Probably the most common example is sendfile() for writing file contents out to a socket without reading them into userspace:
I'm not very familiar with that though.
Then I found out Apache supports it via the EnableSendfile directive. Nice.
>This directive controls whether httpd may use the sendfile support from the kernel to transmit file contents to the client. By default, when the handling of a request requires no access to the data within a file -- for example, when delivering a static file -- Apache httpd uses sendfile to deliver the file contents without ever reading the file if the OS supports it.
That said, it's tricky to use if the server also does TLS termination... then you need kTLS, which is a much bigger can of worms.
* nginx: [1] * Haskell webserver module: [2] * caddy: [3]
[1]: https://nginx.org/en/docs/http/ngx_http_core_module.html#sen... [2]: https://hackage.haskell.org/package/warp-3.3.28/docs/Network... [3]: https://github.com/caddyserver/caddy/pull/5022