400 C programming Interview Questions with Answers 2026
Course Description
Master C with 500+ deep-dive interview questions, memory management, and real-world systems programming.
C Programming Interview Practice Questions and Answers is the definitive resource I’ve built to help you bridge the gap between knowing the syntax and surviving a grueling technical interview at top-tier engineering firms. I have meticulously designed this question bank to challenge your understanding of everything from pointer arithmetic and manual memory management to advanced concurrency with pthreads and secure coding practices. Whether you are a student preparing for your first job or a senior engineer brushing up on low-level mechanics, I provide detailed explanations for every single option to ensure you don't just find the right answer, but actually master the underlying logic. By focusing on "why" code fails or succeeds—covering undefined behavior, memory leaks, and optimization—I’ve created a roadmap that transforms you from a coder into a systems-level professional ready to tackle any technical screening with confidence.
Exam Domains & Sample Topics
Core C Foundations: Syntax, storage classes, and the compilation model.
Memory Management: Heap vs. Stack, dynamic allocation, and pointer safety.
Data Structures & Algorithms: Implementation of linked lists, trees, and bitwise logic.
Systems Programming: Multithreading, IPC, signals, and function pointers.
Engineering & Tooling: GDB, Valgrind, Makefiles, and CERT C secure coding.
Sample MCQ Questions
Question 1: What is the behavior of the following code snippet? int *ptr = (int*)malloc(sizeof(int)); free(ptr); ptr = NULL; free(ptr);
A) Runtime Error: Double Free
B) Segmentation Fault
C) Memory Leak
D) No error; freeing a NULL pointer is safe
E) Compilation Error
F) Undefined Behavior
Correct Answer: D
Overall Explanation: In C, the free() function is explicitly defined by the standard to perform no action if the passed argument is NULL.
Option Explanations:
A: Incorrect; a double free only occurs if you free a non-NULL address twice.
B: Incorrect; free(NULL) does not access restricted memory.
C: Incorrect; the memory was freed in the first call, and the pointer was cleared.
D: Correct; the C standard guarantees that free(NULL) is a "no-op."
E: Incorrect; this is perfectly valid syntax.
F: Incorrect; this behavior is well-defined.
Question 2: Which keyword ensures a variable is always read from physical memory rather than a CPU register?
A) static
B) register
C) extern
D) auto
E) volatile
F) const
Correct Answer: E
Overall Explanation: The volatile qualifier tells the compiler that the value of a variable may be changed by something external to the visible code (like an interrupt or hardware register), preventing aggressive optimization.
Option Explanations:
A: Incorrect; static manages lifetime and visibility, not memory-reading behavior.
B: Incorrect; register is a hint to store it in a register, the opposite of this goal.
C: Incorrect; extern is for cross-file linkage.
D: Incorrect; auto is the default local storage class.
E: Correct; volatile forces a fresh memory read every time the variable is accessed.
F: Incorrect; const makes the variable read-only in the code logic.
Question 3: If ptr is a pointer to an integer, what does ptr++ do?
A) Increments the address by 1 byte
B) Increments the value stored at the address by 1
C) Increments the address by sizeof(int) bytes
D) Points to the previous integer in memory
E) Results in a syntax error
F) Decrements the address by sizeof(int)
Correct Answer: C
Overall Explanation: Pointer arithmetic is scaled by the size of the data type the pointer points to.
Option Explanations:
A: Incorrect; this would only happen if ptr was a char*.
B: Incorrect; that would require (*ptr)++.
C: Correct; the pointer moves to the start of the next integer.
D: Incorrect; ptr++ moves forward, not backward.
E: Incorrect; pointer incrementing is a fundamental C operation.
F: Incorrect; this describes ptr--.
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