RFR: 8354941: Build failure with glibc 2.42 due to uabs() name collision

[Andrew Haley]

On Thu, 17 Apr 2025 12:31:18 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> glibc 2.42 defines its own version of `uabs` when `stdlib.h` is being included. I propose to rename the hotspot version of `uabs` to `custom_uabs` (or some other suitable name) to avoid the name clash. The rename of the function is hidden by using a `UABS` macro instead in the code.
> 
> Testing:
> - [x] GHA (also [here](https://github.com/jerboaa/jdk/actions/runs/14513365723))
> - [x] Builds with this patch with GCC 15 where the build would fail earlier and pass with this patch.
> 
> Thoughts?

So here's the root of the problem: we define _GNU_SOURCE, which pulls in even nonstandard definitions that violate the usual namespace rules. In this case, defining uabs() which is proposed for standardization in C2Y. 

We're always going to be vulnerable to problems like this as long as we define _GNU_SOURCE, because we cannot predict what names will be used in future C and C++ standards. The only way we can really defend ourselves against this is to use a private namespace.

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PR Comment: https://git.openjdk.org/jdk/pull/24723#issuecomment-2833369061