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Doesn't fwrite more or less write directly to the FILE buffer, if buffering is enabled?

I'm curious to take a closer look at fmtlib/fmt, which APIs treat FILE as non-opaque?

I'm curious how much speedup is gained from this.

With fwrite that would be another level of buffering in addition to FILE's buffer. If you are interested in what {fmt} is doing, a good starting point is https://github.com/fmtlib/fmt/blob/35dcc58263d6b55419a5932bd.... It is also possible to bypass stdio completely and get even faster output (https://vitaut.net/posts/2020/optimal-file-buffer-size/) and while it is great for files, it may introduce interleaving problems with things like stdout.