sizeof and memory budgeting in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of sizeof with arrays and pointers

What is the output of this C code snippet?

Embedded C

#include <stdio.h>

int main() {
    int arr[10];
    int *ptr = arr;
    printf("%zu %zu\n", sizeof(arr), sizeof(ptr));
    return 0;
}

A 40 4

B 40 8

C 10 4

D 10 8

💡 Hint

Remember that sizeof on an array gives total bytes, but on a pointer gives pointer size.

The array arr has 10 integers. Each int is 4 bytes on a 32-bit system or 4 bytes on many embedded systems, so total 40 bytes. The pointer ptr size depends on architecture; on a 64-bit system it is 8 bytes. So output is "40 8".

🧠 Conceptual

intermediate

Memory budgeting for struct with padding

Given this struct, what is the total size in bytes on a system where int is 4 bytes and alignment is on 4-byte boundaries?

struct S {
  char a;
  int b;
  char c;
};

A 9

B 10

C 12

D 8

💡 Hint

Think about padding added to align int b and the struct size.

The struct has char a (1 byte), then 3 bytes padding to align int b (4 bytes), then char c (1 byte), then 3 bytes padding to make total size multiple of 4. Total size is 12 bytes.

🔧 Debug

advanced

Why does this memory budgeting code produce wrong size?

What error causes the wrong total size calculation in this code?

struct Data {
  char flag;
  double value;
};

int total_size() {
  return sizeof(char) + sizeof(double);
}

A It ignores padding between <code>flag</code> and <code>value</code> causing underestimation

B It uses wrong types in <code>sizeof</code> causing overestimation

C It should use <code>sizeof(struct Data)</code> instead of summing members

D It returns size in bits instead of bytes

💡 Hint

Think about how structs are aligned in memory.

Adding sizes of members ignores padding bytes added by compiler for alignment. The double usually requires 8-byte alignment, so padding is added after flag. Summing sizes misses this padding, underestimating total size.

📝 Syntax

advanced

Which code correctly uses sizeof for memory allocation?

Which option correctly allocates memory for an array of 5 integers?

A int *arr = malloc(5 * sizeof(arr));

B int *arr = malloc(sizeof(int) * 5);

C int *arr = malloc(5 * sizeof(int));

D int *arr = malloc(sizeof(*arr) * 5);

💡 Hint

Consider what sizeof(arr) and sizeof(*arr) mean.

sizeof(*arr) gives size of one integer (the type pointed to). sizeof(arr) gives size of pointer, which is not correct for allocation. Options A and B are correct but C is preferred for safety and maintainability.

🚀 Application

expert

Calculate total memory for nested structs

Given these structs, what is the total size of struct Container on a system where int is 4 bytes, char is 1 byte, and alignment is on 4-byte boundaries?

struct Inner {
  char x;
  int y;
};

struct Container {
  struct Inner a;
  char b;
};

A 12

B 10

C 8

D 16

💡 Hint

Calculate size of Inner first including padding, then add b with padding.

Inner has char x (1 byte), 3 bytes padding, then int y (4 bytes), total 8 bytes. Container has Inner a (8 bytes), then char b (1 byte), plus 3 bytes padding to align total size to 4 bytes. Total size is 12 bytes.