C Struct Padding

Struct padding is the empty space C adds inside a structure so its parts line up neatly in memory, like gaps in a shelf to fit items right. It makes the structure a little bigger.

In this page:

Why Padding Exists
Seeing Padding with sizeof
Field Order Affects Total Size
Padding at the End of a Struct
Disabling Padding When It Matters

Syntax

struct StructName {
    char member1;   // 1 byte, then padding
    int member2;    // 4 bytes, aligned to a 4-byte boundary
};

printf("%zu", sizeof(struct StructName));   // size includes padding

Why Padding Exists

Most CPUs read multi-byte values fastest when they sit at a memory address that's a multiple of their own size -- a 4-byte int is fastest read from an address divisible by 4 -- so the compiler inserts unused padding bytes between struct members to keep each field aligned this way.

Example: Why Padding Exists

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Define a structure type named Example
struct Example {
	char c;
	int i;
};
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%zu", sizeof(struct Example));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Seeing Padding with sizeof

A struct containing a char followed by an int often reports a sizeof larger than the sum of its members' individual sizes -- the compiler silently inserted 3 padding bytes after the char so the following int lands on a 4-byte boundary.

Example: Seeing Padding with sizeof

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Define a structure type named Mixed
struct Mixed {
	char c;
	int i;
};
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%zu %zu %zu", sizeof(char), sizeof(int), sizeof(struct Mixed));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Field Order Affects Total Size

Reordering a struct's members can shrink or grow its total size even though the fields themselves haven't changed -- grouping larger types together and placing smaller ones like char at the end typically minimizes the padding the compiler needs to insert.

Example: Field Order Affects Total Size

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Define a structure type named Ordered
struct Ordered {
	int i;
	char c;
};
// Define a structure type named Reordered
struct Reordered {
	char c;
	int i;
};
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%zu %zu", sizeof(struct Ordered), sizeof(struct Reordered));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Padding at the End of a Struct

Compilers also add trailing padding after a struct's last member so that the struct's overall size is a multiple of its strictest member's alignment requirement, which matters when the struct is used inside an array so each element stays properly aligned.

Example: Padding at the End of a Struct

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Define a structure type named Trailing
struct Trailing {
	int i;
	char c;
};
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%zu", sizeof(struct Trailing));
	// Return 0 to signal that the program finished successfully
	return 0;
}

Disabling Padding When It Matters

Compiler-specific directives like #pragma pack(1) can force a struct to be packed with no padding at all, which matters when matching an exact binary file format or network protocol layout, but reading unaligned fields this way can be slower or even fault on some hardware.

Example: Disabling Padding When It Matters

// Include standard input/output (printf, scanf)
#include <stdio.h>
#pragma pack(1)
// Define a structure type named Packed
struct Packed {
	char c;
	int i;
};
#pragma pack()
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%zu", sizeof(struct Packed));
	// Return 0 to signal that the program finished successfully
	return 0;
}

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