Re: [Tinycc-devel] Fun little bug...

[Peter Lund]

On Tue, 2007-09-04 at 23:29 -0500, Rob Landley wrote:

> int enable = NULL || 0;
> 
> And NULL, of course, expands to ( ( void * ) 0 ).
> 
> What tcc is saying is that NULL || 0 is illegal.  Which is not, in fact, the 
> case.

You were right!

'||' requires that the two operands are scalars (§6.5.14 Logical OR
operator, paragraph 1).

Paragraph 2 says "The || operator shall yield 1 if either of its
operands compare unequal to 0; otherwise, it yields 0. The result has
type int."

Okay, so what is a scalar type?

§6.2.5 Types, paragraph 21:
"Arithmetic types and pointer types are collectively called scalar
types. Array and structure types are collectively called aggregate
types.[37])"

Sounds good so far.  Arithmetic types comprise integer types and
floating types.

What about "compare unequal to 0"?

§6.3.2.3 Pointers

Paragraph 3:
"An integer constant expression with the value 0, or such an expression
cast to type void *, is called a null pointer constant.[55] If a null
pointer constant is converted to a pointer type, the resulting pointer,
called a null pointer, is guaranteed to compare unequal to a pointer to
any object or function."

Footnote 55: "The macro NULL is defined in <stddef.h> (and other
headers) as a null pointer constant; see 7.17."

That didn't make us any wiser.


§6.5.9 Equality operators

Paragraph 2:
"One of the following shall hold:
— both operands have arithmetic type;
— both operands are pointers to qualified or unqualified versions of
compatible types;
— one operand is a pointer to an object or incomplete type and the other
is a pointer to a
   qualified or unqualified version of void; or
— one operand is a pointer and the other is a null pointer constant."

The first requirement is the one that fits 'NULL != 0'.

How pointers compare to integers is not really specified, as far as I
can see.  But if we can agree that NULL can be compared to 0 -- and I
believe we can -- then your initial statement (int enable = NULL || 0)
is perfectly valid.

Damn, I learned something new today.

-Peter