C Functions Advanced | void, Multiple Parameters, Pass-by-Value

📖 What to learn on this page

✅ Must-know essentials

Split code into functions for reuse
Recursion: a function calls itself
Recursion must have a base case

⭐ Read if you have time

Stack overflow
Tail-call optimization
Callback functions

💪 It's normal not to get it on the first try

Recursion — a function calling itself — feels mysterious until you've seen it in action a few times.

How to approach it again

Make sure you're solid on basic functions first
Trace factorial (n!) on paper: expand fact(5) down to fact(1)
Confirm there's always a base case
Start with small depths (3 to 5)
Notice that the same logic can be written iteratively — try writing both versions

💡 Tip: 90% of recursion bugs come down to a missing or unreachable base case, or forgetting to shrink the argument. Check those first.

Functions with return values

A function takes parameters, performs a computation, and can return a value to the caller.

int square(int n) {    // return type: int
  return n * n;        // return a value
}
int main(void) {
  int r = square(5);  // call the function; r becomes 25
  printf("%d\n", r);
}

Argument

The value passed to the function. Goes inside the parentheses.

Return value

The result the function sends back, via the return statement.

name(args)

A call expression. Its value is whatever the function returns.

void — "nothing"

Use void to indicate "no parameters" or "no return value."

// no return, no parameters
void sayHello(void) {
  printf("Hello!\n");     // return can be omitted
}

// no return, with parameter
void printBox(int n) {
  for(int i=0; i<n; i++) printf("*");
  printf("\n");
}

// returns a value, no parameters
int getAnswer(void) {
  return 42;
}

The main function: int main(void) means "no parameters, returns an int."

Multiple parameters & types

You can pass multiple parameters separated by commas. Each one needs its own type declaration.

double triangleArea(double base, double height) {
  return base * height / 2.0;
}

int main(void) {
  double area = triangleArea(6.0, 4.0);
  printf("area = %f\n", area); // 12.000000
}

Match the types: passing a double (for example, 3.5) to an int parameter truncates the fractional part.

Memory during a call (call stack)

When a function is called, a stack frame is created for it. That frame is released when the function returns.

Click "Next step" to walk through the function call flow.

Key points:

Calling a function creates a new stack frame.
Parameters and local variables live inside that frame.
When the function returns, the frame is freed and control goes back to the caller.
Local variables disappear the moment the function exits.

Try it yourself — advanced functions

funcs.c

Output

Click "Run" to execute...

💡 Try these ideas too

Rewrite the function to compute a factorial recursively
Return both the sum and the product using a struct
Verify that local variables are independent on each call
Count the number of calls using a static local variable

Review Quiz

Check your understanding of this lesson!

Q1. When you pass an array to a C function, what is actually passed?

A copy of the array

The address of the first element

The array size

In C, an array name decays to a pointer to its first element, so the whole array isn't copied.

Q2. What is essential for a recursive function?

A global variable

A base case (stopping condition)

A pointer

A recursive function calls itself, so without a base case you end up with infinite recursion and a stack overflow.

Q3. Which is a typical use of function pointers?

Freeing memory

Implementing callbacks

String manipulation

Function pointers hold the address of a function. They're commonly used for callbacks, dispatch tables, and similar patterns.

Lesson 25: Functions (Advanced)

Functions with return values

void — "nothing"

Multiple parameters & types

Memory during a call (call stack)

Try it yourself — advanced functions

Related Lessons

Review Quiz

Q1. When you pass an array to a C function, what is actually passed?

Q2. What is essential for a recursive function?

Q3. Which is a typical use of function pointers?