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   9 <title>section 1.10: External Variables and Scope</title>
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  15 <H2>section 1.10: External Variables and Scope</H2>
  16
  17 page 31
  18 <p>There's a bit of jargon in this section.
  19 An <dfn>external</dfn> variable is what is sometimes called a global
  20 variable.
  21 The authors introduce the term <dfn>automatic</dfn> to refer to the
  22 local variables we've seen so far; this is a good word to
  23 remember, even if you never use it, because people will spring
  24 it on you when they're being precise, and if you don't know
  25 this usage you'll think they're talking about transmissions or
  26 something.
  27 (To be precise,
  28 ``local'' is a broader category than ``automatic'';
  29 there are both automatic and <TT>static</TT> local variables.)
  30 </p><p>Deep sentence:
  31 <blockquote>If [automatic variables] are not set,
  32 they will contain garbage.
  33 </blockquote>Actually,
  34 if automatic variables <em>always</em> contained garbage,
  35 the situation wouldn't be quite so bad.
  36 In practice,
  37 they often
  38 (though not always)
  39 do contain zero or some other predictable value,
  40 and this happens just often enough to lull you into the
  41 occasional false sense of security,
  42 by making
  43 a program with an inadvertently uninitialized variable
  44 seem
  45 to work.
  46
  47 </p><p>Deep sentence:
  48 <blockquote>An external variable must be <I>defined</I>,
  49 exactly once,
  50 outside of any function; this sets aside storage for it.
  51 The variable must also be <I>declared</I> in each function that
  52 wants to access it;
  53 this states the type of the variable.
  54 </blockquote>The basic rule is ``define once; declare many times.''
  55 As we'll see just below,
  56 it is not necessary for a declaration of an external variable
  57 to appear in every single function;
  58 it is possible for one external declaration to apply to many
  59 functions.
  60 (In the clause
  61 ``the variable must also be declared in each function'',
  62 the word ``declared'' is an adjective, not a verb.)
  63
  64 </p><p>page 33
  65 </p><p>In fact, the ``common practice'' of placing
  66 ``definitions of all external variables at the beginning of the source file''
  67 is so common that it's rare to see external declarations within functions,
  68 as in the functions on page 32.
  69 The authors are using the in-function <TT>extern</TT>
  70 declarations partly because it is an alternative style, and
  71 partly because we haven't talked about separate compilation
  72 (that is, building a single program from several separate
  73 source files) yet.
  74 Rather than jumping the gun and discussing those two topics now,
  75 I'll just mention that the discussion in section 1.10
  76 might be a bit misleading,
  77 and that you should probably wait
  78 until we get to the complete description of the issue in section 4.4
  79 before you commit any of this to memory.
  80 </p><p>Deep sentence:
  81 <blockquote>You should note that we are using the words
  82 <I>definition</I> and <I>declaration</I> carefully when we refer
  83 to external variables in this section.
  84 ``Definition'' refers to the place where the variable is created
  85 or assigned storage;
  86 ``declaration'' refers to places where the nature of the variable
  87 is stated but no storage is allocated.
  88 </blockquote>Do note the careful distinction;
  89 it's an important one and one which I'll be using, too.
  90 </p><p>page 34
  91 </p><p>The authors' criticism of the second (page 32) version of the
  92 longest-line program is accurate.
  93 The revision of the longest-line program to use external variables
  94 was done only to demonstrate the use of external variables,
  95 not to improve the program in any way
  96 (nor does it improve the program in any way).
  97 </p><p>As a general rule,
  98 external variables are acceptable for storing certain kinds of
  99 global state information which never changes,
 100 which is needed in many functions,
 101 and which would be a nuisance to pass around.
 102 I don't think of external variables as
 103 ``communicating between functions''
 104 but rather as
 105 ``setting common state for the entire program.''
 106 When you start thinking of an external variables as being one
 107 of the ways you communicate with a particular function,
 108 and in particular when you find yourself changing the value of
 109 some external variable just before calling some function,
 110 to affect its operation in some way,
 111 you start getting into the troublesome uses of external variables,
 112 which you should avoid.
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