Struct packing and alignment in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of struct size with default alignment

What is the output of the following C code when printed with sizeof(s)?

Embedded C

#include <stdio.h>
#include <stddef.h>

struct S {
    char c;
    int i;
};

int main() {
    struct S s;
    printf("%zu", sizeof(s));
    return 0;
}

A 12

B 5

C 8

D 16

💡 Hint

Think about how the compiler aligns the int after the char.

The struct has a char (1 byte) followed by an int (usually 4 bytes). The compiler adds padding after the char to align the int on a 4-byte boundary. So total size is 8 bytes.

❓ Predict Output

intermediate

Effect of #pragma pack(1) on struct size

What is the output of the following C code when printed with sizeof(s)?

Embedded C

#include <stdio.h>
#pragma pack(1)
struct S {
    char c;
    int i;
};
#pragma pack()

int main() {
    struct S s;
    printf("%zu", sizeof(s));
    return 0;
}

A 16

B 8

C 12

D 5

💡 Hint

Packing removes padding bytes.

Using #pragma pack(1) tells the compiler to pack the struct without padding. So the size is the sum of the sizes of char (1 byte) and int (4 bytes) = 5 bytes.

🔧 Debug

advanced

Why does this struct cause unexpected padding?

Consider this struct and its size output. Why does it have unexpected padding bytes?

Embedded C

struct S {
    char c1;
    double d;
    char c2;
};

// sizeof(S) printed is 24 on a 64-bit system.

A Because double requires 8-byte alignment, padding is added after c1 and c2.

B Because the struct is packed with #pragma pack(1).

C Because the compiler ignores alignment rules for double.

D Because char variables always cause 8 bytes of padding each.

💡 Hint

Think about alignment requirements of double and where padding is inserted.

The double needs to be aligned on an 8-byte boundary. After char c1 (1 byte), 7 bytes of padding are added before double d. After char c2, 7 bytes of padding are added to make the struct size a multiple of 8.

📝 Syntax

advanced

Identify the syntax error in struct packing directive

Which option contains a syntax error in using #pragma pack to pack a struct?

A

#pragma pack(2)
struct S { char c; int i; };

B

#pragma pack 1
struct S { char c; int i; };

C

#pragma pack(1)
struct S { char c; int i; };
#pragma pack()

D

#pragma pack(push, 1)
struct S { char c; int i; };
#pragma pack(pop)

💡 Hint

Check the syntax of #pragma directives carefully.

Option B is invalid because #pragma pack requires parentheses around the argument, e.g., #pragma pack(1). Writing #pragma pack 1 is a syntax error.

🚀 Application

expert

Calculate struct size with mixed types and packing

Given the struct below with #pragma pack(1), what is the size of struct S?

Embedded C

#pragma pack(1)
struct S {
    char c;
    short s;
    int i;
    char arr[3];
};
#pragma pack()

A 10

B 12

C 11

D 14

💡 Hint

Add sizes of each member without padding due to packing.

With #pragma pack(1), no padding is added. Sizes: char (1) + short (2) + int (4) + char[3] (3) = 10 bytes.