bug551.cpp

1     static unsigned word_length = 0;
2
3     unsigned maxr( unsigned u, unsigned len )
4         {
5         unsigned n = 0, m;
6         unsigned lowbit = u & 1;
7
8         if( len == 0 ) return 0;
9         while( (u & 1) == lowbit && n < len )
10            { u >>= 1; n++; }
11        m = maxr( u, len-n );
12        return n > m ? n : m;
13        }
14
15    /* max_run(u) returns the maximum run of 0's or 1's in u */
16    unsigned max_run( unsigned u )
17        {
18        unsigned w = ~0u;
19        while( w ) { word_length++; w >>= 1; }
20        return maxr( u, w );
21        }

The programmer wrote a "wonderful" algorithm to determine the length of the maximum run of 0's or 1's in a word. Unfortunately there is a fatal flaw that renders his algorithm useless. Can you spot it?

bug551.cpp lint Output

--- Module:   bug551.cpp

    --- Wrap-up for Module: bug551.cpp

Warning 551: Symbol 'word_length' (line 1, file bug551.cpp) not accessed





Reference Manual Explanation 


551   Symbol 'Symbol' (Location) not accessed   -- A variable (declared
      static at the module level) was not accessed though the variable
      was referenced. This means that the value of a variable was never
      used.  Perhaps the variable was assigned a value but was never
      used. Note that a variable's value is not considered accessed by
      autoincrementing or autodecrementing unless the autoincrement/decrement
      appears within a larger expression, which uses the resulting value.
      The same applies to a construct of the form: var += expression.  If
      an address of a variable is taken, its value is assumed to be accessed.
      An array, struct or union is considered accessed if any portion thereof
      is accessed.



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