Structs in C

This section introduces Structs, which allow us to group different variables under a single name. In C, understanding structs requires a clear mental model of how they are laid out in the Stack and how they behave when passed to functions.

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1. Defining and Initializing Structs

A struct is a custom data type. Using typedef allows us to use the name Point directly without prepending the struct keyword every time.

typedef struct Point {
    int x;
    int y;
} Point;

When you initialize a struct like Point p = {3, 4};, C allocates a contiguous block of memory on the stack large enough to hold all its members.

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2. Memory Layout and Addressing

Members of a struct are stored in the order they are defined. The address of the struct is the same as the address of its first member.

Variable	Memory Address	Size	Value
p	0x...40	8 bytes	{x: 3, y: 4}
p.x	0x...40	4 bytes	3
p.y	0x...44	4 bytes	4
p2	0x...48	8 bytes	{x: 7, y: 8}

As shown above, p.y starts exactly 4 bytes (the size of an int) after the start of p.

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3. Passing Structs: The “By Value” Trap

In C, when you pass a struct to a function, the entire struct is copied into the function’s stack frame. This leads to a common bug when trying to modify data.

The Problem: updateX

void updateX(Point p, int newX){
    p.x = newX; // This modifies a COPY, not the original p1
}

When updateX(p1, 30) is called:

1. A new Point p is created in updateX’s memory.
2. The values from p1 (3 and 4) are copied into this new space.
3. The function changes the copy’s x to 30.
4. The function returns, and the copy is destroyed. The original p1 in main remains {3, 4}.

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4. Example

Adding 2 points together, which the resulting Point

Point add(Point a, Point b){
	int x = a.x + b.x;
	int y = a.y + b.y;
	Point added = {x, y};
	return added;
}
 
void updateX(Point p, int newX){
	p.x = newX;
}
 
int main(){
	Point p1 = {3, 4};
	Point p2 = {7, 8};
	Point result = add(p1, p2);
	updateX(p1, 30);
}

WARNING

The update to the Point only changed the value in the argument data space, not the main frame value that we wished to change → need a different function definition to actually change the real data stored

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5. Key Takeaways

• Contiguous Memory: Struct members live right next to each other in the order they are declared.
• Pass by Value: Passing a struct to a function copies every byte of that struct. For large structs, this can be slow.
• Modification: To actually change a struct inside a function, you must pass its address (a pointer), which we will explore in the next section.