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   9 <title>4.4 Examples</title>
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  15 <H2>4.4 Examples</H2>
  16
  17 <p>Here is an example demonstrating
  18 almost
  19
  20 everything we've seen so far:
  21 <pre>
  22         int globalvar = 1;
  23         extern int anotherglobalvar;
  24         static int privatevar;
  25
  26         f()
  27         {
  28                 int localvar;
  29                 int localvar2 = 2;
  30                 static int persistentvar;
  31         }
  32 </pre>
  33 </p><p>Here we have six variables, three declared outside and three
  34 declared inside of the function <TT>f()</TT>.
  35 </p><p><TT>globalvar</TT> is a global variable.
  36 The declaration we see is its defining instance
  37 (it happens also to include an initial value).
  38 <TT>globalvar</TT> can be used anywhere in this source file,
  39 and it could be used in other source files, too
  40 (as long as corresponding external declarations are issued in
  41 those other source files).
  42 </p><p><TT>anotherglobalvar</TT> is a second global variable.
  43 It is <em>not</em> defined here;
  44 the defining instance for it
  45 (and its initialization)
  46 is
  47 somewhere else.
  48 </p><p><TT>privatevar</TT> is a ``private'' global variable.
  49 It can be used anywhere within this source file,
  50 but functions in other source files cannot access it,
  51 even if they try to issue external declarations for it.
  52 (If other source files try to declare
  53
  54 a global variable called
  55 ``<TT>privatevar</TT>'', they'll get their own;
  56 they won't be sharing this one.)
  57 Since it has static duration and receives no explicit initialization,
  58 <TT>privatevar</TT> will be initialized to 0.
  59 </p><p><TT>localvar</TT> is a local variable
  60 within the function <TT>f()</TT>.
  61 It can be accessed only within the function <TT>f()</TT>.
  62 (If any other part of the program declares a variable named
  63 ``<TT>localvar</TT>'',
  64 that variable will be distinct from the one we're looking at here.)
  65 <TT>localvar</TT> is conceptually ``created'' each
  66 time <TT>f()</TT> is called, and disappears
  67 when <TT>f()</TT> returns.
  68 Any value which was stored in <TT>localvar</TT> last time
  69 <TT>f()</TT> was running will be lost and will not be
  70 available next time <TT>f()</TT> is called.
  71 Furthermore,
  72 since it has no explicit initializer,
  73 the value of <TT>localvar</TT> will in general be garbage
  74 each time <TT>f()</TT> is called.
  75 </p><p><TT>localvar2</TT> is also local,
  76 and everything that we said about <TT>localvar</TT> applies to it,
  77 except that since its declaration includes an explicit initializer,
  78 it will be initialized to 2
  79 each time <TT>f()</TT> is called.
  80 </p><p>Finally, <TT>persistentvar</TT> is again local to <TT>f()</TT>,
  81 but it <em>does</em> maintain its value between calls to <TT>f()</TT>.
  82 It has static duration but no explicit initializer,
  83 so its initial value will be 0.
  84 </p><p>The defining instances and external declarations we've been
  85 looking at so far have all been of simple variables.
  86 There are also defining instances and external declarations of
  87 functions, which we'll be looking at in the next chapter.
  88 </p><p>(Also, don't worry about static variables for now
  89 if they don't make sense to you;
  90 they're a relatively sophisticated concept,
  91 which you won't need to use at first.)
  92 </p><p>The term <dfn>declaration</dfn> is a general one which
  93 encompasses defining instances and external declarations;
  94 defining instances and external declarations are two different
  95 kinds of declarations.
  96 Furthermore, either kind of declaration suffices to inform the
  97 compiler of the name and type of a particular variable (or function).
  98 If you have the defining instance of a global variable in a
  99 source file, the rest of that source file can use that variable
 100 without having to issue any external declarations.
 101 It's only in source files where the defining instance hasn't
 102 been seen that you need external declarations.
 103 </p><p>You will sometimes hear a defining instance referred to simply
 104 as a ``definition,''
 105 and
 106 you will sometimes hear an external declaration referred to simply
 107 as a ``declaration.''
 108 These usages are mildly ambiguous, in that you can't tell out
 109 of context whether a ``declaration'' is a generic
 110 declaration
 111 (that might be a defining instance or an external declaration)
 112 or whether it's an external declaration that specifically is
 113 not a defining instance.
 114 (Similarly, there are other constructions that can be called
 115 ``definitions'' in C, namely the definitions of
 116 preprocessor macros, structures, and typedefs, none of which
 117 we've met.)
 118 In these notes, we'll try to make things clear by using the
 119 unambiguous terms <dfn>defining instance</dfn>
 120 and <dfn>external declaration</dfn>.
 121 Elsewhere,
 122
 123 you may have to look at the context to determine how
 124 the terms ``definition'' and
 125 ``declaration'' are being used.
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