For example, `clang-format`. I'm finding a code formatter plugin for Geany. But all of the projects I encountered are unmaintained.
You don't need a plugin. Since `clang-format` can act as a filter (read stdin and write stdout) it can be run from a [custom command](https://www.geany.org/manual/current/index.html#sending-text-through-custom-...) and format the selection, or the whole file if no selection.
Same for any other formatter that acts as a filter.
It's also possible to use the LSP plugin https://github.com/techee/geany-lsp which supports formatting and clangd in your case.
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