200 C Language Interview Questions

Basics of C Programming

1. What is C programming language?
   • C is a high-level, general-purpose programming language that was developed by Dennis Ritchie at Bell Labs in 1972.
2. What are the key features of the C language?
   • Key features include simplicity, efficiency, portability, and flexibility. It also allows low-level memory access and supports structured programming.
3. What is a compiler in C?
   • A compiler is a program that translates C code into machine code that can be executed by a computer.
4. What is the difference between a compiler and an interpreter?
   • A compiler translates the entire code at once and creates an executable file, while an interpreter translates code line-by-line and executes it directly.
5. What is a syntax error in C?
   • A syntax error occurs when the code violates the grammatical rules of the C language.
6. What is a variable in C?
   • A variable is a storage location identified by a name that holds data which can be modified during program execution.
7. What are the basic data types in C?
   • Basic data types include int, char, float, and double.
8. What is a constant in C?
   • A constant is a value that cannot be altered during the execution of the program.
9. What is the purpose of the main() function in C?
   • The main() function is the entry point of any C program. Execution starts from the main() function.
10. How do you declare a variable in C?
    • Variables are declared by specifying the data type followed by the variable name, e.g., int a;.

Control Structures

11. What is an if statement?
    • An if statement is a control structure used to execute a block of code if a specified condition is true.
12. What is an else statement?
    • An else statement is used to execute a block of code if the corresponding if condition is false.
13. What is a switch statement?
    • A switch statement allows a variable to be tested against a list of values, each with its own code block.
14. What is a loop in C?
    • A loop is a control structure that repeats a block of code while a condition is true.
15. What is a for loop?
    • A for loop is used to execute a block of code a specific number of times.
16. What is a while loop?
    • A while loop executes a block of code as long as a specified condition is true.
17. What is a do-while loop?
    • A do-while loop executes a block of code once before checking the condition, and then it repeats the loop as long as the condition is true.
18. What is a break statement?
    • A break statement terminates the nearest enclosing loop or switch statement.
19. What is a continue statement?
    • A continue statement skips the remaining code inside the current loop iteration and starts the next iteration.
20. How do you implement nested loops in C?
    • Nested loops are implemented by placing one loop inside the body of another loop.

Functions

21. What is a function in C?
    • A function is a block of code that performs a specific task, can be called by its name, and can be reused.
22. What is the syntax for defining a function?
    • The syntax is: return_type function_name(parameters) { // body }.
23. What is a return type in a function?
    • The return type specifies the type of value that the function returns.
24. What are function parameters?
    • Function parameters are variables passed to the function to use in its operations.
25. What is a function prototype?
    • A function prototype declares a function’s name, return type, and parameters to the compiler before its actual definition.
26. What is the difference between a function declaration and definition?
    • A function declaration specifies the function’s name, return type, and parameters without the body, while the definition includes the actual body of the function.
27. What is recursion in C?
    • Recursion is a process where a function calls itself directly or indirectly.
28. What are the advantages of using functions?
    • Advantages include code reusability, modularity, easier debugging, and improved readability.
29. How do you call a function in C?
    • A function is called by using its name followed by parentheses containing any required arguments, e.g., function_name(arguments);.
30. What is a void function?
    • A void function does not return any value.

Arrays and Strings

31. What is an array in C?
    • An array is a collection of elements of the same type stored in contiguous memory locations.
32. How do you declare an array?
    • An array is declared by specifying the data type, the array name, and the size, e.g., int arr[10];.
33. What is the index of an array?
    • The index is the position of an element in the array, starting from 0.
34. How do you initialize an array?
    • An array can be initialized at the time of declaration, e.g., int arr[3] = {1, 2, 3};.
35. What is a multidimensional array?
    • A multidimensional array is an array of arrays, e.g., a two-dimensional array is like a matrix.
36. How do you declare a two-dimensional array?
    • A two-dimensional array is declared as data_type array_name[rows][columns];, e.g., int matrix[3][4];.
37. What is a string in C?
    • A string is an array of characters terminated by a null character \0.
38. How do you declare a string?
    • A string is declared as char str[size]; or initialized directly as char str[] = "text";.
39. What functions are used for string manipulation in C?
    • Common string functions include strlen(), strcpy(), strcat(), strcmp(), and strstr().
40. What is the difference between char str[] and char *str?
    • char str[] declares an array of characters, while char *str declares a pointer to a character.

Pointers

41. What is a pointer in C?
    • A pointer is a variable that stores the memory address of another variable.
42. How do you declare a pointer?
    • A pointer is declared by specifying the data type followed by an asterisk and the pointer name, e.g., int *ptr;.
43. What is the purpose of the & operator?
    • The & operator is used to get the address of a variable.
44. What is the purpose of the * operator?
    • The * operator is used to dereference a pointer, i.e., to access the value stored at the address held by the pointer.
45. What is a NULL pointer?
    • A NULL pointer is a pointer that does not point to any valid memory location.
46. How do you assign an address to a pointer?
    • You assign an address to a pointer using the address-of operator, e.g., ptr = &variable;.
47. What is pointer arithmetic?
    • Pointer arithmetic involves operations like incrementing, decrementing, and scaling pointers to navigate through memory locations.
48. What is the difference between int *ptr and int **ptr?
    • int *ptr is a pointer to an integer, while int **ptr is a pointer to a pointer to an integer.
49. What is a function pointer?
    • A function pointer is a pointer that points to a function’s address and can be used to call the function.
50. How do you pass a pointer to a function?
    • You pass a pointer to a function by specifying the pointer type in the function’s parameter list, e.g., void function(int *ptr);.

Structures and Unions

51. What is a structure in C?
    • A structure is a user-defined data type that groups related variables of different data types.
52. How do you define a structure?
    • A structure is defined using the struct keyword followed by the structure name and body, e.g., struct Person { char name[50]; int age; };.
53. How do you declare a structure variable?
    • A structure variable is declared by specifying the structure type followed by the variable name, e.g., struct Person person1;.
54. How do you access structure members?
    • Structure members are accessed using the dot operator, e.g., person1.age.
55. What is the purpose of the typedef keyword?
    • The typedef keyword is used to create an alias for a data type, including structures.
56. What is a union in C?
    • A union is a user-defined data type that allows storing different data types in the same memory location.
57. How do you define a union?
    • A union is defined using the union keyword, similar to structures, e.g., union Data { int i; float f; char str[20]; };.
58. How do you declare a union variable?
    • A union variable is declared by specifying the union type followed by the variable name, e.g., union Data data1;.
59. What is the difference between a structure and a union?
    • In a structure, each member has its own storage location, whereas in a union, all members share the same memory location.
60. How do you access union members?
    • Union members are accessed using the dot operator, e.g., data1.i.

Dynamic Memory Allocation

61. What is dynamic memory allocation?
    • Dynamic memory allocation is the process of allocating and freeing memory during the runtime using pointers.
62. What functions are used for dynamic memory allocation in C?
    • Functions include malloc(), calloc(), realloc(), and free().
63. What does the malloc() function do?
    • The malloc() function allocates a specified number of bytes and returns a pointer to the allocated memory.
64. How do you use the malloc() function?
    • malloc() is used as follows: ptr = (cast_type*) malloc(size);.
65. What is the calloc() function?
    • The calloc() function allocates memory for an array of elements, initializes all bytes to zero, and returns a pointer to the allocated memory.
66. How do you use the calloc() function?
    • calloc() is used as follows: ptr = (cast_type*) calloc(num_elements, element_size);.
67. What is the realloc() function?
    • The realloc() function changes the size of previously allocated memory block and returns a pointer to the reallocated memory.
68. How do you use the realloc() function?
    • realloc() is used as follows: ptr = (cast_type*) realloc(ptr, new_size);.
69. What is the free() function?
    • The free() function deallocates previously allocated memory to avoid memory leaks.
70. How do you use the free() function?
    • free() is used as follows: free(ptr);.

File I/O

71. What is file I/O in C?
    • File I/O refers to the operations of reading from and writing to files.
72. How do you open a file in C?
    • You open a file using the fopen() function, which returns a file pointer.
73. What are the different modes to open a file?
    • Modes include “r” (read), “w” (write), “a” (append), “r+” (read/update), “w+” (write/update), and “a+” (append/update).
74. What is the syntax for opening a file?
    • The syntax is: FILE *file_ptr = fopen("filename", "mode");.
75. How do you close a file?
    • A file is closed using the fclose() function, e.g., fclose(file_ptr);.
76. What is the purpose of the fprintf() function?
    • fprintf() is used to write formatted output to a file.
77. How do you use the fprintf() function?
    • fprintf() is used as follows: fprintf(file_ptr, "format", data);.
78. What is the fscanf() function?
    • fscanf() is used to read formatted input from a file.
79. How do you use the fscanf() function?
    • fscanf() is used as follows: fscanf(file_ptr, "format", &data);.
80. What is the fgets() function?
    • fgets() is used to read a string from a file.
81. How do you use the fgets() function?
    • fgets() is used as follows: fgets(buffer, size, file_ptr);.
82. What is the fputs() function?
    • fputs() is used to write a string to a file.
83. How do you use the fputs() function?
    • fputs() is used as follows: fputs(string, file_ptr);.
84. What is the fseek() function?
    • fseek() is used to move the file pointer to a specific location within the file.
85. How do you use the fseek() function?
    • fseek() is used as follows: fseek(file_ptr, offset, origin);.
86. What is the ftell() function?
    • ftell() returns the current position of the file pointer.
87. How do you use the ftell() function?
    • ftell() is used as follows: long pos = ftell(file_ptr);.
88. What is the rewind() function?
    • rewind() sets the file pointer to the beginning of the file.
89. How do you use the rewind() function?
    • rewind() is used as follows: rewind(file_ptr);.

Preprocessor Directives

90. What is a preprocessor directive in C?
    • Preprocessor directives are commands that are executed by the preprocessor before the compilation of the code begins.
91. What is the #include directive?
    • The #include directive is used to include the contents of a file or library in the program.
92. How do you include a standard library header file?
    • Use angle brackets, e.g., #include <stdio.h>.
93. How do you include a user-defined header file?
    • Use double quotes, e.g., #include "myheader.h".
94. What is the #define directive?
    • The #define directive is used to define macros or constants.
95. How do you define a macro?
    • Use #define followed by the macro name and replacement text, e.g., #define PI 3.14.
96. What is a macro with arguments?
    • A macro with arguments is a macro that takes parameters, e.g., #define SQUARE(x) ((x) * (x)).
97. What is the #undef directive?
    • The #undef directive is used to undefine a macro.
98. How do you use the #undef directive?
    • Use #undef followed by the macro name, e.g., #undef PI.
99. What is the #if directive?
    • The #if directive is used to include code based on a condition.
100. What is the #ifdef directive? – The #ifdef directive is used to check if a macro is defined.

Advanced Topics

101. What is a pointer to a function in C? – A pointer to a function points to the address of a function and can be used to call the function.
102. How do you declare a pointer to a function? – Syntax: return_type (*pointer_name)(parameter_list); e.g., int (*func_ptr)(int, int);.
103. What are command-line arguments? – Command-line arguments are parameters passed to the program at the time of execution.
104. How do you access command-line arguments in C? – Using int main(int argc, char *argv[]), where argc is the argument count and argv is the argument vector.
105. What is a static variable? – A static variable retains its value between function calls and has file scope if defined outside a function.
106. How do you declare a static variable? – Use the static keyword, e.g., static int count;.
107. What is a register variable? – A register variable is stored in a CPU register for faster access.
108. How do you declare a register variable? – Use the register keyword, e.g., register int counter;.
109. What is an external variable? – An external variable is declared outside of any function and is accessible to all functions within the file.
110. How do you declare an external variable? – Use the extern keyword in another file to access it, e.g., extern int number;.

Error Handling

111. What is error handling in C? – Error handling refers to the process of anticipating, detecting, and responding to errors in the program.
112. What is a runtime error? – A runtime error occurs during the execution of the program, such as division by zero or accessing an invalid memory location.
113. How do you handle errors in C? – Using error codes, return values, and the errno variable.
114. What is the errno variable? – errno is a global variable set by system calls and library functions in case of an error to indicate what went wrong.
115. How do you use the errno variable? – Include <errno.h> and check the value of errno after a function call.
116. What is the perror() function? – perror() prints a descriptive error message to stderr.
117. How do you use the perror() function? – perror() is used as follows: perror("Error message");.
118. What is the strerror() function? – strerror() returns a pointer to the textual representation of the current errno value.
119. How do you use the strerror() function? – strerror() is used as follows: printf("%s", strerror(errno));.
120. What is the exit() function? – exit() terminates the program and can return an exit status to the operating system.

Miscellaneous

121. What is a bitwise operator? – Bitwise operators perform operations on the binary representations of integers.
122. What are the common bitwise operators in C? – Common operators include & (AND), | (OR), ^ (XOR), ~ (NOT), << (left shift), and >> (right shift).
123. How do you perform a bitwise AND operation? – Use the & operator, e.g., result = a & b;.
124. How do you perform a bitwise OR operation? – Use the | operator, e.g., result = a | b;.
125. How do you perform a bitwise XOR operation? – Use the ^ operator, e.g., result = a ^ b;.
126. How do you perform a bitwise NOT operation? – Use the ~ operator, e.g., result = ~a;.
127. What is a left shift operation? – A left shift operation shifts bits to the left and fills the rightmost bits with zeros.
128. How do you perform a left shift operation? – Use the << operator, e.g., result = a << 1;.
129. What is a right shift operation? – A right shift operation shifts bits to the right and fills the leftmost bits with zeros or the sign bit (in case of signed integers).
130. How do you perform a right shift operation? – Use the >> operator, e.g., result = a >> 1;.

Advanced Data Structures

131. What is a linked list? – A linked list is a data structure consisting of nodes, where each node contains data and a pointer to the next node.
132. What are the types of linked lists? – Types include singly linked list, doubly linked list, and circular linked list.
133. How do you create a node in a linked list? – A node is created using a struct that contains data and a pointer to the next node.
134. What is a stack? – A stack is a linear data structure that follows the Last In First Out (LIFO) principle.
135. What are the basic operations of a stack? – Basic operations include push (insert), pop (delete), and peek (retrieve top element).
136. What is a queue? – A queue is a linear data structure that follows the First In First Out (FIFO) principle.
137. What are the basic operations of a queue? – Basic operations include enqueue (insert), dequeue (delete), and front (retrieve front element).
138. What is a binary tree? – A binary tree is a hierarchical data structure where each node has at most two children, referred to as left and right.
139. What is a binary search tree (BST)? – A BST is a binary tree in which the left child contains values less than the parent node, and the right child contains values greater than the parent node.
140. What are the common tree traversal methods? – Common methods include in-order, pre-order, and post-order traversal.

Concurrency and Multithreading

141. What is concurrency in C? – Concurrency refers to the ability to run multiple parts of a program simultaneously.
142. What is a thread in C? – A thread is a lightweight process that can run concurrently with other threads within the same program.
143. What library is commonly used for threading in C? – The POSIX Threads (pthreads) library.
144. How do you create a thread using pthreads? – Use pthread_create(), e.g., pthread_create(&thread_id, NULL, function, arg);.
145. What is the pthread_join() function? – pthread_join() waits for a thread to terminate.
146. What is a mutex? – A mutex is a synchronization primitive used to prevent multiple threads from accessing a shared resource simultaneously.
147. How do you initialize a mutex? – Use pthread_mutex_init(), e.g., pthread_mutex_init(&mutex, NULL);.
148. What is a deadlock? – A deadlock is a situation where two or more threads are blocked forever, each waiting for the other to release a resource.
149. How do you avoid deadlocks? – By using techniques like resource ordering, avoiding circular wait, and using timeout mechanisms.
150. What is a condition variable in pthreads? – A condition variable is used to block a thread until a particular condition is met.

More Topics

151. What is type casting in C? – Type casting is the conversion of one data type to another.
152. How do you perform explicit type casting? – By using the cast operator, e.g., int x = (int) 3.14;.
153. What is implicit type casting? – Implicit type casting is automatically performed by the compiler when compatible types are used.
154. What is a macro? – A macro is a fragment of code defined using the #define directive that can be inserted wherever its name appears.
155. What is the difference between #define and const? – #define is a preprocessor directive, while const is a keyword to define constant variables with type checking.
156. What is an enumeration in C? – An enumeration is a user-defined data type consisting of named integer constants.
157. How do you define an enumeration? – Using the enum keyword, e.g., enum Days {Sunday, Monday, Tuesday};.
158. What is the size of an int data type? – The size of an int is platform-dependent, typically 2 or 4 bytes.
159. What is a segmentation fault? – A segmentation fault occurs when a program tries to access an invalid memory location.
160. What is a buffer overflow? – A buffer overflow occurs when data exceeds the storage capacity of a buffer, leading to adjacent memory corruption.

Input/Output

161. What is the printf() function? – printf() is used to print formatted output to the standard output (stdout).
162. What is the scanf() function? – scanf() is used to read formatted input from the standard input (stdin).
163. How do you print a single character using printf()? – Use %c format specifier, e.g., printf("%c", ch);.
164. How do you print a string using printf()? – Use %s format specifier, e.g., printf("%s", str);.
165. How do you read an integer using scanf()? – Use %d format specifier, e.g., scanf("%d", &num);.
166. What is the putchar() function? – putchar() writes a single character to stdout.
167. What is the getchar() function? – getchar() reads a single character from stdin.
168. What is the puts() function? – puts() writes a string to stdout and appends a newline character.
169. What is the gets() function? – gets() reads a string from stdin until a newline character or EOF.
170. Why should gets() be avoided? – gets() should be avoided due to the risk of buffer overflow.

Advanced Memory Management

171. What is memory leak? – A memory leak occurs when allocated memory is not freed, causing the program to consume more memory over time.
172. How do you avoid memory leaks? – By ensuring that every allocated memory is properly freed using free().
173. What is the stack memory? – Stack memory is a region of memory used for static memory allocation, function calls, and local variables.
174. What is heap memory? – Heap memory is a region of memory used for dynamic memory allocation.
175. What is the difference between stack and heap memory? – Stack memory is limited, automatically managed, and faster, while heap memory is larger, manually managed, and slower.
176. What is fragmentation? – Fragmentation is the inefficient use of memory that leads to wasted space and reduced performance.
177. What is the difference between internal and external fragmentation? – Internal fragmentation occurs when allocated memory exceeds the requested memory, while external fragmentation occurs when free memory is scattered and non-contiguous.
178. How do you minimize fragmentation? – By using memory allocation techniques like compaction, paging, and segmentation.

Algorithms and Data Structures

179. What is an algorithm? – An algorithm is a step-by-step procedure for solving a problem or performing a task.
180. What are the characteristics of a good algorithm? – Characteristics include correctness, efficiency, clarity, and maintainability.
181. What is Big O notation? – Big O notation is used to describe the upper bound of the time complexity of an algorithm.
182. What are common time complexities in Big O notation? – Common time complexities include O(1), O(n), O(log n), O(n log n), O(n^2).
183. What is the difference between time complexity and space complexity? – Time complexity measures the time an algorithm takes to run, while space complexity measures the memory it uses.
184. What is a recursive function? – A recursive function is a function that calls itself.
185. What is base case in recursion? – The base case is the condition under which the recursive calls stop.
186. What is a sorting algorithm? – A sorting algorithm arranges elements in a specific order (ascending or descending).
187. What are common sorting algorithms? – Common sorting algorithms include bubble sort, selection sort, insertion sort, merge sort, and quick sort.
188. What is binary search? – Binary search is a search algorithm that finds the position of a target value within a sorted array.
189. How does binary search work? – Binary search repeatedly divides the search interval in half and compares the target value with the middle element until it finds the target or the interval is empty.
190. What is a hash table? – A hash table is a data structure that maps keys to values using a hash function.
191. What is a hash function? – A hash function converts a given key into an index in the hash table.
192. What is collision in a hash table? – A collision occurs when two keys are hashed to the same index.
193. How do you handle collisions in a hash table? – Collision handling techniques include chaining and open addressing.
194. What is a graph? – A graph is a data structure consisting of vertices (nodes) connected by edges (links).
195. What are the types of graphs? – Types include directed, undirected, weighted, and unweighted graphs.
196. What is a depth-first search (DFS)? – DFS is a graph traversal algorithm that explores as far as possible along each branch before backtracking.
197. What is a breadth-first search (BFS)? – BFS is a graph traversal algorithm that explores all nodes at the present depth before moving on to nodes at the next depth level.
198. What is a tree? – A tree is a hierarchical data structure with a root node and child nodes, where each child node can have its own child nodes.
199. What is a binary heap? – A binary heap is a complete binary tree where the value of each node is greater than or equal to (max-heap) or less than or equal to (min-heap) the values of its children.
200. What is dynamic programming? – Dynamic programming is an optimization technique that solves complex problems by breaking them down into simpler subproblems and storing the results of subproblems to avoid redundant computations.