C interview questions

1. What is C and who made it?

C is a general-purpose high-level language. Dennis Ritchie made C at Bell Labs for the UNIX operating system. C first worked in 1972.

Full lesson: C intro

2. What is a token in C?

A token is the smallest unit the compiler can read. The compiler breaks source code into tokens. Keywords, names, and numbers are tokens.

Full lesson: C basic syntax

3. How do you declare a variable in C?

You write a type and then a name. You can give the variable a value at the same time. You must declare a variable before you use it.

int count = 10;
float price = 2.5;

Full lesson: C variables

4. What are the main data types in C?

C has basic types such as int, float, double, and char. Each type has a size and a range. The size can change on different systems.

int n = 5;
char c = 'A';
double d = 1.5;

Full lesson: C data types

5. How does an if statement work in C?

The if statement tests a condition. If the condition is true, the code block runs. C treats zero as false and any non-zero value as true.

int a = 10;
if (a > 5) {
    printf("big\n");
}

Full lesson: C if

6. What is the difference between if and if...else?

The if block runs when the condition is true. The else block runs when the condition is false. Only one block runs.

if (a > 5) {
    printf("big\n");
} else {
    printf("small\n");
}

Full lesson: C if else

7. How does a for loop work in C?

A for loop has three parts: start, condition, and step. The loop runs while the condition is true. The step runs after each pass.

int i;
for (i = 0; i < 3; i++) {
    printf("%d\n", i);
}

Full lesson: C for loop

8. How does a while loop work in C?

A while loop tests a condition before each pass. The body runs while the condition is true. If the condition is false at the start, the body never runs.

int i = 0;
while (i < 3) {
    printf("%d\n", i);
    i++;
}

Full lesson: C while loop

9. How do you declare an array in C?

You write a type, a name, and a size in brackets. The first element has index 0. The elements sit in contiguous memory.

int nums[3] = {1, 2, 3};
printf("%d\n", nums[0]);

Full lesson: C arrays

10. How does C store a string?

C stores a string as a char array with a null character at the end. The null character marks the end of the string. The array needs one extra space for it.

char site[7] = {'R', 'U', 'N', 'O', 'O', 'B', '\0'};
printf("%s\n", site);

Full lesson: C strings

11. How do you define and call a function in C?

A function has a return type, a name, and parameters. You call it by its name and pass the arguments. The function returns a value to the caller.

int add(int a, int b) {
    return a + b;
}
int sum = add(2, 3);

Full lesson: C functions

12. What does typedef do in C?

The typedef keyword gives a new name to a type. The new name works as a short form of the old type. People often use capital letters for the new name.

typedef unsigned char BYTE;
BYTE b1, b2;

Full lesson: C typedef

13. What does fopen do in C?

The fopen function opens a file or creates a new file. It returns a FILE pointer for the stream. It returns NULL if the open fails.

FILE *fp = fopen("file.txt", "r");
if (fp == NULL) {
    printf("open failed\n");
}

Full lesson: C file io

14. What is a common mistake with a for loop condition?

A common mistake is a condition that never becomes false. Then the loop runs forever. Check that the step moves the value toward the end.

int i;
for (i = 0; i < 3; i--) {  /* wrong: i goes down */
    printf("%d\n", i);
}

Full lesson: C loops

15. What is a common mistake with a string array size?

A common mistake is a char array with no space for the null character. Then the string has no end mark. Make the array one element larger than the text.

char s[3] = "abc";  /* wrong: no room for '\0' */
char t[4] = "abc";  /* correct */

Full lesson: C strings

16. How do you write a nested if statement in C?

You put one if statement inside another if block. The inner if runs only when the outer condition is true.

if (a == 100) {
    if (b == 200) {
        printf("Both true\n");
    }
}

Full lesson: C nested if

17. How does a nested loop work in C?

A nested loop has one loop inside another loop. The inner loop runs completely for each iteration of the outer loop.

for (i = 1; i <= 3; i++) {
    for (j = 1; j <= 2; j++) {
        printf("%d %d\n", i, j);
    }
}

Full lesson: C nested loops

18. What does the abs() function do in C?

The abs() function returns the absolute value of an integer. It is in the <stdlib.h> header. Use fabs() for floating-point numbers.

int x = -5;
printf("%d\n", abs(x)); // prints 5

Full lesson: C function abs

19. What does the div() function return in C?

The div() function divides two integers and returns a structure. The structure has two members: quot for the quotient and rem for the remainder.

div_t d = div(27, 4);
printf("quot = %d, rem = %d\n", d.quot, d.rem);

Full lesson: C function div

20. How do you use the cos() function in C?

The cos() function returns the cosine of an angle in radians. It is in the <math.h> header. You must link the math library when you compile.

#include <math.h>
double result = cos(0.0); // returns 1.0

Full lesson: C function cos

21. How do you check for a file error with ferror() in C?

The ferror() function checks a file stream for an error. It returns a non-zero value if an error occurred. Use it after a file operation.

FILE *fp = fopen("test.txt", "r");
if (ferror(fp)) {
    printf("File error\n");
}

Full lesson: C function ferror

22. What does the perror() function do in C?

The perror() function prints a message to stderr. The message describes the last error. You give a prefix string as the argument.

FILE *fp = fopen("nofile.txt", "r");
if (fp == NULL) {
    perror("Error opening file");
}

Full lesson: C function perror

23. How do you get a string for an error number with strerror() in C?

The strerror() function returns a pointer to a string. The string describes the error code you pass. It is in the <string.h> header.

#include <string.h>
printf("%s\n", strerror(2)); // prints "No such file or directory"

Full lesson: C function strerror

24. How do you create a temporary file with tmpfile() in C?

The tmpfile() function creates a temporary binary file. The file is removed automatically when you close it or when the program ends. It returns a FILE pointer or NULL on error.

FILE *tmp = tmpfile();
if (tmp != NULL) {
    fputs("data", tmp);
    fclose(tmp);
}

Full lesson: C function tmpfile

25. How do you measure a time difference with difftime() in C?

The difftime() function returns the difference in seconds between two time_t values. The result is a double. It is in the <time.h> header.

time_t start = time(NULL);
// ... some work ...
time_t end = time(NULL);
double diff = difftime(end, start);

Full lesson: C function difftime

26. What is a common mistake with the while loop condition?

A common mistake is to forget to change the condition inside the loop. The loop then runs forever. Always update the loop variable.

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++; // without this, infinite loop
}

Full lesson: C while loop

27. What is a common mistake with nested if statements?

A common mistake is to forget braces for the inner if. The else then binds to the wrong if. Always use braces to make the logic clear.

if (a > 0) {
    if (b > 0) {
        printf("both positive\n");
    }
}

Full lesson: C nested if

28. What is a common mistake when using abs() with floating-point numbers?

A common mistake is to use abs() on a float or double. The abs() function works only on integers. Use fabs() from <math.h> for floating-point values.

double x = -3.5;
// wrong: abs(x) truncates to int
printf("%f\n", fabs(x)); // correct

Full lesson: C function abs

29. What is a common mistake when checking the result of fopen()?

A common mistake is to use the file pointer without checking for NULL. If fopen() fails, it returns NULL. Always check before you read or write.

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    perror("fopen");
    return 1;
}

Full lesson: C file io

30. What is a common mistake with the div() function?

A common mistake is to use div() with a zero denominator. This causes undefined behavior. Always check the denominator before you call div().

int denom = 0;
if (denom != 0) {
    div_t d = div(10, denom);
}

Full lesson: C function div

31. What is the basic structure of a C program?

A C program has a main function. The main function is the entry point. It contains statements inside braces.

#include <stdio.h>
int main() {
    printf("Hello");
    return 0;
}

Full lesson: C basic syntax

32. How do you include a header file in C?

Use the #include directive. It copies the header contents into your file. Use angle brackets for standard headers.

#include <stdio.h>

Full lesson: C header files

33. How does an if statement check a condition in C?

The if statement runs a block when the condition is true. A non-zero value is true. A zero value is false.

if (x > 0) {
    printf("positive");
}

Full lesson: C if

34. How does a while loop repeat a statement in C?

The while loop runs a block while the condition is true. It checks the condition before each run. The loop can run zero times.

while (i < 5) {
    i++;
}

Full lesson: C while loop

35. How do you access an element in a C array?

Use the array name and an index in brackets. The first element has index 0. The last element has index size minus 1.

int a[3] = {1, 2, 3};
printf("%d", a[0]);

Full lesson: C arrays

36. How do you read a string with scanf in C?

Use the %s format specifier. The array must have enough space. The input stops at a space.

char name[20];
scanf("%s", name);

Full lesson: C strings

37. How do you open a file for reading in C?

Call fopen with mode "r". It returns a FILE pointer. Check the pointer for NULL.

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) { /* handle error */ }

Full lesson: C file io

38. How do you use the pow() function in C?

Include the math.h header. Call pow with the base and the exponent. It returns a double result.

#include <math.h>
double r = pow(2.0, 3.0);

Full lesson: C function pow

39. How do you use the sin() function in C?

Include the math.h header. Pass the angle in radians. The function returns the sine value.

#include <math.h>
double s = sin(1.0);

Full lesson: C function sin

40. How do you use the exp() function in C?

Include the math.h header. Pass the exponent as a double. The function returns e raised to that power.

#include <math.h>
double v = exp(1.0);

Full lesson: C function exp

41. What is a common mistake with the log() function in C?

The log function needs a positive input. A zero or negative input gives an error or NaN. Check the input before the call.

if (x > 0) {
    double r = log(x);
}

Full lesson: C function log

42. What is a common mistake with a while loop that never ends?

The loop condition stays true if the body does not change it. Update the control variable inside the loop. A missing update causes an infinite loop.

int i = 0;
while (i < 5) {
    i++;
}

Full lesson: C while loop

43. What is a common mistake with array index bounds in C?

C does not check array bounds. An index outside the array causes undefined behavior. Keep the index from 0 to size minus 1.

int a[3];
a[3] = 5; /* wrong: out of bounds */

Full lesson: C arrays

44. What is a common mistake with the scanf %s format and string size?

The %s format has no width limit. A long input can overflow the array. Use a width limit in the format.

char name[20];
scanf("%19s", name);

Full lesson: C strings

45. What is a common mistake with the sin() function and degree input?

The sin function expects radians. A degree value gives a wrong result. Convert degrees to radians first.

double rad = deg * 3.14159 / 180.0;
double s = sin(rad);

Full lesson: C function sin