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   9 <title>5.2 Function Prototypes</title>
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  15 <H2>5.2 Function Prototypes</H2>
  16
  17 <p>In modern C programming,
  18 it is considered good practice to use prototype declarations
  19 for all functions that you call.
  20 As we mentioned, these prototypes help to ensure that the
  21 compiler can generate correct code for calling the functions,
  22 as well as allowing the compiler to catch certain mistakes you
  23 might make.
  24 </p><p>Strictly speaking, however, prototypes are optional.
  25 If you call a function for which the compiler has not seen a
  26 prototype,
  27 the compiler will do the best it can,
  28 assuming that you're calling the function correctly.
  29 </p><p>If prototypes are a good idea,
  30 and if we're going to get in the habit of writing
  31 function prototype declarations for
  32 functions we call that we've written (such as <TT>multbytwo</TT>),
  33 what happens for library functions such as <TT>printf</TT>?
  34 Where are their prototypes?
  35 The answer is in that boilerplate line
  36 <pre>
  37         #include &lt;stdio.h&gt;
  38 </pre>
  39 we've been including at the top of all of our programs.
  40 <TT>stdio.h</TT> is conceptually a file full of
  41 external declarations and other information
  42 pertaining to the ``Standard I/O''
  43 library functions, including <TT>printf</TT>.
  44 The <TT>#include</TT> directive
  45 (which we'll meet formally in a later chapter)
  46 arranges that all of the declarations within <TT>stdio.h</TT>
  47 are considered
  48
  49 by the compiler,
  50 rather as if we'd typed them all in ourselves.
  51 Somewhere within these declarations
  52 is an external function prototype declaration for <TT>printf</TT>,
  53 which satisfies the rule that there should be a prototype
  54 for each function we call.
  55 (For other standard library functions we call,
  56 there will be other ``header files'' to include.)
  57 Finally, one more thing about external function prototype declarations.
  58 We've said that the distinction between external declarations and
  59 defining instances of normal variables
  60 hinges on the presence or absence of the keyword <TT>extern</TT>.
  61 The situation is a little bit different for functions.
  62 The ``defining instance'' of a function is the function,
  63 including its body (that is, the brace-enclosed list of
  64 declarations and statements implementing the function).
  65 An external declaration of a function, even without the keyword
  66 <TT>extern</TT>, looks nothing like a function declaration.
  67 Therefore, the keyword <TT>extern</TT> is optional in
  68 function prototype declarations.
  69 If you wish,
  70 you can write
  71 <pre>
  72         int multbytwo(int);
  73 </pre>
  74 and this is just as good an external function prototype
  75 declaration as
  76 <pre>
  77         extern int multbytwo(int);
  78 </pre>
  79 (In the first form,
  80 without the <TT>extern</TT>,
  81 as soon as the compiler sees the semicolon, it knows it's not
  82 going to see a function body, so the declaration can't be a definition.)
  83 You may want to stay in the habit of using <TT>extern</TT>
  84 in all external declarations, including function declarations,
  85 since ``<TT>extern</TT> = external declaration''
  86 is an easier rule to remember.
  87 </p><hr>
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  96 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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