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Re: [Qemu-block] [PATCH 1/2] block/qapi: make two printf() formats liter

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 1/2] block/qapi: make two printf() formats literal
Date: Mon, 21 Mar 2016 15:14:48 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 03/09/2016 06:46 PM, Peter Xu wrote:
> On Wed, Mar 09, 2016 at 03:14:03PM -0700, Eric Blake wrote:
>>> +        func_fprintf(f, "%*s[%i]:%c", indentation * 4, "", i,
>>> +                     composite ? '\n' : ' ');
>> [The nerd in me wants to point out that you could avoid the ternary by
>> writing '"\n "[composite]', but that's too ugly to use outside of IOCCC
>> submissions, and I wouldn't be surprised if it (rightfully) triggers
>> clang warnings]
> Do you mean something like:
> int i = 0;
> printf("%c", '"\n "[i]');

You mean:

printf("%c", "\n "[i]);

(no '').  But with your declaration of 'i' as int, it is only defined if
i <= 2 (whereas in my example above, "\n "[composite] is always defined
because composite is bool rather than int).

> Is this a grammar btw?

Yes, C has an ugly grammar, because [] is just syntactic sugar for
deferencing pointer addition with nicer operator precedence.  Quoting

"The definition of the subscript operator [] is that E1[E2] is identical
to (*((E1)+(E2))).  Because of the conversion rules that apply to the
binary + operator, if E1 is an array object (equivalently, a pointer to
the initial element of an array object) and E2 is an integer, E1[E2]
designates the E2-th element of E1 (counting from zero)."

And a string literal is just a fancy way of writing the address of an
array of characters (where the address is chosen by the compiler).

Thus, it IS valid to dereference the addition of an integer offset with
the address implied by a string literal in order to obtain a character
within the string.  And since the [] operator is commutative (even
though no one in their right mind commutes the operands), you can also
write the even-uglier:

composite["\n "]

But now we've gone far astray from the original patch review :)

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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