Endianness

Source: CS:APP3e, Bryant and O’Hallaron, practice problem 2.5

• Description
• Notes
• Solution

Description

Verify endianness by checking the byte ordering of a four-byte integer value.

Notes

On a little-endian system (e.g., Intel x86), the low-order (least significant) byte will be stored first, so 0x12345678 will be represented by the byte order 78 56 34 12.

On a big-endian system (e.g., Motorola 68000), the low-order (least significant) byte will be stored last, so 0x12345678 will be represented by the byte order 12 34 56 78.

Solution

Create a pointer to the first byte in a four-byte integer constant by casting to unsigned char. Dereference and increment this pointer, checking its value each time and returning false if byte order is not little-endian.

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#include <assert.h>

int is_little_endian() {
    int x = 0x12345678;
    unsigned char * byte_ptr = (unsigned char *) &x;
    if (*byte_ptr++ != 0x78) return 0;
    if (*byte_ptr++ != 0x56) return 0;
    if (*byte_ptr++ != 0x34) return 0;
    if (*byte_ptr != 0X12) return 0;
    return 1;
}

int main(void) {
    assert(is_little_endian());
    return 0;
}