The Essential C Language Interview Questions for Developers

C is one of the most popular and widely used programming languages in the world. As a system programming language, it offers low-level memory access, a small set of keywords and clean style – making C an excellent choice for programming systems software like operating systems, databases, and compilers.

With its longevity spanning over 50 years mastery of C is a must-have skill for any programmer. This means C language questions are almost guaranteed to come up in tech interviews especially for software engineer roles.

To help you prepare and shine in your next coding interview here are some of the most common and important C language interview questions that assess your fundamental knowledge

Basic C Language Interview Questions

Q1: What are the key features of C language?

Some of the most important features of C include:

• Portability – C is portable and not tied to any specific hardware or platform, enabling code reuse across different systems.

• Speed and efficiency – C code can be very fast and memory-efficient as it allows direct access to low-level structures.

• Low-level control – C provides a great degree of control over hardware, offering direct manipulation of registers and pointers.

• Modularity – C programs can be modularized into separate functions and files, improving code organization.

• Rich function libraries – The C standard library provides many built-in functions for common operations.

• Extensibility – C is highly extensible and can be embedded into many domains and applications.

Q2: What are tokens in C?

Tokens are the basic elements in a C program that are meaningful to the compiler. There are 5 main types of tokens:

• Keywords – Reserved words that have special meaning like if, else, while, etc.

• Identifiers – Programmer defined names for variables, functions, etc.

• Constants – Fixed values like numbers (5, 10.5), characters (‘a’, ‘$’), or strings (“Hello”).

• Operators – Symbols representing actions like +, -, *, /, etc.

• Special symbols – Symbols like semicolon ;, brackets { }, parenthesis ( ), etc.

Correct tokenization is the first phase of compilation in C.

Q3: What is the difference between macros and functions in C?

The main differences between macros and functions are:

• Macros are preprocessed directives that insert code during compilation while functions are true executables.

• Macro definitions are inserted by simple text substitution while functions need to be called explicitly.

• Macros don’t obey scope or type checking while functions are type-safe and follow scoping rules.

• Macros are faster as they are inserted at compile time but give less control vs runtime functions.

So macros are used when code needs to be inserted rapidly but functions provide better organization, maintainability, and type safety.

Q4: What is a pointer and how is it declared in C?

A pointer is a variable that stores the memory address of another variable or value. Pointers provide direct memory access which is often necessary in low-level programming.

Pointers are declared by specifying the pointer variable type followed by an asterisk *. For example:

int* ptr; //pointer to an integerchar* cptr; //pointer to a chardouble* dptr; //pointer to a double

To get the actual value being pointed to, the dereference operator * is used like *ptr.

Q5: What is the difference between ++i and i++ in C?

Both ++i and i++ increment the value of the integer variable i by 1. However, the key difference is:

• ++i increments i before returning the value of i.

• i++ returns the value of i first and then increments i.

So if i = 5, then ++i will return 6 whereas i++ will return 5. This pre and post increment difference is important in certain situations like nested function calls.

Intermediate C Language Interview Questions

Q6: What are the differences between structured and unstructured programming?

Structured programming follows a modular, top-down approach with hierarchical code organization, predefined code blocks like functions and selective branching constructs like if-else. This makes code easier to understand, test and maintain.

Unstructured programming does not follow logical hierarchy, modularization or flow control constructs. Code can branch arbitrarily using goto statements, making it spaghetti code that is hard to follow.

So modern software engineering uses structured programming principles for better code quality.

Q7: What are different storage classes in C?

Some common storage classes in C are:

• Automatic – Default local variables declared inside function. Allocated on stack and scope limited to function.

• External – Global variables. Visible to entire program. Scope is whole program lifetime.

• Static – Local variables that retain their value between function calls. Allocated in static storage instead of stack.

• Register – Variables stored in CPU registers for fastest access. Must have small scope.

• Extern – Used to declare external global variables already declared in another file.

Choosing the right storage class is important based on variable scope, lifespan and speed requirements.

Q8: What is recursion and when is it useful?

Recursion is the technique of a function calling itself repeatedly to solve a problem. The function keeps calling itself recursively until a base condition is reached.

Recursion is useful when the problem can be broken down into simpler repetitive sub-problems, like in tree traversal, sorting, divide-and-conquer, and various algorithms. It also provides an elegant, compact solution in such cases.

However, recursion is not always optimal and can sometimes be replaced by iterative approaches for efficiency.

Q9: How are precedence and associativity of operators used in C?

Operator precedence determines the order in which operators are evaluated in an expression. Operators with higher precedence like * and / are evaluated first before those with lower precedence like + and -.

Associativity determines the order of evaluation of operators with the same precedence. For example, * and / are left associative so they group from left to right.

Understanding these rules allow you to write unambiguous expressions without needing extra parentheses.

Q10: What is memory management in C?

Memory management in C refers to the effective allocation, use and deallocation of memory. Some key aspects are:

• Careful use of local vs global vs dynamic memory based on scope and size requirements.

• Allocating dynamic memory during runtime using malloc() and free() fromstdlib.h.

-Ensuring no memory leaks by freeing allocated memory that is no longer required.

• Avoiding bugs like dangling pointers by careful pointer management.

• Making efficient use of available memory using optimal data structures.

Careful memory management is crucial for building robust programs in C.

Advanced C Language Interview Questions

Q11: What are unions and how are they different from structs in C?

Both unions and structs allow aggregating different data types. The key difference is:

• Members of a struct occupy different memory locations.

• All members of a union share the same memory location.

So changing one member value effectively changes other members in a union. Unions save space but require more care while accessing members.

Q12: How can you access hardware using C?

Some ways to access hardware using C include:

• Direct memory access using pointers.

• Setting CPU registers for low-level control.

• Inline assembly code to execute machine instructions.

• Using special compiler intrinsics that map to native instructions.

• Interacting with peripheral devices via their memory-mapped address space.

• Leveraging Operating System APIs to interact with hardware drivers.

• Using hardware libraries like OpenCL, Vulkan, OpenGL for easier access.

Careful hardware access is needed for system programming tasks in C.

Q13: What are bitfields and when are they useful in C?

Bitfields allow packing multiple related flags/options in a compact integer variable without needing one bit per flag. Individual flag bits can then be examined or toggled using bitwise operators.

For example, representing 4 boolean options in one char:

struct options {  unsigned int bold : 1;  unsigned int italic : 1;   unsigned int underline : 1;  unsigned int strikeout : 1;} flags; 

Bitfields are useful for memory optimization and efficiently packing flags and options.

Q14: How can you debug C programs?

Some ways to debug C programs are:

• Using a debugger like GDB to step through code, set breakpoints and inspect variables.

• Liberal use of printf() statements to print out variable values and monitor program flow.

• Using logging libraries to log debug information to files that can be analyzed later.

• Performing sanity checks on values passed into an