Re: lun - Lucky Number

On 16/03/2025 14:57, Marcel Logen wrote:

Richard Heathfield in sci.crypt:
 

<snip>

#define BIT_QRY(x,i) (((x)[(i)>>3] & (1<<((i)&7)))!=0)

 This is actually another C question:
 Why does it have to be "(x)" and "(i)" everywhere instead of
"x" and "i"? What kind of "protection" is meant here?

Turn to Number 3 in your songsheets if you will, to this rather simpler macro:
#define BAD_DOUBLE(x) 2*x
and this invocation:
int e = 1;
int f = 29;
int g = BAD_DOUBLE(e+f);
We'd hope for a result of 60, but what we get is 31 because BAD_DOUBLE(e+f) expands to 2*e+f which is 2*1+29, and precedence requires the * to happen before the +.
Parentheses solve the problem:
#define GOOD_DOUBLE(y) 2*(y)
int h = GOOD_DOUBLE(e+f);
which expands to 2*(e+f) and gives us the 60 we were after.
They are not, however, a complete solution:
#define DODGY_DOUBLE(x) (x)+(x)
looks solid enough, but DODGY_DOUBLE(e++)
will expand to (e++)+(e++)
which modifies a value twice between sequence points, and the resultant behaviour is undefined by the rules of the language.
If in doubt, steer well clear of side effects in macros.
--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
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