GATE CS 2019 Q.27 (CS Q.17): Processes, Threads and System Calls

The following C program is executed on a Unix/Linux system:

#include <unistd.h>
int main()
{
    int i;
    for (i = 0; i < 10; i++)
        if (i % 2 == 0) fork();
    return 0;
}

The total number of child processes created is ________.

Your answer

Official answer and solution

Solution

Trace the loop to count the number of fork() calls and determine the total child processes created.

The loop runs for $i = 0, 1, 2, \dots, 9$ :

for (i = 0; i < 10; i++)
    if (i % 2 == 0) fork();

The condition i % 2 == 0 evaluates to true for even values of $i$ :

i \in \{0, 2, 4, 6, 8\}

Thus, the fork() statement is executed exactly $5$ times across the loop iterations.

Each fork() call duplicates all existing processes. Starting from $1$ initial parent process:

The total number of processes is $32$ . Since $1$ of them is the original parent process, the number of newly created child processes is:

N_{\text{child}} = 2^5 - 1 = 32 - 1 = 31

Final answer

Common trap

Reporting 32 instead of 31 by forgetting to subtract the initial parent process.

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