Question

I’d like to write a small C program that reads from a file while it is actively being written to. Any ideas?

Answer 1

If you have control over the writing process you should use mmap() with MAP_SHARED in both reader and writer. This way the reader will see the changes done by the writer practically immediately.

Also, note that Linux does not make any snapshot of the data in the file when you open the file, so you should see the changes that are being made in the file even if you just use read() and lseek().

In order to determine whether a file was modified/opened/accessed/etc in Linux you can use inotify API (see inotify manpage). This allows you to make your process wait for an event you’re interested in until it occurs (as opposed to polling it regularly). You can also use epoll() or more traditional select() to accomplish similar result.