What are common examiner traps in System Calls?

Assuming malloc() always issues a system call, ignoring user-space heap arena reuse Confusing a fast CPU mode switch (register/stack pointer save) with a heavy process context switch (PCB switch, page directory switch, TLB flush) Treating normal user-space function calls as kernel mode transitions Off-by-one errors when counting printf outputs in fork() loops by misidentifying when the loop counter updates or terminates across parent and child processes Forgetting that child processes inherit the current state of local variables like loop counters

GATE CSE System Calls PYQs

Standard yield2.6% of Operating Systems

GATE tests System Calls through two primary angles: conceptual understanding of dual-mode operation (distinguishing user-level library routines and mode switche… Read the guide

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Questions: 4
Asked: 2011–2024
Total marks: 5
Marks / paper: 0.2

4 GATE CSE System Calls PYQs with solutions, from Operating Systems. Browse all GATE CSE PYQs.

Questions

Every official paper that asked this topic, newest first.

GATE CSE 2024 Set 1 · Q57Consider 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--; }…NAT2 marksMediumSolve
GATE CSE 2023 · Q23Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?MSQ1 markEasySolve
GATE CSE 2021 Set 1 · Q14Which of the following standard C library functions will always invoke a system call when executed from a single-threaded process in a UNIX/Linux operating system?MSQ1 markMediumSolve
GATE CSE 2011 · Q6Let the time taken to switch between user and kernel modes of execution be $t_1$ while the time taken to switch between two processes be $t_2$ . Which of the following is TRUE?MCQ1 markEasySolve

What GATE tests in System Calls

Concepts, patterns, formulas and traps distilled from past papers.

GATE tests System Calls through two primary angles: conceptual understanding of dual-mode operation (distinguishing user-level library routines and mode switches from true kernel-level traps and process switches) and algorithmic tracing of process control system calls like fork() and wait(). Recent papers feature multi-select questions (MSQs) probing which events or C standard library functions strictly trigger kernel transitions, alongside numerical answer type (NAT) questions involving state-tree tracing over loops of system calls.

Core concepts

User mode vs Kernel mode transition triggers
Mode switch vs Process context switch overheads
C standard library memory management (malloc, heap arena) vs kernel system calls (brk, mmap)
Blocking/timer system calls (sleep, nanosleep)
Process termination (exit, exit)
Process hierarchy, fork() tree generation, and wait() semantics
Hardware traps, exceptions (page faults), and software interrupts

Recurring question patterns

Dual Mode Execution & Mode Switching Overheads
Commonmixed1 marks2023, 2011
Comparing the overhead and semantics of a user-to-kernel mode switch against a full process context switch, or identifying conditions (traps, page faults, interrupts) that mandate switching CPU execution levels.
Standard Library vs Kernel System Call Invocation
OccasionalMSQ1 marks2023, 2021
Evaluating standard C library functions (e.g., malloc, exit, sleep, strlen) to determine whether they execute entirely in user space or strictly necessitate invoking underlying system calls (brk, mmap, exit_group, nanosleep).
Process Creation & Control Flow Tracing (fork/wait)
CommonNAT2 marks2024
Tracing loop structures or sequential invocations containing fork(), wait(), and I/O statements to compute total processes generated or total execution counts of specific statements.

Formulas GATE uses

Total Processes after n successive fork() calls without condition branches

N = 2^n

Used when $n$ unconditional fork() calls execute in sequence or across $n$ iterations of a loop.

Cumulative Loop Statement Executions across Doubling Iterations

\text{Total Executions} = \sum_{i=1}^{k} 2^i = 2^{k+1} - 2

Used when each process in iteration $i$ executes a fork() followed by a print statement for $k$ iterations starting from 1 initial process.

Examiner traps in System Calls

Assuming malloc() always issues a system call, ignoring user-space heap arena reuse
Confusing a fast CPU mode switch (register/stack pointer save) with a heavy process context switch (PCB switch, page directory switch, TLB flush)
Treating normal user-space function calls as kernel mode transitions
Off-by-one errors when counting printf outputs in fork() loops by misidentifying when the loop counter updates or terminates across parent and child processes
Forgetting that child processes inherit the current state of local variables like loop counters

How questions are asked

Multiple-choice comparative analysis
Multi-select identification of system call / mode transition triggers
Code tracing and count calculation (NAT)

Paper trends

Early questions tested straightforward comparative qualitative properties of context switching via single-choice MCQs.
2011
The introduction of MSQs led to deeper testing of POSIX/C standard library behavior vs actual OS system calls, repeatedly using malloc() as a key distractor.
2023, 2021
Recent exams shifted quantitative tracing of fork() and wait() loops into 2-mark NAT format requiring rigorous step-by-step tree tracing.
2024

Difficulty notes

Easy questions test direct definitions (e.g., mode switch is faster than process switch, or system call transitions mode). Medium questions require distinguishing user-space library buffering/allocation (malloc) from kernel calls (sleep, exit) or tracing concurrent fork() loops over multiple iterations without arithmetic slips.