lang/C/the.ansi.c.programming.language/notes.accompany.ansi.c/sx7d.html

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   9 <title>section 4.4: Scope Rules</title>
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  15 <H2>section 4.4: Scope Rules</H2>
  16
  17 page 80
  18 <p>With respect to the ``practical matter''
  19 of splitting the calculator program up into multiple source files,
  20 though it's certainly small enough
  21 to fit comfortably into a single source file,
  22 it's not so small that there's anything wrong
  23 with splitting it up into multiple source files,
  24 especially if we start adding functionality to it.
  25 </p><p>The <dfn>scope</dfn> of a name is what we have been calling its ``visibility.''
  26 When we say things like ``calling a function with a prototype in scope''
  27 we mean that a prototype is visible, that a declaration is in effect.
  28 </p><p>The variables <TT>sp</TT> and <TT>val</TT> can be used by the
  29 <TT>push</TT> and <TT>pop</TT> routines because they're
  30 defined in the same file
  31 (and the definitions appear before <TT>push</TT> and <TT>pop</TT>).
  32 They can't be used in <TT>main</TT> because no declaration for
  33 them appears in <TT>main.c</TT>
  34 (nor in <TT>calc.h</TT>, which <TT>main.c</TT> <TT>#include</TT>s).
  35 If <TT>main</TT> attempted to refer to <TT>sp</TT> or <TT>val</TT>,
  36 they'd be flagged as undefined.
  37 (Don't worry about the visibility of
  38 ``<TT>push</TT> and <TT>pop</TT> themselves.'')
  39 </p><p>The paragraph beginning ``On the other hand''
  40 is explaining how global (``external'') variables like <TT>sp</TT>
  41 and <TT>val</TT> could be accessed in a file other than the file
  42 where they are defined.
  43 In the examples we've been looking at,
  44 as we've said,
  45 <TT>sp</TT> and <TT>val</TT> can be used in <TT>push</TT> and <TT>pop</TT>
  46 because the variables are
  47 defined above the functions.
  48 If the variables were defined elsewhere
  49 (i.e. in some other file),
  50 we'd need a
  51 declaration above--and
  52 that's exactly what <TT>extern</TT> is for.
  53 (See page 81 for an example.)
  54 <br></p><p>page 81
  55 </p><p>A definition <em>creates</em> a variable,
  56 and
  57 for any given global variable,
  58 you only want to do that once.
  59 Anywhere
  60
  61 else,
  62 you want to refer to an existing variable,
  63 created elsewhere,
  64 without creating a new, conflicting one.
  65 Referring to an existing variable or function
  66 is exactly
  67 what a
  68 declaration is for.
  69 </p><p>Note also that the definition may optionally initialize the variable.
  70 (Don't worry about why a declaration may optionally include an
  71 array dimension.)
  72 </p><p>``This same organization would also be needed
  73 if the definitions of <TT>sp</TT> and <TT>val</TT>
  74 followed their use in one file''
  75 means that we could conceivably have,
  76 in one file,
  77 <pre>           extern int sp;
  78                 extern double val[];
  79 <br>
  80 <br>
  81                 void push(double f) { ... }
  82                 double pop(void) { ... }
  83 <br>
  84 <br>
  85                 int sp = 0;
  86                 double val[MAXVAL];
  87 </pre>So ``<TT>extern</TT>'' just means ``somewhere else'';
  88 it doesn't <em>have</em> to mean ``in a different file,''
  89 though usually it does.
  90 </p><hr>
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  99 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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