C5: Functions

Question 1

What is a function in C?

Answer 1

A function in C: a series of statements that has
been grouped (in a block) and given a name. Functions are the main building blocks of C
programs
• They modularize a program, making it easier to read
• They avoid code duplication and allow code reuse

Question 2

What is a function prototype?

Answer 2

It is a declaration of a function before it is defined so that it can be used before it’s defined. (“Only needed if function definition after function call”)

Question 3

What is: 
double max(double, double);

Answer 3

function prototype

Question 4

What is “call by value”?

Answer 4

When a function gets its own copy of arguments (passed in as parameter list). The function ONLY gets a copy of the original values, not the actual data. changes to copy do not change original.

Question 5

What is “call by reference”?

Answer 5

Function can directly access data and changes affect original. In C, the pointer is copied (i.e. the value of the pointer).
Note: there is no actual call by
reference in C
§ We just simulate it with pointers

Question 6

Give an example of simulating calling by reference in C.

Answer 6

Example of passing by reference by copying address of a variable and passing address to function, which creates a pointer that holds the same address (points to the address):

void myFunction(int * data){ *data = 10;}
//...
int main(void){
int x =5;
myFunction(&x);
//now x = 10
}

Question 7

What is an activation record?

Answer 7

An activation record is the portion of the stack used for an invocation of a function and is also called stack frame.

Question 8

What is a stack?

Answer 8

Calls to a function requires its own instantiation of local variables. Functions can only return when all functions it has called are returned following LIFO principle. This structure is a LIFO structure and is called the stack and it holds each (function) instantiation.

Question 9

Is it ok to return an array instantiated in block scope?

Answer 9

No, if you need to return an array:
–create a struct with an array attribute and return the struct

bad: 
char * getMessage (void) {
  char aMessage[] = “hello world”;
  char *result = &(aMessage[0]);
  return result;
}

Question 10

Is it ok to return a pointer pointing to a variable in block scope?

Answer 10

No,
if you need to return a pointer in general:
–instantiate the pointer on the heap and not in the function’s stack (char * result; result = malloc(6); … return result;)
OR
–instantiate the pointer on the caller’s stack and pass by [simulated] reference (char str[6];
getMessage(&str); )

Question 11

What is a static function?

Answer 11

It is a function declared with the keyword static. It is only locally visible within the file defining it. (It uses internal linkage.)