Implementing tail -f is surprisingly difficult and complicated…

Quora

The basic problem is to use the available system calls on the host operating system to detect when a file has new content in it.

For efficiency, both the GNU version and the NetBSD version make use of kernel support for notification of file modification if possible; GNU uses inotify and NetBSD uses kqueue. This involves registering interest in the specified file(s) with the kernel and then making a blocking call to the kernel which will return when something interesting (i.e., an append) has occured.

GNU & bsd sources

inotify

inotify.cr

c example

man