C4: C Variables and types

Question 1

What is a variable?

Answer 1

It is an abstraction of memory cells. It has an identifier, a value, a type …(at least)

Question 2

What is data type?

Answer 2

data type describes a variable, which value, how represented in memory, which operators are possible. There are two types: basic (e.g. integers) and user-defined (structs)

Question 3

What is the difference between variable definition and declaration?

Answer 3

Variable declarations can be duplicated, have static name type binding done at compile time and specifiy identifier and type. Variable definition: only one allowed, assigns fixed address to variable (at runtime?). Definition and declaration of VARIABLE looks the same, possible: int a; but not so for functions.

Question 4

What is initialization of a variable?

Answer 4

Initialization: assigns initial value to a variable, value not needed at compile time. Assigns scope-

Question 5

What is scope?

Answer 5

Scope is the portion of program where name of variable (identifier) can be used to refer to a variable.

Question 6

What is implicit dynamic memory management?

Answer 6

Auto variables (Storage class Auto) in memory. Memory is automatically allocated when program enters a block and deallocated when program leaves block.

Question 7

What are the four kinds of scope_

Answer 7

1. File Scope (identifiers declared outside blocks and function prototypes)
   2 Block scope (used for identifiers declared inside a block and for function parameters = local variables)
2. Function scope (only used for label names (case:) not for variables
3. function prototype scope (unsure what this is)

Question 8

Why do we need storage classes and what are they?

Answer 8

Storage classes determine a variable’s lifetime, when and where memory is allocated/deallocated. The classes are
stored in stack:
1. Auto - default for block scope
2. Register - useful for often-used items in block scope

stored in bss or data segment:
3. Static - keeps value between function calls, has internal linkage

no memory assigned:
4. Extern - variable declared with extern keyword, has no definition. it is defined in another translation unit., A defined extern variable must have file scope. Undefined - it is not allocated memory.

Memory assigned to thread:
5. _Thread_local - each Thread gets a copy. Deallocated memory when Thread dies.

Question 9

What is internal linkage?

Answer 9

Internal linkage (storage class static) is created when explictly writing “static” in front of a variable (see static storage class). The variable is not exported by the compiler to the inker. (It’s like a private variable.)

Question 10

What is external linkage?

Answer 10

External linkage (in storage class extern) is create when the extern keyword is used to declare a variable. It means the variable’s definition is in another translation unit. No memory is assigned, compiler simply exports identifier.

Question 11

What is ‘const’?

Answer 11

‘const’ is a keyword. constant int a = b for example. Declares, defines and initiates a constant variable. These constants must be initialized at definition time and cannot be modified later.
const int b; would create an error becasue b must be initialized at definition time.

Question 12

What is ‘volatile’?

Answer 12

It is a keyword in declaration of a variable that is a hint to the compiler that the value of the variable may change at any time.

Question 13

What is variable alignment?

Answer 13

it is a restriction on the starting location of a variables memory- The first memory position must start at the beginning of a particular block.
For exmample: alignas(4) int b; means that b can start at position 0, 4, 8, 12, …. or multiples of 4.

Question 14

What are the basic data types in C?

Answer 14

Integer
floating point
characters
Booleans
Complex Numbers
Enumerations
Pointer

Question 15

What is a pointer?

Answer 15

It is a basic data type that contains the address of a variable that has specific value

Question 16

What is an array?

Answer 16

It is a composite data type that is a consecutive group of data elements of the same type.

Question 17

What is a struct?

Answer 17

a struct definition creates a new data type.

Question 18

What is typedef?

Answer 18

typedef makes synonyms for previously defined data types.
typedef struct Books {
   char title[50];
   char author[50];
   char subject[100];
   int book_id;
} Book;

int main( ) {
   Book book;
}

Question 19

Given the following code:
struct Time currentTime = {20,15,0};
struct Time * timePtr = &currentTime;
is timePtr ->hour = 12; or timePtr.hour -12 correct?

Answer 19

timePtr->hour =12 or (*timePtr).hour =12 would be correct.

But not timePtr.hour

Question 20

What are the drawbacks of structures (composite data type struct)?

Answer 20

• no information hiding: all elements of a struct are public
• no interface: if implmentation changes, all programs using that struct must change,
• no inheritance

Question 21

What is Union?

Answer 21

Union is a composite data type similar to struct, but data members share memory/storage space. It only contains one data member at a time, which means, if we set one attribute of a union, then the other(s) attribute is set too.
union test {
int x, y;
};

int main() 
{ 
    // A union variable t 
    union test t; 

t.x = 2; // t.y also gets value 2 
printf("After making x = 2:\n x = %d, y = %d\n\n", 
       t.x, t.y); 

t.y = 10; // t.x is also updated to 10 
printf("After making y = 10:\n x = %d, y = %d\n\n", 
       t.x, t.y); 
return 0;  }  output after making x = 2:  x = 2, y = 2 ...after making y = 10: x = 10, y = 10