Functions, Parameter Passing & Pass-by-Value vs Pointers

1Concept

In C, ALL function arguments are passed strictly by value (copied). To modify a caller's variable or pass large structs without copy overhead, developers pass pointer addresses (pass-by-pointer). Function declarations (prototypes) inform the compiler of parameter types and return values.

2Architecture Diagram

Pass-by-Value:
 Caller: [ x = 10 ]  --Copy Val 10-->  Callee: [ a = 10 ] (Modifying 'a' doesn't affect 'x')

Pass-by-Pointer:
 Caller: [ x = 10 (Addr 0x1000) ]  --Pass Addr 0x1000--> Callee: [ ptr = 0x1000 ]
 Dereferencing *ptr = 20 directly modifies Caller's 'x'!

3Code Example

#include <stdio.h>

// Pass-by-value: caller's variable remains unchanged
void swap_by_value(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}

// Pass-by-pointer: modifies caller's memory directly
void swap_by_pointer(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main(void) {
    int x = 100, y = 200;
    printf("Original: x = %d, y = %d\n", x, y);

    swap_by_value(x, y);
    printf("After swap_by_value: x = %d, y = %d (Unchanged!)\n", x, y);

    swap_by_pointer(&x, &y);
    printf("After swap_by_pointer: x = %d, y = %d (Swapped!)\n", x, y);

    return 0;
}

4Expected Output

Original: x = 100, y = 200
After swap_by_value: x = 100, y = 200 (Unchanged!)
After swap_by_pointer: x = 200, y = 100 (Swapped!)

5Key Takeaways

• ✓C has no native pass-by-reference (&ref syntax in C++); it simulates it via pointers.
• ✓Use const T* ptr for input-only pointer parameters to guarantee caller data is read-only.
• ✓Never return a pointer to a local stack variable from a function (dangling pointer!).