alt Hacker Newsmmap is not a C feature, but POSIX. There are C platforms that don't provide mmap, and on those that do you can use mmap from other languages (there's mmap module in the Python's standard library, for example).
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I think this is sort of missing the point, though. Yes, mmap() is in POSIX[1] in the sense of "where is it specified".
But mmap() was implemented in C because C is the natural language for exposing Unix system calls and mmap() is a syscall provided by the OS. And this is true up and down the stack. Best language for integrating with low level kernel networking (sockopts, routing, etc...)? C. Best language for async I/O primitives? C. Best language for SIMD integration? C. And it goes on and on.
Obviously you can do this stuff (including mmap()) in all sorts of runtimes. But it always appears first in C and gets ported elsewhere. Because no matter how much you think your language is better, if you have to go into the kernel to plumb out hooks for your new feature, you're going to integrated and test it using a C rig before you get the other ports.
[1] Given that the pedantry bottle was opened already, it's worth pointing out that you'd have gotten more points by noting that it appeared in 4.2BSD.
And it's not just mmap(), all the functions in the code snippet except printf() are not actually C stdlib functions.