HowTo use "diff" and "patch"

You have created a program "prog.c", saved as this name and also copied to "prog.c.old".
You post "prog.c" to users. Next, you make changes to prog.c and want to release a patch

$ diff -c prog.c.old prog.c > prog.patch

Now, users can get the latest updates by running.

$ patch < prog.patch