bug579.cpp

1    #include <stdio.h>
2
3    void print_out( char n, ... )
4        {
5        if( n == 'i' )
6            printf( "%d", *(int *)(&n + 1) );
7        else if( n == 's' )
8            printf( "%s", *(char **)(&n + 1) );
9        }
10
11   int main()
12       {
13       print_out( 's', "June hath " );
14       print_out( 'i', 30 );
15       print_out( 's', " days.\n" );
16       return 0;
17       }

Our programmer is printing a message using a function with a variable argument list but something is going awry. Can you spot the problem?

bug579.cpp lint Output

--- Module:   bug579.cpp
                        _
void print_out( char n, ... )
bug579.cpp(3) : Warning 579: parameter preceding ellipsis has invalid type
                                        _
        printf( "%d", *(int *)(&n + 1) );
bug579.cpp(6) : Info 826: Suspicious pointer-to-pointer conversion (area too small)
                                          _
        printf( "%s", *(char **)(&n + 1) );
bug579.cpp(8) : Info 826: Suspicious pointer-to-pointer conversion (area too small)

Reference Manual Explanation

  
579   parameter preceding ellipsis has invalid type  -- When an ellipsis is used, the type 
      preceding the ellipsis should not be a type that would undergo a default promotion such 
      as char, short or float.  The reason is that many compilers' variable argument schemes 
      (using stdarg.h) will break down.