On Sat, May 15, 1999 at 10:25:30PM +0100, Ben Laurie wrote:
>> typedef unsigned char des_cblock[8];
>> typedef const unsigned char const_des_cblock[8];
>> int des_set_key(const_des_cblock *key,des_key_schedule schedule);
>>
>> unsigned char key[7] = {...};
>> unsigned char deskey[8] = {...};
>> des_key_schedule ks;
>>
>> des_set_key(&key, ks); /* gets a warning */
>> des_set_key(&deskey, ks); /* no warning */
> I really don't like this. I can't quite figure out why, but it seems
> messy. What happens if you use the two definitions for des_cblock and
> const_des_cblock, but don't introduce the *s?
>
> Actually, I can figure out why. &key doesn't mean anything sensible: you
> can't have a pointer to key, [...]
Of course you can. key is an array, but except when used with sizeof
or &, an array is automagically turned into a pointer to the first
element of that array; this is usually O.K, but when you write
deskey key;
some_function(key);
it is not obvious from the program text that a pointer is passed and
not the value of key (which might be something else than an array,
after all).
(Another magic C rule turns array types in function prototypes into
the corresponding element-pointer types, so that
void some_function(deskey key);
is just the same as
void some_function(char *key);
but it's _not_ the same as
void some_function(deskey *key);.)
&key is a pointer to an array of a specific size, which means that the
compiler can detect more problems. For the encryption functions, most
of the arguments tend to be pointers (usually to char), so there's not
much the compiler can detect otherwise.
> because it is already as pointery as it can be.
When you define
deskey key;
(except in a function argument list), then key is not a pointer,
obviously; it's just that in most circumstances C rules convert it
into a pointer to the first array element. Function calls
some_function(key) and some_other_function(&key) are the same only if
you look at them at assember level; but as far as C types are
concerned, they are quite different:
void some_function(deskey key);
void some_other_function(deskey *key);
void yet_another_function(void)
{
deskey key = "12345678";
some_function(key);
some_other_function(&key);
}
void some_function(deskey key)
{
/* sizeof *key is 1, key[1] is '2' */
}
void some_other_function(deskey *key)
{
/* sizeof *key is 8, (*key)[1] is '2' */
}
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