Common C Interview Questions

C interviews frequently test a candidate's understanding of low-level concepts such as memory management, pointers, and storage classes, since these topics reveal how well a candidate understands what is happening 'under the hood' in a program. This guide covers the most commonly asked C interview questions, each explained clearly with the reasoning an interviewer expects to hear. Mastering these concepts will help you confidently answer both theoretical and practical C interview rounds.

Q: What is the difference between a pointer and an array in C? A: An array name decays into a pointer to its first element in most expressions, but they are not identical. sizeof(arr) gives the total size of the array in bytes, while sizeof(ptr) gives the size of a pointer (typically 8 bytes on 64-bit systems). Arrays allocate fixed, contiguous memory at declaration and cannot be reassigned to point elsewhere, whereas pointers are variables that can be reassigned to point to different memory locations at any time.

Q: What is the difference between malloc() and calloc()? A: Both allocate memory dynamically on the heap, but malloc(size) allocates a single block of 'size' bytes with indeterminate (garbage) initial values, while calloc(n, size) allocates memory for 'n' elements of 'size' bytes each and initializes all bytes to zero. calloc() also internally guards against multiplication overflow, whereas malloc() requires the caller to compute the total size correctly.

Q: C only supports call by value, so how do we simulate call by reference? A: C passes arguments by value, meaning a function receives a copy of the argument. To let a function modify the caller's original variable, you pass the address of that variable (a pointer) instead. The function then dereferences the pointer to read or modify the original value, effectively simulating pass-by-reference behavior, as seen in functions like scanf("%d", &x) or a swap(int *a, int *b) function.

Q: What is the difference between a static variable and a global variable? A: A global variable is declared outside any function and has file scope with external linkage by default, meaning it can be accessed from other source files using extern. A static variable declared at file scope has internal linkage, restricting its visibility to the file it's declared in. A static variable declared inside a function retains its value between function calls (unlike a normal local variable) but is only visible within that function.

Q: What is structure padding, and why does it happen? A: Structure padding is the insertion of unused bytes between structure members (or after the last member) by the compiler to satisfy each member's memory alignment requirements, which allows the CPU to access data more efficiently. For example, a struct with a char followed by an int may have 3 padding bytes inserted after the char so the int starts at a 4-byte aligned address, making sizeof(struct) larger than the sum of its members' individual sizes.

Q: What does the const keyword guarantee, and where can it be placed with pointers? A: const marks a variable as read-only after initialization; any attempt to modify it causes a compile-time error. With pointers, placement matters: 'const int *p' means the data pointed to cannot be modified through p (but p itself can point elsewhere), 'int *const p' means p itself cannot be reassigned (but the data it points to can be modified), and 'const int *const p' means neither the pointer nor the pointed-to data can change.

Q: What does the volatile keyword do, and when is it needed? A: volatile tells the compiler that a variable's value may change at any time outside the normal flow of the program (for example, by hardware, an interrupt service routine, or another thread), so the compiler must not optimize away or cache reads/writes to that variable. It is commonly used for memory-mapped hardware registers and variables shared with interrupt handlers.

Q: Why do header files use include guards like #ifndef/#define/#endif? A: Header guards prevent a header file's contents from being included more than once in the same translation unit, which would otherwise cause 'redefinition' compile errors for types, functions, or macros if the header is directly or indirectly included multiple times (common with nested #include chains). #pragma once achieves the same effect with non-standard but widely supported compiler-specific syntax.

Q: What is a dangling pointer, and how can it be avoided? A: A dangling pointer points to memory that has already been freed or that has gone out of scope (such as returning the address of a local variable from a function). Dereferencing it causes undefined behavior. It can be avoided by setting a pointer to NULL immediately after calling free() on it, and by never returning addresses of automatic (stack) local variables from a function.