GATE CS 2024 Set 1 Q.57: Processes, Threads and System Calls

Consider the following code snippet using the fork() and wait() system calls. Assume that the code compiles and runs correctly, and that the system calls run successfully without any errors.

int x = 3;
while(x > 0) {
  fork();
  printf("hello");
  wait(NULL);
  x--;
}

The total number of times the printf statement is executed is _______

Your answer

Official answer and solution

Official answer: 14
From: Official GATE 2024 answer key

Solution

Count the number of active processes after each fork() call across the 3 loop iterations (x = 3, 2, 1).

Tracing the number of processes executing the loop body across each value of x:

Iteration 1 (x = 3): $1$ initial process calls fork(), resulting in $2$ processes. Both execute printf("hello") (2 times) and decrement their copy of x to $2$ .
Iteration 2 (x = 2): All $2$ processes call fork(), resulting in $4$ processes. All $4$ execute printf("hello") (4 times) and decrement x to $1$ .
Iteration 3 (x = 1): All $4$ processes call fork(), resulting in $8$ processes. All $8$ execute printf("hello") (8 times) and decrement x to $0$ , terminating the loop.

Total number of times printf is executed $= 2 + 4 + 8 = 14$ .

Final answer

Common trap

Confusing the total number of printf executions ( $2 + 4 + 8 = 14$ ) with the final number of processes ( $2^3 = 8$ ) or child processes ( $2^3 - 1 = 7$ ).

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 1 (CS1)
Question: 57 of 65
Section: Computer Science and Information Technology

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