GATE CS 2023 Q26 — Runtime Environments

An activation tree depicts the execution flow of a program, where each node represents an activation of a function, and an edge from node $A$ to node $B$ indicates that function $A$ calls function $B$. Sibling nodes are ordered from left to right in the order of their invocation.

Let us trace the sequence of function calls starting from main():

1. main() first calls f1().
2. main() then calls f2(2):
   • Inside f2(2), f3() is called first.
   • Since $X = 2 \neq 1$, the else branch is executed, calling f2(1).
     • Inside f2(1), f3() is called first.
     • Since $X = 1$, the if condition is met, calling f1().
3. Finally, main() calls f3().

Note: The semicolon after if (X==1) in the code is a typographical error in the original GATE question and should be ignored for the logical flow.

The resulting hierarchical activation tree is:

      main
    /   |   \
   f1   f2   f3
       /  \
      f3  f2
         /  \
        f3  f1

This hierarchical structure corresponds to the tree shown in Option A.