辨别系统是64位 or 32位以及大小端

endian.c

#include <stdio.h>

int main(void)
{
    int a = 0x80000000;
    printf("os is %d\t%d\n", a, sizeof(int));
    union ut {
        short s;
        char c[2];
    } u;
    if(sizeof(short) == 2){
        u.s = 0x0102;
        if (u.c[0] == 1 && u.c[1] == 2) {
            printf("big endian.\n");
        }
        else if (u.c[0] == 2 && u.c[1] == 1) {
            printf("little endian.\n");
        }
    }
    return 0;
}