Re: [hackers] [libgrapheme] Mark likely branches || Laslo Hunhold

From: Laslo Hunhold <dev_AT_frign.de>
Date: Wed, 5 Jan 2022 11:31:37 +0100

On Wed, 5 Jan 2022 02:24:01 +0600
NRK <nrk_AT_disroot.org> wrote:

Dear NRK,

> Answering my own question: because it fails if `__has_builtin` is not
> defined. I was expecting the 2nd expression wouldn't get evaluated at
> all. Should probably take some time and learn more about the
> pre-processor sometimes.

yes exactly. If you use normal non-function-like-macros that don't
exist, it works out, as they are simply replaced with 0 in such an
expression. At least GCC, from what I know of, always evaluates all
macros in each expression, and it seems to be undefined in the standard
if you do that beforehand.

It's different for function-like-macros: If they do not exist, it
throws an error instead of replacing it with 0, which can easily be
confirmed by trying to compile a file test.c containing

   #if defined (idontexist) && idontexist(test)
   #endif

yielding

   $ cc -o test test.c
   test.c:1:29: error: function-like macro 'idontexist' is not defined
   #if defined (idontexist) && idontexist(test)
                               ^
   1 error generated.
   $

This is also probably a good reason to always use nested ifdefs instead
of ifs, as using if-constructs leads to such surprises given the
macro-logic-operators don't seem to behave like the ones in the
language itself. Using ifdef forces you to only evaluate one condition
per line.

With best regards

Laslo
Received on Wed Jan 05 2022 - 11:31:37 CET