Re: How can Autoconf help with the transition to stricter compilation defaults?

[Paul Eggert <eggert@cs.ucla.edu>]

On 2022-11-10 10:19, Aaron Ballman wrote:
> In terms of the Clang side of things, I don't think we've formed any
> sort of official stance on how to handle that yet. It's UB (you can
> declare the C standard library interface without UB but calling any
> function with a mismatched signature is UB)

The Autoconf-generated code is never executed, so this is not a runtime 
issue; it's merely an issue of undefined behavior during translation. A 
problem could occur with a picky compiler or linker that rejects modules 
with mismatched function type declarations. Does Clang do that, or 
require or use such a linker? If not, there's no practical problem here. 
If so, it'd be helpful if Clang continued to support its traditional 
behavior that doesn't reject Autoconf's test cases, and for this to be 
the default.

Autoconf arose because one cannot ask something like "Can I call the 
renameat2 function with its usual signature?" in standard C code and one 
must escape into something like the shell to handle such questions. C23 
and GCC and Clang have added a few features to answer such questions, 
such as __has_include. But these features don't go nearly far enough. 
For example, __has_include tells me only whether the include file 
<foo.h> exists; it won't tell me whether I can successfully include 
<foo.h>, or whether <foo.h> will declare the function 'bar', or whether 
'bar' will have a signature compatible with my code's calls to 'bar'.

If I could request a single thing from the C23/GCC/Clang side, I'd ask 
for better facilities to be able to ask such questions within C code, 
without using the shell. Then a good chunk of Autoconf could dry up and 
blow away.

I realize that I'm asking for a lot. For example, a traditional 
implementation cannot answer the renameat2 question without consulting 
the linker. That being said, this ability is essential for modular 
programming at the low level, and if compilers don't provide this 
ability Autoconf will simply have to do the best it can, regardless of 
whether it generates source code that relies on undefined behavior.