C Quiz

1. What does a C compiler produce from your source code?

• A script that an interpreter reads line by line
• Machine code for a particular processor and operating system
• Bytecode for a virtual machine
• An HTML page

C is compiled ahead of time into a native executable (lessons 01 and 04).

2. What kind of error is "undefined reference to `sqrt'" on Linux?

• A syntax error
• A warning
• A linker error — the maths library wasn't linked (add -lm)
• A run-time crash

The code compiled, but the linker couldn't find the function's definition (lessons 04 and 25).

3. What does 7 / 2 evaluate to in C?

• 3.5
• 3
• 4
• A compile error

Integer division discards the fractional part (lesson 07).

4. Which statement about the size of int is true?

• It is always exactly 4 bytes
• The standard only guarantees at least 16 bits; check with sizeof
• It is always the same as long
• It depends on the value stored

Use stdint.h types when you need an exact size (lessons 07–08).

5. What happens when a signed int overflows past INT_MAX?

• It wraps around to INT_MIN, guaranteed
• It stops at INT_MAX
• The behaviour is undefined
• The program prints an error

Unsigned arithmetic wraps; signed overflow is undefined (lessons 08 and 54).

6. Why is comparing 0.1 + 0.2 == 0.3 unreliable?

• Most decimal fractions can't be represented exactly in binary floating point
• == doesn't work on doubles
• The compiler rounds to integers
• Doubles only hold two decimal places

Compare with a tolerance instead (lesson 09).

7. Which scanf specifier reads a double?

• %d
• %f
• %lf
• %lld

%f is for float in scanf (although printf uses %f for both) (lesson 15).

8. Why should you check the return value of scanf?

• It returns the value that was read
• It returns how many items were successfully stored; bad input stores nothing
• It returns the length of the input
• It's not necessary

(lesson 15)

9. In if (x = 5), what happens?

• x is compared with 5
• 5 is assigned to x, and the condition is always true
• A compile error
• x is unchanged

Use == to compare; compilers warn about this (lesson 04).

10. What happens in a switch case without break?

• The switch ends
• Execution falls through into the next case
• The default case runs
• A compile error

(lesson 17)

11. Arguments to C functions are passed…

• by value: the function gets copies
• by reference: the function gets the originals
• by name
• by value for ints and by reference for structs

To let a function change a variable, pass its address (lessons 22 and 31).

12. What is wrong with returning the address of a local variable?

• Nothing
• It's slower
• The variable no longer exists after the function returns, leaving a dangling pointer
• Functions can't return pointers

(lessons 23 and 34)

13. What does static int count = 0; inside a function do?

• Makes count a constant
• Keeps its value between calls; it's initialised only once
• Makes it visible from other files
• Puts it on the stack

(lesson 23)

14. How many bytes does char word[] = "cat"; occupy?

• 3
• 4
• 8
• It depends on the compiler

Three letters plus the '\0' terminator (lesson 28).

15. How do you correctly compare the text of two strings?

• a == b
• strcmp(a, b) == 0
• strlen(a) == strlen(b)
• a.equals(b)

== compares addresses (lesson 29).

16. Why is snprintf preferred over sprintf and strcpy?

• It's faster
• It never writes more than the buffer size you give it
• It supports more format specifiers
• It allocates memory automatically

(lessons 11 and 29)

17. If int *p = &x;, what does *p = 7; do?

• Changes p to point to address 7
• Stores 7 in x
• Stores 7 in p
• A compile error

* dereferences the pointer (lesson 31).

18. Inside void f(int a[]), what is sizeof a?

• The size of the whole array
• The size of a pointer
• The number of elements
• 0

Arrays decay to pointers when passed; pass the length separately (lesson 32).

19. If double *p points to values[0], what does p + 2 point to?

• The byte two after values[0]
• values[2]
• values[0] + 2.0
• Nothing; you can't add to pointers

Pointer arithmetic moves by whole elements (lesson 33).

20. What does const char *s promise?

• The characters can't be modified through s
• s can never point anywhere else
• The string is stored in read-only memory
• Nothing

char *const s would be a pointer that can't move (lesson 35).

21. What must you do after a successful malloc?

• Nothing, memory is freed automatically
• Eventually call free on the pointer exactly once
• Call free immediately
• Cast the result to int

Otherwise the memory leaks (lesson 36).

22. Why shouldn't you write p = realloc(p, n);?

• realloc never returns a new pointer
• If realloc fails it returns NULL and the original block would be leaked
• It's a syntax error
• It frees p twice

Assign to a temporary first (lesson 37).

23. Which tool finds buffer overflows and use-after-free bugs while your program runs?

• -O2
• make
• AddressSanitizer (-fsanitize=address)
• -pedantic

(lesson 38)

24. What does the comparison function passed to qsort return?

• true or false
• A negative, zero or positive number
• The larger element
• The index of the smaller element

(lessons 39 and 46)

25. With a pointer p to a struct, how do you access its score member?

• p.score
• p->score
• *p.score
• p[score]

p->score means (*p).score (lesson 41).

26. How big is a union?

• The sum of its members' sizes
• At least as big as its largest member
• Always 8 bytes
• The size of its first member

All members share the same memory (lesson 42).

27. Which fopen mode erases an existing file?

• "r"
• "a"
• "w"
• "r+"

(lesson 47)

28. In int main(int argc, char *argv[]), what is argv[0]?

• The first argument after the program name
• The program's name or path
• The number of arguments
• Always NULL

(lesson 49)

29. Why should a function-like macro wrap its parameters in parentheses?

• Text substitution with operator precedence can otherwise change the meaning
• The preprocessor requires it
• It makes the macro faster
• To avoid a linker error

SQUARE(2 + 3) without parentheses gives 11 (lesson 51).

30. What does static mean on a function at file scope?

• The function can't be changed
• The function runs only once
• The function is private to its source file
• The function is stored on the stack

Internal linkage (lessons 23 and 52).