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

💪 Not getting it first time is normal

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

How to retry

Solidify 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 (3-5)
Notice the same logic can be written iteratively; write both

💡 Tip: 90% of recursion bugs come from "no / unreachable base case" or "forgot to shrink the argument." Check those first.

Functions with return values

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

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

Value passed to the function. Goes in the parentheses.

Return value

Result the function returns, via the return statement.

name(args)

Call expression. Its value is the return value.

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;
}

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

Multiple parameters & types

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

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 (e.g. 3.5) to an int parameter truncates the decimal part.

Memory during a call (call stack)

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

Click "Next step" to follow the function-call flow

Key points:

Calling a function creates a new stack frame.
Parameters and local variables live inside that frame.
Returning frees the frame and control returns to the caller.
Local variables disappear when the function exits.

Try it yourself — advanced functions

funcs.c

Output

Click "Run" to execute...

💡 Try these ideas too

Rewrite the function to compute factorial recursively
Return both sum and product using a struct
Verify local variables are independent per call
Count call count 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. The whole array is not 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 get 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 and are used for callbacks, dispatch tables, and similar patterns.

Recommended Books to Deepen Your Understanding

Combine this interactive site with books for more practice.

📘

Painfully Learning C

by MMGames

A classic C book for beginners. Builds solid fundamentals with clear explanations.

View on Amazon

📗

New Clear C Language (Beginner)

by Bohyoh Shibata

Plenty of diagrams and exercises. Widely used as a university textbook.

View on Amazon

📙

The C Programming Language, 2nd Ed.

by Brian Kernighan & Dennis Ritchie

Known as K&R. The original C book. Ideal after completing the basics.

View on Amazon

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Lesson 14: 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?

Recommended Books to Deepen Your Understanding