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C Basics, Data Types and Operators Interview Questions for C Programming with DSA

25 C Basics, Data Types and Operators questions with answers for c programming with dsa roles — 11 easy, 11 medium, 3 hard.

About C Basics, Data Types and Operators

Start with how a C program is compiled and linked, then learn data types, operators, control flow, functions, storage classes, macros and the preprocessor. You also learn undefined behaviour, integer promotion, bit manipulation and safe compiler flags, which interviewers use to check whether you truly understand how C code behaves on real machines.

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01

What are the main stages of compiling a C program?

C Basics, Data Types and OperatorsEasy
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Preprocessing expands #include files and macros and removes comments. Compilation turns the preprocessed code into assembly. Assembly converts it into an object file of machine code. Linking combines object files with libraries such as libc to produce the executable. With gcc you can stop at each stage using -E, -S and -c, which helps when debugging macro or linker errors.
02

What are the basic data types in C and their typical sizes?

C Basics, Data Types and OperatorsEasy
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C has char (1 byte), short (usually 2), int (usually 4), long (4 on Windows, 8 on 64-bit Linux), long long (at least 8), float (4) and double (8). The standard only fixes minimum ranges, so sizes depend on the platform. Use sizeof to check and the fixed-width types from stdint.h such as int32_t and uint8_t when exact sizes matter.
03

What is the difference between a declaration and a definition in C?

C Basics, Data Types and OperatorsEasy
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A declaration tells the compiler a name and its type, such as extern int count; or a function prototype, without allocating storage. A definition actually creates the object or provides the function body, such as int count = 0;. A variable can be declared many times but defined only once across the program, otherwise the linker reports a multiple definition error.
04

What does the static keyword do in different contexts?

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Inside a function, a static local variable keeps its value between calls and is initialised only once. At file level, a static global variable or function has internal linkage, so it is visible only in that source file, which helps hide implementation details. Static variables are stored in the data or BSS segment, not the stack, and default to zero.
05

What is the difference between ++i and i++?

C Basics, Data Types and OperatorsEasy
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++i increments i and then yields the new value, while i++ yields the old value and then increments. As a standalone statement both have the same effect, and modern compilers generate the same code. The difference matters inside expressions: if i is 5, a = i++ sets a to 5, while a = ++i sets a to 6. Never use both on the same variable in one expression.
06

A program prints garbage values for a local variable. What is the likely cause?

C Basics, Data Types and OperatorsEasy
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Local automatic variables are not initialised in C, so they hold whatever bytes were on the stack. Reading them is undefined behaviour and gives random results. Initialise every variable at declaration, such as int sum = 0;. Compiling with -Wall -Wuninitialized and running under Valgrind or with -fsanitize=memory helps detect such reads early.
07

Write a C program to swap two numbers without using a third variable.

C Basics, Data Types and OperatorsEasy
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Using arithmetic or XOR through pointers: void swap(int *a, int *b) { if (a == b) return; *a = *a ^ *b; *b = *a ^ *b; *a = *a ^ *b; } The a == b check matters because XOR swapping a variable with itself sets it to zero. In real code a temporary variable is clearer and just as fast.
08

Write a C function to check whether a number is an Armstrong number.

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Sum of digits raised to the number of digits equals the number. int isArmstrong(int n) { int digits = 0, t = n, sum = 0; while (t) { digits++; t /= 10; } t = n; while (t) { int d = t % 10, p = 1; for (int i = 0; i < digits; i++) p *= d; sum += p; t /= 10; } return sum == n; } For example 153 = 1 + 125 + 27.
09

Write a header guard and explain why it is needed.

C Basics, Data Types and OperatorsEasy
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A header guard stops a header from being included twice in one translation unit, which would cause redefinition errors. #ifndef MATH_UTILS_H #define MATH_UTILS_H int add(int a, int b); #endif Most compilers also support #pragma once, which is shorter but not part of the C standard.
10

Why do you want to work in C and systems programming?

C Basics, Data Types and OperatorsEasy
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Situation: during college I built a small shell in C for an operating systems lab. Task: make it handle pipes and background jobs. Action: I learned fork, exec and file descriptors and debugged crashes with gdb. Result: I enjoyed understanding how software talks to hardware and the OS, and I want roles in embedded, systems or performance critical software where C skills matter.
11

What is the difference between break and continue in C?

C Basics, Data Types and OperatorsEasy
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break immediately exits the nearest enclosing loop or switch statement, and execution continues after it. continue skips the rest of the current loop iteration and moves to the next one; in a for loop the update expression still runs. In nested loops both affect only the innermost loop, so a flag or goto is needed to exit outer loops early.
12

Explain storage classes in C.

C Basics, Data Types and OperatorsMedium
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auto is the default for local variables, stored on the stack with block lifetime. register hints the variable should live in a CPU register, and its address cannot be taken. static gives a local variable lifetime for the whole program, or limits a global to its file. extern declares a variable defined in another file. Storage class decides scope, lifetime and linkage.
13

What is the difference between a macro and an inline function?

C Basics, Data Types and OperatorsMedium
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A macro is plain text substitution by the preprocessor, with no type checking, and arguments may be evaluated more than once, so SQUARE(i++) misbehaves. An inline function is a real function with type checks and single evaluation of arguments, which the compiler may expand in place for speed. Prefer static inline functions and use macros only for things like conditional compilation.
14

What is the volatile qualifier used for in C?

C Basics, Data Types and OperatorsMedium
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volatile tells the compiler that a variable can change outside normal program flow, for example a hardware register, a variable changed in an interrupt handler or memory shared with another device. The compiler must read it from memory every time and cannot cache it in a register or remove repeated reads. It does not provide atomicity or thread safety on its own.
15

What is the difference between const int *p, int *const p and const int *const p?

C Basics, Data Types and OperatorsMedium
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const int *p is a pointer to a constant int: you cannot change the value through p, but p can point elsewhere. int *const p is a constant pointer: p always points to the same address, but the value can change. const int *const p fixes both. Read declarations from right to left to understand them. const pointers to data are common in function parameters.
16

Explain the memory layout of a C program.

C Basics, Data Types and OperatorsMedium
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From low to high address, a typical process has the text segment holding code, the initialised data segment for globals and statics with values, BSS for uninitialised or zero globals, the heap growing upward for malloc memory, and the stack growing downward for function frames and local variables. Knowing this helps explain stack overflow, memory leaks and where variables live.
17

A loop that sums large numbers gives a negative result. What is happening?

C Basics, Data Types and OperatorsMedium
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The sum overflowed the range of int, and signed overflow in C is undefined behaviour, often wrapping to a negative value. Use a wider type like long long or int64_t for the accumulator, check before adding with a condition like if (sum > INT_MAX - x), or use compiler builtins such as __builtin_add_overflow. Compile with -fsanitize=undefined to catch it during testing.
18

Two source files both define a global variable named count and the build fails. How do you fix it?

C Basics, Data Types and OperatorsMedium
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Each global may be defined only once. Define it in one .c file, such as int count = 0;, and declare it in a shared header as extern int count;, which other files include. If the variable should be private to each file, mark it static in both files instead. Since GCC 10, -fno-common is default, so duplicate tentative definitions also fail to link.
19

Write C code to count the number of set bits in an integer.

C Basics, Data Types and OperatorsMedium
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Brian Kernighan's trick clears the lowest set bit each step. int countBits(unsigned int n) { int count = 0; while (n) { n &= n - 1; count++; } return count; } It runs once per set bit. GCC and Clang also offer __builtin_popcount(n), which uses a CPU instruction when available.
20

How do you set, clear and toggle a particular bit in C?

C Basics, Data Types and OperatorsMedium
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Use bit masks with shift operators. x |= (1u << n); // set bit n x &= ~(1u << n); // clear bit n x ^= (1u << n); // toggle bit n int bit = (x >> n) & 1u; // read bit n Use unsigned types so shifts are well defined. These operations are common in embedded code for configuring hardware registers.
21

Which gcc flags would you use to build and debug a C program safely?

C Basics, Data Types and OperatorsMedium
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A good development build is: gcc -std=c17 -Wall -Wextra -Wpedantic -g -O1 \ -fsanitize=address,undefined \ -o app main.c utils.c -Wall and -Wextra turn on useful warnings, -g adds debug symbols for gdb, and sanitizers catch memory errors and undefined behaviour at runtime. For release, drop sanitizers and use -O2.
22

Tell me about a time you debugged a hard crash in C code.

C Basics, Data Types and OperatorsMedium
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Situation: our data logger crashed randomly after hours of running. Task: find the cause before a client demo. Action: I rebuilt with AddressSanitizer, ran a long stress test and found a buffer overflow in a string copy that corrupted a nearby pointer. I replaced it with a bounded snprintf and added a length check. Result: the logger ran for a week without a crash and we added sanitizer builds to CI.
23

What is undefined behaviour in C? Give examples.

C Basics, Data Types and OperatorsHard
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Undefined behaviour means the C standard places no requirement on what happens, so the program may crash, give wrong results or appear to work. Examples are signed integer overflow, dereferencing a null or freed pointer, reading an uninitialised variable, accessing an array out of bounds and modifying a variable twice without a sequence point, such as i = i++. Compilers optimise assuming it never happens.
24

What is integer promotion and why does it matter?

C Basics, Data Types and OperatorsHard
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In expressions, types smaller than int, like char and short, are promoted to int before arithmetic. In mixed signed and unsigned comparisons, the signed value may convert to unsigned. So if (-1 < sizeof(int)) is false, because -1 becomes a huge unsigned number. These rules cause subtle bugs in embedded and systems code, so compile with -Wall -Wextra and use explicit casts carefully.
25

Code works with -O0 but fails with -O2. How do you approach this?

C Basics, Data Types and OperatorsHard
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This usually means the code relies on undefined behaviour that the optimiser exploits, such as uninitialised variables, out-of-bounds access, strict aliasing violations or missing volatile on shared hardware variables. Build with -Wall -Wextra and -fsanitize=address,undefined, then fix each report. If a variable is changed by an interrupt or another thread, add volatile or proper atomics.

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How important is C Basics, Data Types and Operators in C Programming with DSA interviews?

Start with how a C program is compiled and linked, then learn data types, operators, control flow, functions, storage classes, macros and the preprocessor. You also learn undefined behaviour, integer promotion, bit manipulation and safe compiler flags, which interviewers use to check whether you truly understand how C code behaves on real machines.

How should I use the C Basics, Data Types and Operators sheet?

Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.

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