From https://helloacm.com/how-to-find-out-whether-a-machine-is-big-endian-or-little-endian-in-cc/

little_big_endian.txt

// https://helloacm.com/how-to-find-out-whether-a-machine-is-big-endian-or-little-endian-in-cc/
#include <stdio.h>
 
#define BIG_ENDIAN 0
#define LITTLE_ENDIAN 1
 
int TestByteOrder() {
        short int word = 0x0001;
        char *b = (char *)&word;
        return (b[0] ? LITTLE_ENDIAN : BIG_ENDIAN);
}
 
int main() {
        int r = TestByteOrder();
        printf("%s\n", r == LITTLE_ENDIAN ? "Little Endian" : "Big Endian");
        return r;
}