C

Question 1

What does the volatile keyword do in C?

Answer 1

Volatile keyword tells the compiler that the variable can be changed externally (e.g. not via code, like with a GPIO) and keeps the compiler from using optimizations that center around the variable not changing

Question 2

What does the static keyword do in C? What value do they have if not initialized?

Answer 2

Static local variables preserve data values between function calls even after they’re out of scope. Static global variables (or functions) can only be accessed in the same file as they’re declared. A static variable is 0 if not initialized and they can only be initialized with constant literals. They are stored in the data segment of memory, not stack.

Question 3

What are malloc() and calloc() in C?

Answer 3

malloc() and calloc() are used to allocate dynamic memory (runtime / heap). malloc() takes the number of bytes to allocate as an argument and does not initialize them. calloc() allocates contiguous memory and initializes it. It takes number of blocks and size of each block as an argument. Null return value indicates failure, otherwise a pointer to a block of memory is returned

Question 4

What are tokens in C?

Answer 4

Identifiers, Keywords, Constants, Operators, Special Characters, Strings

Question 5

What’s the difference between a global and a static variable in C?

Answer 5

Globals are visible to all functions in the same source file. If in a header, visible to all source files that include it. Static variables have local scope and retain state between function calls, useful for counters

Question 6

What is a pointer?

Answer 6

Pointers store the address of a variable in memory. They are used primarily to save memory space and speed up execution. They are used to pass arguments by reference, access array elements, return multiple values.

Question 7

What’s the difference between const char* p and char const* p?

Answer 7

const char* p is a pointer to a const char* p is a pointer to a const char*, since const char and char const are the same, they are the same.

Question 8

Why does n++ execute faster than = n + 1

Answer 8

n++ is a unary operator and n = n + 1 is a binary operator. In modern compilers, n = n + 1 will be converted to n++ automatically

Question 9

What is typecasting and how can it be achieved in C?

Answer 9

modifies interpretation of value without changing binary. (type_name) expression. Typecasting can result in loss of data or precision if new type is smaller than old type. Floating point to int would truncate fractional part.

Question 10

What are macros? What are the advantages of macros over functions?

Answer 10

Macros are preprocessed and their contents are inserted anywhere the macro name is seen in the code. Macros increase code length over functions, but are faster to execute. Macros don’t check compile-time errors whereas functions do.

Question 11

What are preprocesor directives in C?

Answer 11

Macros, include statements, conditional compilation, directives like #ifdef, #pragma

Question 12

What’s #pragma?

Question 13

What’s the memory layout of C programs?

Question 14

What’s a dangling pointer and how is it different from a memory leak?

Question 15

How can you dereference a pointer in C?

Answer 15

int* ptr p; declares ptr, &ptr dereferences it accessing the data at the memory location

Question 16

What is a typedef in C?

Answer 16

A typedef is a keyword that defines an alias for an existing type. E.g. typedef long lg

Question 17

What’s the difference between type-casting and type-conversion?

Answer 17

Type casting is explicit whereas type conversion can be implicit (via compiler) or explicit (via type-casting) type c asting is a type of conversion

Question 18

What is a struct in c?

Answer 18

A struct is a user defined composite data type

Question 19

What is a union?

Answer 19

a union allows storing different data types in the same location in memory. A union allocates enough memory to hold the largest member of the union. All members share the space. Changing one value can affect the other values

Question 20

What are r-values and l-values?

Answer 20

l-values represent memory locations where a value can be stored. r-values are expressions that can be read, not assigned to

Question 21

How can the address of a variable be accessed?

Answer 21

int num = 2;
foo( &num );

Question 22

What is a memory leak? How to avoid?

Answer 22

When dynamically allocated memory is created but not destroyed, this is a memory leak. Memory is added to the heap but never freed. Call free on memory that is dynamically allocated

Question 23

What are near, far, and huge pointers in C?

Answer 23

16bit, 32bit, and

Question 23

What’s the difference between a pointer and a reference?

Answer 23

Pointers store the address of a variable in memory. References are an alias to another variable.

char c = ‘z’; // variable
char* char_ptr = *c; // ptr
char& ref = c // reference changing ref would change c and vise-versa
const char& ref = c // const reference, cannot change c through ref but can change c directly which changes ref

Question 23

What does the extern keyword do?

Answer 23

extern is a storage specifier that extends visiblity of c variables to other source files. Functions are available by default.

Question 24

Where do globals exist in memory?

Answer 24

data segment typically