Re: cast to 'void *'

blargg.h4g@xxxxxxxxxxxxx (blargg) writes:
Harald van =?UTF-8?b?RMSzaw==?= wrote:
On Fri, 05 Dec 2008 14:02:57 -0600, blargg wrote:
Rolf Magnus wrote:
Mark wrote:
[...]
void *p;
int *x;

p = x;
x = p;

printf("p=%p, x=%p\n", p, (void *)x);
return 0;
}

When else is implicit conversion to 'void *' done, except assignment
like above written?

Basically everywhere except variable argument lists.

void* to/from function pointer is also not implicit.

Implementations commonly provide both implicit and explicit conversions
between void * and function pointers as extensions, and neither is
available in standard C.

Argh! My statement above was about as relevant as

void* to/from struct tm is also not implicit.

Apologies.

Except that conversion between void* and struct tm is a constraint
violation. There is no implicit conversion, so such a conversion
would have to done via a cast operator. C99 6.5.4p2-3 says:

Constraints

2 Unless the type name specifies a void type, the type name shall
specify qualified or unqualified scalar type and the operand shall
have scalar type.

3 Conversions that involve pointers, other than where permitted by
the constraints of 6.5.16.1, shall be specified by means of an
explicit cast.

Conversion between void* and a function pointer, as I just wrote
elsethread, violates no constraint, but its behavior is undefined (but
of course an implementation is free to define it).

Incidentally, it seems to me that paragraph 3 is superfluous; any case
where it would apply is already covered by other constraints.

--
Keith Thompson (The_Other_Keith) kst-u@xxxxxxx <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
.

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