Fixed-Sized Struct

In C systems programming, a Fixed-Sized Struct is a structure where all data is stored directly within the struct itself on the Stack. This is contrasted with dynamic structs where data is stored via pointers on the Heap.

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Analysis of ListOfDoubles Memory

#include <stdio.h>
 
#define MAX_LIST_SIZE 20
 
typedef struct ListOfDoubles {
	int size;
	double contents[MAX_LIST_SIZE];
} ListOfDoubles;
 
ListOfDoubles concat(ListOfDoubles d1, ListOfDoubles d2){
	ListOfDoubles result;
	result.size = d1.size + d2.size;
	
	for(int i = 0; i < d1.size; i++){
		result.contents[i] = d1.contents[i];
	}
	for(int i = 0; i < d2.size; i++){
		result.contents[i + d1.size] = d2.contents[i];
	}
	return result;
}
 
int main(){
	ListOfDoubles lod = {2, {3.0, 4.0}};
	ListOfDoubles lod2 = {3, {6.0, 9.0, 10.0}};
	ListOfDoubles added = concat(lod, lod2);
	for(int i = 0; i < added.size; i++){
		printf("%f ", added.contents[i]);
	}
	printf("\n");
}

1. Size Calculation

The size of a fixed-sized struct is the sum of its members’ sizes (plus potential padding for alignment). In the provided example:

\begin{align*} \text{sizeof(ListOfDoubles)} &= \text{sizeof(int)} + (\text{MAX\_LIST\_SIZE} \cdot \text{sizeof(double)}) \\ &= 4 + (20 \cdot 8) \\ &= 4 + 160 \\ &= \mathbf{164 \text{ bytes}} \end{align*}

2. Passing and Returning by Value

When you pass a fixed-sized struct to a function or return it (as seen in your concat function), the entire block of memory is copied.

• Function Call: concat(lod, lod2) creates full copies of lod and lod2 on the concat stack frame.
• Return: The ListOfDoubles added variable in main receives a complete copy of the result struct from the concat function.

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Stack Constraints and Design

1. The Stack Size Limit

Most Operating Systems and processes impose a strict Stack Size Limit, typically around a few megabytes ($\approx$ 1,000,000 bytes).

• If you define a fixed-sized struct that is too large (e.g., an array of 1,000,000 doubles), you risk a Stack Overflow.
• Because of this limit, fixed-sized structs are best suited for data with a known, fixed maximum size that remains relatively small.

2. Advantages and Use Cases

Feature	Fixed-Sized Struct
Allocation Speed	Extremely fast (managed by the CPU stack pointer).
Memory Locality	High (all data is contiguous in memory).
Cleanup	Automatic (memory is reclaimed when the function returns).
Best For	Small datasets, coordinate systems (x, y, z), or fixed-length headers.

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Related Files in Memory Management

• Size of Struct: Detailed look at how alignment and padding might change that 164-byte calculation to a larger multiple (like 168 or 176).
• Dynamic Struct: How to use malloc to move large data to the Heap when it exceeds stack limits.
• Structs in C: General syntax and initialization patterns.