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Re: definition of NULL correct?
Arnaud Lacombe <lacombar%gmail.com@localhost> wrote:
> On Tue, Aug 11, 2009 at 4:32 PM, Greg Troxel<gdt%ir.bbn.com@localhost> wrote:
>> We define NULL as "(void *)0" rather than "((void *)0)", and that causes
>> sizeof NULL
>> to fail.
> Wouldn't it more be a gcc issue where it treats the expression as what
> c99 defines as "sizeof (type-name)" (cf section 6.5.3) leaving the "0"
> as a syntax error rather than interpreting the cast and treating the
> sizeof expression as "sizeof unary-expression" ? I'm not sure about
> the precedence of the cast operation over the 'sizeof' operator.
My reading of footnote 71 is that gcc gets the precedence right.
71)The syntax specifies the precedence of operators in the
evaluation of an expression, which is the same as the
order of the major subclauses of this subclause, highest
precedence first. Thus, for example, the expressions
allowed as the operands of the binary + operator (6.5.6)
are those expressions defined in 6.5.1 through 6.5.6.
The exceptions are cast expressions (6.5.4) as operands
of unary operators (6.5.3), [...]
sizeof is unary expression - 6.5.3, cast is 6.5.4. The exception
mentioned is for the production:
unary-expr: unary-operator cast-expr
and doesn't apply to sizeof. So
sizeof (void *) 0
is correctly parsed as uanry-expr: sizeof ( type-name ) leaving the
"0" as the syntax error.
Neat. I was always bothered by lack of parens around that cast and
always wondered if there is a context where it gonna bite us...
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