' in your ``examples'' directory (or <A HREF="../examples/struct4.c#struct4c">view it directly</A>). How does it differ from ``struct3.c'' from the last lesson? <P> <P><A NAME="tex2html5" HREF="answer25.html">Click here for the answer</A><P> <P> This is all fairly straightforward, assuming that you already understand pointers. Take a look at the ``printf'' that makes up the body of ``writePerson''. Where we used to write, for example, ``p.first'', we now write ``(*p).first''. Why? <P> <P><A NAME="tex2html6" HREF="answer28.html">Click here for the answer</A><P> <P> Try two experiments. First change ``(*p).first'' to ``p.first.'' You should receive an error message as explained in the last paragraph. Next try changing it to ``*p.first'' (i.e., leave out the parentheses). Why do you still get the error message? <P> <P><A NAME="tex2html7" HREF="answer31.html">Click here for the answer</A><P><HR><center> <table border=1> <td><A NAME="tex2html22" HREF="c28.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html28" HREF="c28.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html30" 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><P><ADDRESS> <I><A NAME="tex2html1" HREF="http://www.cs.utah.edu/~hamlet">Hamlet Project</A> <BR> <A NAME="tex2html2" HREF="mailto:hamlet@cs.utah.edu">hamlet@cs.utah.edu</A></I> </ADDRESS> </BODY>