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   9 <title>The <cw>sizeof</> operator</title>
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  15 <H2>The <cw>sizeof</> operator</H2>
  16
  17 page 135
  18 <p>This
  19 may seem like an excessively roundabout or low-level way
  20 of finding the number of elements in an array,
  21 but it is the way it's done in C,
  22 and it's perfectly safe and straightforward
  23 once you get used to it.
  24 (I would, however, be hard-pressed to defend against the
  25 accusation that it's a bit too low-level.)
  26 </p><p>Note that <TT>sizeof</TT> works on both type names
  27 (things like <TT>int</TT>, <TT>char *</TT>, <TT>struct key</TT>, etc.)
  28 and variables
  29 (strictly speaking, any expression).
  30 Parentheses are required
  31 when you're using <TT>sizeof</TT> with a type name
  32 and optional
  33 when you're using it with a variable or expression
  34 (just like <TT>return</TT>),
  35 but it's safe to just always use parentheses.
  36 </p><p><TT>sizeof</TT> returns the size counted in <dfn>bytes</dfn>,
  37 where the C definition of ``byte'' is ``the size of a <TT>char</TT>.''
  38 In other words, <TT>sizeof(char)</TT> is always 1.
  39 (It turns out that it's not necessarily the case,
  40 though,
  41 that a byte or a <TT>char</TT> is 8 bits.)
  42 When we start doing our own dynamic memory allocation
  43 (which will be pretty soon),
  44 we'll always be needing to know the size of things
  45 so that we can allocate space for them,
  46 so it's just as well
  47 that we're meeting and getting used to the <TT>sizeof</TT> operator now.
  48 </p><p>The sentence
  49 ``But the expression in the <TT>#define</TT>
  50 is not evaluated by the preprocessor''
  51 means that,
  52 as far as the preprocessor is concerned,
  53 the ``value'' of the macro <TT>NKEYS</TT>
  54 (like the value of any macro)
  55 is just a string of characters like
  56 <pre>   (sizeof(keytab) / sizeof keytab[0])
  57 </pre>which it replaces wherever <TT>NKEYS</TT> is used,
  58 and which will then be evaluated by the compiler as usual,
  59 so it doesn't matter
  60 that the preprocessor wouldn't have known
  61 how to deal with the <TT>sizeof</TT> operator,
  62 or how big the <TT>keytab</TT> array or a <TT>struct key</TT> were.
  63 </p><p>A third way of defining <TT>NKEYS</TT> would be
  64 <pre>   #define NKEYS (sizeof(keytab) / sizeof *keytab)
  65 </pre></p><p>Note that
  66 the definition of <TT>NKEYS</TT>
  67 depends on
  68 the definition of the <TT>keytab</TT> array
  69 (which appears on page 133),
  70 and
  71 both of them
  72 will have to precede the use of <TT>NKEYS</TT>
  73 in <TT>main</TT> on page 134.
  74 (Also, all three will have to be in the same source file,
  75 unless other steps are taken.)
  76 </p><p>page 136
  77 </p><p>Notice that <TT>getword</TT> has a lot in common with the
  78 <TT>getop</TT> function of the calculator example
  79 (section 4.3, page 80).
  80 </p><hr>
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  89 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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