如何判断机器的endianness

字节层次上

_Bool is_little_endian() {
  int a = 1;
  char* p = (char*)&a;
  return *p == 1;
}

或

_Bool is_little_endian() {
  union {
    int a;
    char b;
  } u;
  u.a = 1;
  return u.b == 1;
}

位层次上

_Bool is_little_endian() {
  union {
    unsigned char a;
    struct {
      unsigned char b0 : 1;
      unsigned char : 7;
    } b;
  } u;
  u.a = 1;
  return u.b.b0 == 1;
}

比较同一块内存的不同表示

big endian:

+----+----+----+----+
| 00 | 00 | 00 | 01 |
+----+----+----+----+

-------- 内存地址增长 -------->

little endian:

+----+----+----+----+
| 01 | 00 | 00 | 00 |
+----+----+----+----+

-------- 内存地址增长 -------->