Top 40 Most Asked C Programming Interview Questions
1. What makes C a mid-level programming language, and why?
C is often termed a mid-level programming language because it incorporates features of both high-level and low-level languages. It allows for direct manipulation of hardware through pointers (low-level) while also providing constructs such as functions and control structures (high-level). This versatility makes C suitable for system programming, including operating systems and embedded systems, as well as application development.
2. What characteristics does the C language have?
Some of the C language’s features include:
- Easy to Use and Effective: C provides a clear structure and is relatively easy to learn.
- Machine Independent/Portable: C code can be compiled on different machines with minimal changes.
- Middle-Level Language: Combines features of both high and low-level languages.
- Structured Programming: Promotes clear and structured code through functions.
- Rich Function Library: Includes a variety of built-in functions for various tasks.
- Dynamic Memory Management: Supports allocation and deallocation of memory at runtime.
- Speed: C programs are typically fast due to low-level operations.
- Pointers: Allows for direct memory access and manipulation.
- Expandable: Can be extended with libraries and frameworks.
3. What do tokens mean?
Tokens are the smallest elements of a program in C. The six types of tokens are:
- Identifiers: Names for variables and functions.
- Keywords: Reserved words with special meaning (e.g., int, return).
- Constants: Fixed values that do not change (e.g., 100, 3.14).
- Operators: Symbols that perform operations (e.g., +, -, *).
- Special Characters: Characters with special functions (e.g., {, }, ;).
- Strings: Sequences of characters enclosed in double quotes (e.g., “Hello”).
4. Why are the functions printf() and scanf() used? Describe format specifiers as well?
- printf(): Used for outputting formatted data to the console.
- scanf(): Used for reading formatted input from the user.
Format specifiers are used in these functions to define the type of data being handled. Common format specifiers include: - %d: Integer values.
- %s: Strings.
- %c: Character values.
- %f: Float values.
5. What does the expression 5[“abcdef”] represent?
The expression 5[“abcdef”] evaluates to ‘f’. This is due to the way arrays are indexed in C; it can be understood as *(5 + “abcdef”), which accesses the sixth element of the string.
6. What in C is a built-in function?
Built-in functions, or library functions, are predefined functions provided by the C standard library to perform specific tasks. Examples include printf(), scanf(), strcpy(), strlen(), and strcmp(), among others.
7. What is a preprocessor, exactly?
A preprocessor is a tool that processes source code before compilation. It handles directives (e.g., #include, #define, #ifdef) to include files, define macros, and manage conditional compilation.
8. What does the #line in C do?
The #line directive is used to set the current line number in the source file. This can be useful for debugging or when generating code dynamically, as it allows you to specify a new line number for error reporting.
9. How do you turn a number into a string?
You can use the sprintf() function to format a number as a string. For example:
Code Snippet:
char buffer[50];
sprintf(buffer, “Value of Pi = %f”, 3.141593);
10. In C, what is recursion?
Recursion occurs when a function calls itself directly or indirectly to solve a problem. A recursive function typically has a base case to terminate the recursion and reduce the problem size in each call. For example:
Code Snippet:
void do_recursion() {
// Base case
if (condition) return;
do_recursion(); // Recursive call
}
11. Why is function overloading not supported in C?
Function overloading is not supported in C because the language does not provide a mechanism to differentiate between functions with the same name based on their parameter types. C maintains the names of functions as they are compiled, which would lead to ambiguity without name mangling, a technique used in C++.
12. What distinguishes a static int declaration from a global int declaration?
A global variable has a global scope, meaning it can be accessed from any file within the program. In contrast, a static variable has local scope and retains its value between function calls, but cannot be accessed outside of its defining function.
Code Snippet:
static int my_static_var = 0; // Local to this file
int my_global_var = 0; // Accessible from any file
13. In C, what is a pointer?
A pointer is a variable that stores the memory address of another variable. It allows for direct manipulation of memory and is used to create dynamic data structures, among other purposes. For example:
Code Snippet:
int a = 10;
int *p = &a; // p holds the address of a
14. What is the difference between char const and const char p?**
Both char const* p and const char* p are equivalent; they declare a pointer to a constant character. This means you cannot modify the character being pointed to, but you can change the pointer to point to another character.
15. What in C is pointer to pointer?
A pointer to a pointer is a variable that holds the address of another pointer. This allows for multiple levels of indirection. For example:
Code Snippet:
int a = 5;
int *p = &a; // p points to a
int **pp = &p; // pp points to p
16. Why does n + + execute more quickly than n + 1?
The n++ operation is typically more efficient because it is a unary operation and involves less overhead than the binary operation n = n + 1, which requires reading and writing the value of n. However, the actual performance difference can be negligible on modern hardware.
17. What does typecasting in C mean?
Typecasting is the process of converting a variable from one data type to another. This can be done explicitly in C using the syntax:
Code Snippet:
int x = (int)3.14; // Casts a float to an int
18. What benefits does macro have over function?
Macros are expanded in place by the preprocessor, which can result in faster execution since there’s no function call overhead. However, they can increase the size of the compiled code and lack type safety. Functions, on the other hand, provide better type checking and debugging capabilities.
19. What are enumerations, exactly?
Enumerations (enums) are a user-defined data type in C that consists of integral constants, making the code easier to read and maintain. For example:
Code Snippet:
enum Days {Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday};
20. When should the register storage specifier be used?
The register storage specifier suggests to the compiler that the variable will be heavily used, and it may store it in a CPU register for faster access. However, the compiler can ignore this request.
21. What are the various categories of decision control statements?
Control statements allow for branching in the execution flow based on conditions. Categories include:
- If statement: Executes code if a condition is true.
- Else statement: Executes code if the preceding if condition is false.
- Else-if statement: Checks multiple conditions.
- Switch statement: Executes code based on the value of a variable.
- Nested if-else statement: If statements within another if statement.
22. What do an r-value and a l-value mean?
An r-value is a value that resides on the right side of an assignment operator, representing a temporary value that cannot be assigned to. An l-value refers to an object that occupies some identifiable location in memory (e.g., variables), allowing it to appear on both sides of an assignment.
23. What distinguishes calloc() from malloc()?
malloc() allocates a specified number of bytes in memory, while calloc() allocates memory for an array of elements and initializes them to zero.
- malloc(): void* malloc(size_t size);
- calloc(): void* calloc(size_t num, size_t size);
24. What is the distinction between a union and a struct in C?
A struct allocates separate memory for each of its members, allowing simultaneous access to all members. A union, however, shares the same memory location for all its members, meaning only one member can hold a value at any time.
Code Snippet:
struct myStruct {
int a;
char b;
}; // Both a and b can be used simultaneously
union myUnion {
int a;
char b;
}; // Only one of a or b can be used at a time
25. What does a function’s call by reference mean?
In call by reference, the function receives a reference (or address) of the argument rather than a copy. This allows the function to modify the original variable. This is typically done using pointers in C. For example:
c
Code Snippet:
void modify(int *x) {
*x = 20; // Modifies the original variable
}
26. What does function pass by reference mean?
Pass by reference means passing the address of the argument to the function. The function can modify the value at that address. This is achieved by passing pointers as parameters, allowing for changes to the original data.
27. What is a memory leak, exactly? How do you stop it?
A memory leak occurs when allocated memory is not properly deallocated, leading to wasted memory resources. To prevent memory leaks, always use free() to release memory allocated with malloc() or calloc() once it is no longer needed. Regularly check for and eliminate unneeded allocations.
28. What in C is dynamic memory allocation? The dynamic allocation methods should be named?
Dynamic memory allocation refers to allocating memory at runtime using functions like malloc(), calloc(), and realloc(). This allows for flexible memory management, adapting to the program’s needs. Memory must be freed using free() to prevent leaks.
29. Describe typedef.
The typedef keyword is used in C to create an alias for an existing data type, enhancing code readability. For example:
Code Snippet:
typedef unsigned long ulong;
ulong var = 5; // Now ulong can be used instead of unsigned long
30. Why is using gets() typically a poor idea? Offer a solution.
Using gets() is risky because it does not check buffer overflow, leading to potential security vulnerabilities. Instead, use fgets(), which limits the number of characters read:
Code Snippet:
char buffer[100];
fgets(buffer, sizeof(buffer), stdin);
31. What distinguishes #include “…” from #include <…>?
The #include “filename” directive tells the preprocessor to look for the file in the current directory first, while #include <filename> looks in the system directories. This distinction affects how files are included based on their location.
32. What distinguishes the letters “g” and “g” in the C language?
In C, a single-quoted ‘g’ represents a character type, while double-quoted “g” represents a string type (character array). This distinction affects how they are stored and manipulated in memory.
33. What do the terms “near pointer” and “far pointer” in C mean?
A near pointer can address memory within a single segment, typically 16 bits, whereas a far pointer can address memory across multiple segments, allowing for larger memory addressing in older architectures (not commonly used in modern C).
34. What kind of structure connects the program and the operating system?
The file structure connects the program and the operating system, allowing for data storage and retrieval. It defines how data is organized, accessed, and managed on storage devices.
35. Assume a local and global variable has the same name. Can you access a global variable from a block that defines local variables?
Yes, you can access a global variable even if a local variable with the same name exists. However, within the local scope, the local variable takes precedence. To explicitly refer to the global variable, use the extern keyword or access it from outside the local block.
36. Which is preferable, #define or enum?
Using enum is generally preferable because it provides type safety, can be scoped, and allows for better debugging. #define does not have a type, which can lead to errors and harder-to-read code. Enums can also automatically assign integer values.
37. Why does every program statement terminate with a semicolon (;) ?
In C, the semicolon serves as a statement terminator, indicating the end of a statement. This is necessary for the compiler to correctly parse and understand the code structure.
38. Distinguish between source and object codes.
Source code is the human-readable code written by programmers in a programming language. Object code is the machine-readable output generated by a compiler after processing the source code, which can be executed by a computer.
39. What are header files and how are they used in C programming?
Header files contain function declarations, macros, and definitions that can be shared across multiple source files. They are included in programs using #include and help in organizing code and avoiding redundancy.
40. How do you define a dynamic data structure?
A dynamic data structure is a structure that can grow and shrink at runtime as needed, allowing for flexible memory usage. Common examples include linked lists, trees, and dynamic arrays, which utilize pointers to manage memory allocation dynamically.
41. What are the characteristics of a good programming language?
A good programming language should possess several key characteristics:
- Ease of Learning: Should have a clear and straightforward syntax.
- Efficiency: Should allow for optimized performance and resource management.
- Portability: Should be able to run on various platforms with minimal changes.
- Robustness: Should provide error handling and prevent undefined behaviors.
- Flexibility: Should support multiple programming paradigms (e.g., procedural, object-oriented).
- Strong Community Support: Should have extensive documentation and active user support.