C Developer Interview Questions &amp; Answers

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typedef creates an alias for a type. typedef struct Node { int data; struct Node *next; } Node; lets you write Node *head instead of struct Node *head. It is also useful for function pointer types and platform specific types. Some style guides avoid hiding pointers behind typedefs because it makes ownership and const usage less clear.

After calling realloc, the program sometimes crashes. What could be wrong?

Pointers and Memory ManagementMedium

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realloc may move the block and returns the new address, so old pointers to the block become invalid. Writing ptr = realloc(ptr, n) also leaks the original memory if realloc fails and returns NULL. Store the result in a temporary pointer, check for NULL, then assign. Make sure no other pointers still refer to the old location after a successful move.

What is the two-pointer technique and when is it used?

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Two pointers keep two indexes that move through an array according to a rule, often from both ends of a sorted array or at different speeds. It solves pair sum on sorted arrays, removing duplicates in place, reversing, merging sorted arrays and partitioning. It usually gives O(n) time and O(1) extra space instead of nested loops.

Merge two sorted arrays into a third sorted array.

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Walk both arrays with two indexes. void merge(const int *a, int n, const int *b, int m, int *out) { int i = 0, j = 0, k = 0; while (i < n && j < m) out[k++] = (a[i] <= b[j]) ? a[i++] : b[j++]; while (i < n) out[k++] = a[i++]; while (j < m) out[k++] = b[j++]; } Time O(n + m). Using <= keeps the merge stable.

What is the difference between a complete, full and perfect binary tree?

Trees and GraphsEasy

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A full binary tree has every node with zero or two children. A complete binary tree has all levels filled except possibly the last, which is filled from left to right; heaps use this shape so they fit an array. A perfect binary tree has all internal nodes with two children and all leaves at the same level, so it has 2 to the power h+1 minus 1 nodes.

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

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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.

What is the difference between char s[] = "hello" and char *s = "hello"?

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char s[] = "hello" creates a modifiable array of 6 bytes, usually on the stack, initialised with a copy of the literal. char *s = "hello" makes a pointer to a string literal stored in read-only memory; modifying it is undefined behaviour and often crashes. Declare such pointers as const char * so the compiler catches accidental writes.

Write a C program that counts lines, words and characters in a file.

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Read character by character with fgetc. FILE *fp = fopen(path, "r"); if (!fp) { perror("fopen"); return 1; } long lines = 0, words = 0, chars = 0; int c, inWord = 0; while ((c = fgetc(fp)) != EOF) { chars++; if (c == '\n') lines++; if (isspace(c)) inWord = 0; else if (!inWord) { inWord = 1; words++; } } fclose(fp); printf("%ld %ld %ld\n", lines, words, chars); c is int so EOF can be distinguished from valid bytes.

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.

You need to update one record in the middle of a large binary file without rewriting it all. How?

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Use fixed-size records so each one starts at index times record size. Open the file in r+b mode, fseek to that offset, fwrite the new record and fflush. Read it back to verify if needed. For crash safety, write to a temporary file and rename, or keep a simple journal. Databases like SQLite solve this robustly if requirements grow.

How do you pass a structure to a function efficiently?

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Passing a struct by value copies every member, which is costly for large structs. Pass a pointer instead, and add const when the function should only read it, such as void print(const Student *s). Access members with s->name. Small structs of a few bytes can be passed by value safely, and returning small structs by value is also fine.

Tell me about a time you explained a complex data structure to your team.

Linked ListsMedium

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Situation: our team needed an LRU cache in a C service and several members had not built one. Task: align everyone before coding. Action: I drew the hash table and doubly linked list on a whiteboard, walked through get and put with a small example and shared a short test suite covering edge cases. Result: the team implemented it in two days with no major review issues.

Implement your own strcmp function.

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Compare character by character until a difference or the end. int my_strcmp(const char *a, const char *b) { while (*a && *a == *b) { a++; b++; } return (unsigned char)*a - (unsigned char)*b; } Casting to unsigned char matches the standard behaviour for characters above 127. It returns zero if equal, negative if a sorts first and positive otherwise.

What does sizeof return for a pointer and for an array?

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sizeof on a pointer returns the size of the pointer itself, typically 8 bytes on a 64-bit system, regardless of what it points to. sizeof on an array declared in the same scope returns the total bytes of all elements, so sizeof arr / sizeof arr[0] gives the element count. Inside a function receiving an array parameter, sizeof returns pointer size, so pass the length separately.

Reverse a string in place in C.

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Swap characters from both ends moving inward. void reverse(char *s) { size_t n = strlen(s); if (n < 2) return; for (size_t i = 0, j = n - 1; i < j; i++, j--) { char t = s[i]; s[i] = s[j]; s[j] = t; } } Time O(n) and O(1) extra space. The string must be a modifiable array, not a literal.

Define a struct for a student and print all students with marks above 75.

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Use an array of structs and the dot operator. typedef struct { int roll; char name[50]; float marks; } Student; void printToppers(const Student *s, int n) { for (int i = 0; i < n; i++) { if (s[i].marks > 75.0f) printf("%d %s %.1f\n", s[i].roll, s[i].name, s[i].marks); } } Passing a const pointer avoids copying the whole array.

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.

How did you improve your problem solving skills?

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Situation: in my second year I could not solve even easy array problems in time. Task: become placement ready in six months. Action: I followed a topic-wise sheet, solved two problems daily in C, wrote down the pattern behind each one and revised mistakes every Sunday. Result: I solved over 300 problems, cleared three online coding tests and now explain complexity confidently.

A program reads a file with while (!feof(fp)) and processes the last line twice. Why?

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feof() becomes true only after a read has already failed at end of file. The loop checks feof before reading, so the final failed read leaves the old buffer content, which is processed again. Loop on the read function's return value instead, such as while (fgets(line, sizeof line, fp) != NULL), and check ferror after the loop.

What is the difference between text and binary file modes?

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In text mode, the C library may translate line endings, for example \r\n on Windows to \n, and a Ctrl+Z byte can mean end of file. Binary mode reads and writes bytes exactly as stored. Use text mode with fprintf and fgets for human readable files and binary mode with fread and fwrite for images, structs and any exact byte format. On Linux both behave the same.

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.

How is a 2D array stored in memory in C?

Arrays and StringsMedium

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C stores multidimensional arrays in row-major order: all elements of row 0, then row 1 and so on, in one contiguous block. Element a[i][j] of an array with C columns is at offset i * C + j. Traversing row by row is cache friendly and much faster than column by column for large matrices. When passing a 2D array to a function, every dimension except the first must be specified.

What are null, dangling, wild and void pointers?

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A null pointer holds NULL and points to nothing; dereferencing it crashes. A dangling pointer points to memory that has been freed or a local that went out of scope. A wild pointer is uninitialised and holds a random address. A void pointer is a generic pointer that can hold any object address but must be cast before dereferencing, as malloc returns void *.

Why is gets() dangerous and what should you use instead?

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gets() reads a line into a buffer without knowing its size, so long input overflows the buffer and can be used for code execution attacks. It was removed in C11. Use fgets(buf, sizeof buf, stdin), which stops at the buffer size and keeps the newline, then strip the newline if needed. For formatted input, give scanf field widths like %19s.

What is a monotonic stack?

Stacks, Queues and HashingMedium

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A monotonic stack keeps elements in increasing or decreasing order by popping elements that break the order before pushing a new one. Each element is pushed and popped at most once, so problems run in O(n). It solves next greater element, daily temperatures, stock span and largest rectangle in a histogram, which would otherwise need nested loops.

What are the main stages of compiling a C program?

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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.

Write a function that swaps two integers using pointers.

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Pass addresses so the function can modify the caller's variables. void swap(int *a, int *b) { int temp = *a; *a = *b; *b = temp; } int main(void) { int x = 3, y = 7; swap(&x, &y); printf("%d %d\n", x, y); // 7 3 return 0; } C passes everything by value, so pointers are needed to change caller data.

A file opened with fopen is NULL in production but fine on your machine. How do you debug?

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Always check the return value and print the reason with perror or strerror(errno), which shows errors like No such file or directory or Permission denied. Common causes are a relative path resolved from a different working directory, missing permissions for the service user, a full disk or too many open files. Use absolute or configured paths and close every file you open.

Write a C function to count words in a sentence.

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Count transitions from space to non-space. #include <ctype.h> int countWords(const char *s) { int count = 0, inWord = 0; for (; *s; s++) { if (isspace((unsigned char)*s)) { inWord = 0; } else if (!inWord) { inWord = 1; count++; } } return count; } This handles multiple spaces, tabs and leading or trailing spaces correctly.

What is an enum in C and what are its advantages?

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An enum defines a set of named integer constants, such as enum Color { RED, GREEN, BLUE };, where values start at 0 unless set. It makes code readable and lets compilers warn about missing cases in a switch with -Wswitch. Enum values are int sized in practice, and C23 allows specifying the underlying type. Use them for states, commands and error codes.

What is a pointer in C?

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A pointer is a variable that stores the memory address of another variable. int *p = &x; makes p hold the address of x, and *p reads or writes the value at that address. Pointers allow passing large data efficiently, modifying caller variables, working with arrays and strings, building dynamic data structures like linked lists and allocating memory at runtime.

Describe how you approach a coding problem you have never seen before.

Arrays and StringsMedium

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Situation: in a coding round I got an unfamiliar matrix problem. Task: solve it within 30 minutes. Action: I restated the problem, worked through a small example by hand, explained a brute force first, then spotted that a prefix sum would remove repeated work and coded it in C while talking through edge cases like empty rows. Result: I finished in 25 minutes and the interviewer praised my clear reasoning.

Write a function that dynamically allocates an array of n integers and fills it with squares.

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Check the allocation and let the caller free it. int *makeSquares(size_t n) { int *arr = malloc(n * sizeof *arr); if (arr == NULL) return NULL; for (size_t i = 0; i < n; i++) { arr[i] = (int)(i * i); } return arr; } // caller int *a = makeSquares(10); if (a) { /* use */ free(a); }

What is the difference between break and continue in C?

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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.

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

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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.

What is the difference between stack and heap allocation?

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Stack memory holds local variables, is allocated and freed automatically when functions enter and return, and is very fast but limited in size, often 1 to 8 MB. Heap memory is allocated with malloc and freed manually, can be large and outlive the function, but is slower and can fragment or leak. Large or variable sized data and data that must outlive a function belong on the heap.

What is the difference between fseek, ftell and rewind?

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fseek(fp, offset, origin) moves the file position relative to SEEK_SET, SEEK_CUR or SEEK_END. ftell(fp) returns the current position as a long. rewind(fp) moves to the start and clears error flags. Together they let you find file size, jump to a record in a fixed-size record file or re-read data. For very large files, use fseeko and ftello with off_t.

What is the difference between malloc, calloc, realloc and free?

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malloc(size) allocates uninitialised memory. calloc(n, size) allocates memory for n elements and sets all bytes to zero, and it checks n times size for overflow. realloc(ptr, size) resizes a block, possibly moving it, and returns the new address. free(ptr) releases memory back to the allocator. All allocators return NULL on failure, so always check the result.

What is the difference between an array and a pointer?

Pointers and Memory ManagementMedium

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An array is a block of contiguous elements whose size is fixed and known to the compiler; sizeof gives the full array size. A pointer is a variable holding an address; sizeof gives the pointer size. An array name decays to a pointer to its first element in most expressions, but it cannot be reassigned. When passed to a function, an array always becomes a pointer, losing its length.

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