The order of evaluation in such an expression is entirely unspecified. There are points in C programs that separate a "before" and an "after". The semicolon i

The order of evaluation in such an expression is entirely unspecified. There are points in C programs that separate a "before" and an "after". The semicolon is a typical such point. There are others. Now between two adjacent points anything goes. In this particular example the evaluations and effects are allowed to be interleaved: pre = I + 1; // evaluating the pre-increment post = I; // evaluating the post-increment I++; // effect of pre-increment I++; // effect of post-increment return pre + post; // evaluating the sum Simply put, pre and post-increment are allowed to occur "simultaneously".