Consider the following C program. Assume parameters to a function are…

Concept

In C, the order in which a function's argument expressions are evaluated is unspecified by the standard; a compiler may pick any order. This problem fixes that order: the argument expressions of a call are evaluated right to left. Each call to g or h prints its value at the moment it executes, so the printed sequence is exactly the order in which the calls fire.

Application — trace the calls in firing order

Start at f(g(10), h(20)). Its two argument expressions are evaluated right to left before f runs, then f's body runs top to bottom.

1. h(20) is the right argument, so it fires first: it prints 20 and returns 20.

2. g(10) is the left argument, fires next: it prints 10 and returns 10.

3. With both arguments evaluated, f(10, 20) is invoked. Inside f, g(x) runs first and prints 10.

4. Then h(y) runs and prints 20.

Cross-check

Reading the four prints in firing order gives 20, 10, 10, 20. Concatenated with no separators (each printf uses "%d"), the program writes 20101020. Note the symmetry: argument evaluation contributes "20 10" (right-to-left), and f's body contributes "10 20" (the parameters x, y in declaration order), so the two halves are mirror images.