[drysart]

Hopping in here to suggest that instead of running a persistent whole-ass daemon, you could just configure a systemd service, set it up to trigger off a write to a fifo, and then use filesystem permissions to restrict access to who can write to the fifo to whatever user/group should be allowed to perform the operation. (You can also do it by giving those users sudo access specifically to be able to trigger the service via systemctl; but if our goal here is to eliminate the use of setuid then any solution that uses sudo fails the assignment).

The systemd service executable is just your simple C program as-is.

Persistent whole-ass daemons aren't really the way it should be done even over in Windows, because in Windows you can attach ACLs to give permissions to start a Windows service to any arbitrary users that should be able to do so; which is spiritually equivalent to the Linuxy systemd solution.