> > It was explained to me that the !pointer test wasn't guaranteed to be > > equivalent because of the way that the test is handled. > > Whoever explained that to you was wrong. 6.5.3.3 is the final word on > how "!x" is interpreted, and it *says* in the *text* that > "!x" === "x!=0". I don't see how this could be any clearer. > > > The spec fragments above don't address how the boolean test is > > coerced. Does it cast pointer to an integer and perform the test, or > > does it cast the 0 to a pointer and perform the test. The C++ spec I > > have is vague on this point. The only reference it makes to pointers > > is that the operand for ! may be a pointer. > > Because of the equivalence *given in the text of 6.5.3.3* we can simply > follow the money. (I'm not concerned, here, about what ambiguities the > C++ folks may or may not have introduced into their monstrosity. The > Linux kernel is written in C, and the C standard is unambiguous on this > point. Though frankly I'd be suprised if C++ breaks something so > straightforward and useful.) > > The section that defines != says > > 6.5.9 Equality operators > Syntax > (1) equality-expression: > relational-expression > equality-expression == relational-expression > equality-expression != relational-expression > Constraints > > (2) One of the following shall hold: > ... > -- one operand is a pointer and the other is a null pointer constant. > > (5) ... If one operand is a pointer and the other is a null pointer > constant, the null pointer constant is converted to the type of the > pointer. ... > > So: > 1. !x is defined equivalent to x!=0. > 2. 0 is a "null pointer constant". > 3. (assuming x is a pointer) 0 will be promoted to pointer type in the > expression "x!=0".
You are right to 99.9% ;) The last 0.1% of wrongness comes from linux/stddef.h: ... #define NULL ((void *)0) Thus, !ptr is equivalent to ptr==0 but not equivalent to ptr==NULL in general case for the kernel code (it is equivalent if ptr is a variable of a _pointer type_ because ptr then implicitly converted to (void*)). Our NULL isn't 0 by design. it's not a bug, regardless what Stroustrup says about NULL define. -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
