GATE CS 1998 Q2.13 — Programming in C

To understand the output of the program, let's analyze the behavior of the expression result := f(x) * f(x) where x is initially 5:

• Standard Call-by-Reference: If the compiler evaluates the expression sequentially, the first call f(x) increments the global x to $6$ and returns $6$. The second call f(x) then increments the global x to $7$ and returns $7$, yielding $6 \times 7 = 42$.
• Call-by-Value-Result / Deferred Side-Effects: If the compiler uses call-by-value-result (copy-in, copy-out) or evaluates both operands before updating the global state, both calls to f(x) receive the initial value of x = 5. Each call increments its local copy to $6$ and returns $6$, yielding $6 \times 6 = 36$.

Since C ($36$) is the designated correct option, the evaluation model assumes call-by-value-result or deferred side-effect propagation where the global state is not updated between the operand evaluations.