C Stack Bug Hunt: Fix Two Pop Errors in an RPN Calculator (Pre/Post-Decrement & Underflow)

C stack implementation exercise for a postfix RPN expression evaluator — debug two bugs in a hand-rolled integer stack backed by a fixed-size array. The first bug is a decrement-before-read error in the pop function: using pre-decrement returns the element below the actual top instead of the top element itself. The second bug is a missing underflow guard that causes undefined behavior when an operator consumes more operands than were pushed. Learn the critical difference between pre-decrement and post-decrement in index arithmetic, how stack-based RPN evaluation works token by token, and how to add a defensive bounds check to low-level C data structures. Both the broken evaluator and the corrected version with detailed explanation of each bug are provided.