如何用C语言代码判断字节编址是大端模式还是小端模式，请举三种方法

方法一：使用联合体实现大小端判断

#include <stdio.h>

int main() {
    union {
        short s;
        char c[sizeof(short)];
    } un;
    un.s = 0x0102;
    if (sizeof(short) == 2) {
        if (un.c[0] == 1 && un.c[1] == 2) {
            printf("big-endian\n");
        } else if (un.c[0] == 2 && un.c[1] == 1) {
            printf("little-endian\n");
        } else {
            printf("unknown\n");
        }
    } else {
        printf("sizeof(short) = %lu\n", sizeof(short));
    }
    return 0;
}

方法二：使用指针强制类型转换实现大小端判断

#include <stdio.h>

int main() {
    short s = 0x0102;
    char *p = (char*)&s;
    if (*p == 1 && *(p + 1) == 2) {
        printf("big-endian\n");
    } else if (*p == 2 && *(p + 1) == 1) {
        printf("little-endian\n");
    } else {
        printf("unknown\n");
    }
    return 0;
}

方法三：使用位运算实现大小端判断

#include <stdio.h>

int main() {
    short s = 0x0102;
    char *p = (char*)&s;
    if ((p[0] << 8 | p[1]) == s) {
        printf("big-endian\n");
    } else if ((p[1] << 8 | p[0]) == s) {
        printf("little-endian\n");
    } else {
        printf("unknown\n");
    }
    return 0;
}