Require use of Modern C

[david-a-wheeler]

It's possible to use flags to enforce use if "modern" C (C89 or later). More info here:

https://fedoraproject.org/wiki/Changes/PortingToModernC

Perhaps those should be added. Some changes may cause autoconf problems, we shoukd document how to address them, e.g., define exit().

[david-a-wheeler]

One interesting aspect is that this recommends using -WError with specific values instead of "any warning". That has very different effects - turning all warnings into errors creates strong dependencies on specific compiler versions, but enumerating a few cases does not do the same thing. This would become something like (for GCC, I suspect clang is similar/identical):

-Werror=implicit-function-declaration -Werror=int-conversion
-Werror=incompatible-pointer-types
-std=gnu2x -Werror=old-style-definition

When not using autoconf (maybe we can create a workaround by declaring exit() and friends?):

-Werror=implicit-int

Maybe:

 -Werror=return-types

This needs more research. But expressly preventing dangerous constructs that often no longer work, and have been supported (or required) by C99, seems reasonable. 24 years is time enough to fix bad code :-).

[david-a-wheeler]

I just checked clang 14.0.3; it accepts:

-Werror=implicit-function-declaration -Werror=int-conversion
-Werror=incompatible-pointer-types
-std=gnu2x -Werror=old-style-definition
-Werror=implicit-int

It does not accept -Werror=return-types

[david-a-wheeler]

I contacted the autoconf developers. Looks like the latest version of autoconf will address some of these. It's planned to be released next week (December 2023). A discussion about this should soon show here: https://lists.gnu.org/archive/html/autoconf/2023-12/threads.html