Is this undefined C behaviour?

Question

Our class was asked this question by the C programming prof:

You are given the code:

int x=1;
printf(\"%d\",++x,x+1);

What output w

Answer 1

The points made about undefined behavior are correct, but there is one additional wrinkle: printf may fail. It's doing file IO; there are any number of reasons it could fail, and it's impossible to eliminate them without knowing the complete program and the context in which it will be executed.