Endianness (big-endian vs little-endian) in Embedded C

Introduction

Endianness tells us how computers store numbers in memory. It helps us understand how data is arranged when we read or write it.

When sending data between different devices that may store numbers differently.

When reading raw data from files or network that use a specific byte order.

When debugging low-level programs that work with memory directly.

When writing code for embedded systems that communicate with hardware.

When converting data formats between systems with different endianness.

Syntax

Embedded C

/* No direct syntax in C, but we check endianness like this: */
#include <stdio.h>

int main() {
    unsigned int x = 1;
    char *c = (char*)&x;
    if (*c) {
        // little-endian
    } else {
        // big-endian
    }
    return 0;
}

Endianness is about byte order in memory, not a language keyword.

We use pointers to check how bytes are stored.

Examples

This prints the first byte of the number in memory. If it prints 0x78, the system is little-endian. If 0x12, big-endian.

Embedded C

unsigned int x = 0x12345678;
char *c = (char*)&x;
printf("First byte: 0x%x\n", (unsigned char)*c);

This simple check tells the endianness of the system.

Embedded C

unsigned int x = 1;
char *c = (char*)&x;
if (*c == 1) {
    printf("Little-endian\n");
} else {
    printf("Big-endian\n");
}

Sample Program

This program shows how the bytes of a number are stored in memory. It prints each byte and then tells if the system is little-endian or big-endian.

Embedded C

#include <stdio.h>

int main() {
    unsigned int x = 0x12345678;
    char *c = (char*)&x;

    printf("Memory bytes of 0x12345678:\n");
    for (int i = 0; i < sizeof(x); i++) {
        printf("Byte %d: 0x%x\n", i, (unsigned char)c[i]);
    }

    if (*c == 0x78) {
        printf("System is little-endian\n");
    } else if (*c == 0x12) {
        printf("System is big-endian\n");
    } else {
        printf("Unknown endianness\n");
    }

    return 0;
}

Important Notes

Most modern PCs use little-endian format.

Network protocols often use big-endian (called network byte order).

Always check endianness when sharing binary data between different systems.

Summary

Endianness is about the order of bytes in memory for numbers.

Little-endian stores the smallest byte first; big-endian stores the largest byte first.

Knowing endianness helps when working with low-level data or different devices.