On 13/05/2025 12:04 am, Demi Marie Obenour wrote: > On 5/12/25 2:25 PM, Elliott Mitchell wrote: >> On Mon, May 12, 2025 at 03:00:18PM +0200, Jan Beulich wrote: >>> On 12.05.2025 14:09, Andrew Cooper wrote: >>>> Now for the (new) controversial part. Since sending this, Linux has >>>> decided to just #define auto __auto_type for C < 23, in order to start >>>> writing C23 compatible code from now. It's more succinct, and has >>>> better longevity. >>>> >>>> We might want to consider the same, although it will introduce a new >>>> example of defining a keyword, which we'd have to call out in the >>>> MISRA/Eclair config. >>> I'm not outright opposed, as I don't think we use "auto" with its >>> original semantics, but it feels somewhat odd. >> Problem is "auto" already has a defined meaning in C.Having this will >> subtly break contributions from authors who weren't familiar with >> everything in Xen's headers. For anyone who does anything with projects >> besides Xen this will encourage bad habits. >> >> I believe many projects have a rule of *never* #define C keywords. I'm >> surprised such made it into the Linux kernel. I expect it will be ripped >> out in the near future. >> >> MISRA *doesn't* absolutely forbid this? > I'm no expert on the C standard, but my understanding is that "auto" was > redundant starting in C89, so it is almost entirely unused. C++11 and later > *do* heavily use "auto", and they use it for roughly the same purpose as C23 > does, so I suspect that contributors are far more likely to be familiar with > the C23 "auto" than they are with the pre-C23 version,
auto in older versions of C is a storage classifier, so grouped with static, extern and register. It is inherited from B, and along with K&R's having implicit int types, was there for familiarity of code to existing programmers. e.g. "auto a, b, c;" was B's way of saying "I'd like 3 ints on the stack please". It is very rare to see in C these days. C++11 repurposed 'auto' as a type, and C23 has followed suit. This is compatible with the prior meaning, and 'auto' can still be used as a storage classifier in C23. You can't however use 'auto auto'. In GCC/Clang prior to C23, the same behaviour is available from __auto_type. So. auto as a type inference keyword will be commonplace C in few years, just like it is already commonplace C++ for a decade. Right now in Xen, we can choose to either use something that is on the brink of becoming normal, or we can use the older form which will get changed at some point in the future. One of these makes far more sense than the other, considering that it is already standardised C23. ~Andrew
