Lesson 10 of 20

6 mins read

Functions, Prototypes & Pass-by-Pointer

In Plain English

C evaluates arguments strictly pass-by-value. To mutate a caller variable, pass its memory address (&val) to a pointer parameter (int *val). Forward function prototypes inform the compiler of parameter types and return signatures before main().

The Mental Model

"Pass-by-value is photocopying your passport and handing the copy to an officer; pass-by-pointer is giving the officer your actual passport to stamp."

Deep Dive: How It Works

Function Prototypes: Informs the compiler of signatures before implementation definitions.

Pass-by-Value: Placing arguments onto the stack creates local copies.

Pass-by-Pointer: Passing addresses enables direct mutation of caller scope variables.

Syntax Blueprint

void doubleValue(int *val) {
    *val *= 2;
}

doubleValue(&score);

Define function accepting pointer parameter and pass address with & at callsite.

Core Rules to Remember

Forward Declarations: Prototypes prevent implicit declaration warnings and enforce type safety.

Pass-by-Pointer: Enables multiple return values from a single function call.

Stack Scope: Never return pointers to local stack variables from functions.

Common Beginner Traps & How to Fix Them

Mistake:Returning the address of a local stack variable: int* getVal() { int x = 10; return &x; }.

Why it happens: x is popped off the stack when the function returns; the pointer becomes immediately dangling.

How to fix: Allocate dynamic heap memory (malloc) or have the caller provide a destination buffer.

Pro Tip: Use const on pointer parameters (const char *str) when the function only reads and does not mutate the buffer.

Live Interactive Example

Hit Run Code to see it live

In-Place Mutation via Pointer

C11 / GCC 9.2

Output Console

Click "Run Code" to view the rendered output.

How it works: doubleValue dereferences the pointer and doubles the original score in place.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Call doubleValue a Second Time

Call doubleValue(&score) a second time.
Print "Quadrupled Score: %d\n" with score.

Sandbox Output

Click "Run & Check" to test your solution.

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