C Programming Interview Questions

Question 1

What are pointers in C?

Accepted Answer

Pointers are variables that store memory addresses, enabling direct memory manipulation, dynamic allocation, and efficient data passing. Declaration: int *ptr = &variable. Dereference: *ptr accesses the value. Pointer arithmetic: ptr++ moves by sizeof(type). NULL pointer: points to nothing. Use: dynamic memory, arrays, function pointers, data structures. Common errors: dangling pointers, memory leaks, buffer overflows.

Question 2

What is the difference between malloc and calloc?

Accepted Answer

malloc allocates uninitialized memory; calloc allocates and initializes to zero. Both return void* requiring casting. malloc(size): allocates size bytes, returns uninitialized memory. calloc(count, size): allocates count*size bytes, initializes all to 0. Both return NULL on failure. free() releases memory. realloc() resizes allocation. Always check return value. Memory leaks if free() forgotten.

Question 3

What are structures in C?

Accepted Answer

Structures group related variables of different types under one name, enabling custom data types. struct Point { int x; int y; }; Access via dot (.) or arrow (->) for pointers. typedef for aliases. Structures can contain other structures, arrays, function pointers. sizeof may include padding for alignment. Bit fields for compact storage.

Question 4

What is the difference between stack and heap memory?

Accepted Answer

Stack: automatic, fast, limited, LIFO for local variables. Heap: manual allocation, larger, slower, for dynamic data. Stack: function call frames, local variables, auto-freed on return, limited size (1-8MB typically), very fast. Heap: malloc/calloc/realloc, must free() manually, larger, slower due to allocation overhead. Stack overflow: deep recursion. Heap fragmentation: many small allocations/frees.

Question 5

What are function pointers?

Accepted Answer

Function pointers store addresses of functions, enabling callbacks, dynamic dispatch, and runtime function selection. Declaration: int (*fptr)(int, int) = &add. Call: fptr(3, 4). Use: callback functions, strategy pattern, dispatch tables, qsort comparator. typedef simplifies: typedef int (*operation)(int, int). Arrays of function pointers for state machines.

Question 6

How does memory management work in C?

Accepted Answer

Manual management with malloc/calloc/realloc/free. Programmer responsible for allocation, deallocation, and avoiding leaks. No garbage collector. Common issues: (1) Memory leaks: allocated but never freed. (2) Dangling pointers: freed memory still referenced. (3) Double free: freeing same memory twice. (4) Buffer overflow: writing beyond allocated bounds. Tools: Valgrind (leak detection), AddressSanitizer (bounds checking). Best practices: set pointers to NULL after free, use RAII-like patterns.

Question 7

What are volatile and const qualifiers?

Accepted Answer

const prevents modification; volatile tells compiler the value may change externally — don't optimize away reads. const int x = 5: cannot modify. const int *p: can't modify pointed value. int *const p: can't change pointer itself. volatile: used for hardware registers, signal handlers, shared memory — forces re-read every access. const volatile: read-only but externally changeable (read-only hardware register).

Question 8

Your C program crashes with segfault. How do you debug?

Accepted Answer

Use GDB debugger with backtrace, Valgrind for memory errors, and AddressSanitizer for compile-time detection. Steps: (1) Compile with -g flag. (2) Run in GDB: bt (backtrace), print variables, breakpoints. (3) Valgrind --tool=memcheck for memory errors. (4) Compile with -fsanitize=address for AddressSanitizer. (5) Common causes: NULL dereference, buffer overflow, use-after-free, stack overflow. (6) Check return values of malloc.

Question 9

Qualcomm: What are the differences between processes and threads?

Accepted Answer

Processes have separate memory spaces; threads share memory within a process. Threads are lighter but need synchronization. Process: own address space, own resources, IPC needed (pipes, shared memory). Thread: shared address space, shared file descriptors, lighter than processes. Process creation: fork(). Thread creation: pthread_create(). Threads need mutex/semaphore for shared data. Context switch: faster for threads. Use processes for isolation, threads for parallelism.

Question 10

Free C Programming Course - Learn Systems Programming & Algorithms

What You'll Learn

Career Opportunities

Course Completion Certificate

Course Modules Overview

Introduction to C Programming

Variables, Data Types & I/O

Operators & Expressions

Control Flow & Decision Making

Loops & Iteration

Functions & Modular Programming

Arrays & Strings

Pointers & Memory Management

Structures & File Handling

Data Structures in C

Algorithms & Problem Solving

Advanced C & Systems Programming

Course Curriculum

Introduction to C Programming

Variables, Data Types & I/O

Operators & Expressions

Control Flow & Decision Making

Loops & Iteration

Functions & Modular Programming

Arrays & Strings

Pointers & Memory Management

Structures & File Handling

Data Structures in C

Algorithms & Problem Solving

Advanced C & Systems Programming

Your Learning Path

Beginner

Intermediate

Advanced

Tools & Technologies

GCC

VS Code

Valgrind

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