But there's no harm in doing that.

You can return the function atoi's output directly without defining an integer. Nice work. Keep it up

Grammar: Pointers is NOT equal! should be Pointers are NOT equal!

Yes needs, but i don't know how to do this tests.

Needs more tests (preferably randomized).

I think any non-zero exit code is failure by default, any different behavior is a rare exception that has to be dealt with manually.

function must call exit() with FAILURE parameter

There are no standards for exit code and hence "FAILURE parameter" is not telling what should be passed in to exit.

Don't think var name matters in such simple solutions. 'tot'|'sum' definetely better here but...

Best practice? Misleading variable name avg, should be sum or tot or x, or anything but avg