__typeof__(type)的用法（gcc擴展）

    __typeof__(struct s *) xxx;

等同於:

   struct s * xxx;

下面的例子可以證明:

#include <stdio.h>
#include <stdlib.h>

struct s {
    int x;
    int y;
    int z;
};

int main(void)
{
    __typeof__(struct s *) xxx;
    struct s * yyy;
    struct s zzz;
    printf("sizeof(xxx) is %d\n", sizeof(xxx));
    printf("sizeof(yyy) is %d\n", sizeof(yyy));
    printf("sizeof(zzz) is %d\n", sizeof(zzz));
}

運行結果：

$ gcc a.c && ./a.out
sizeof(xxx) is 8
sizeof(yyy) is 8
sizeof(zzz) is 12

參考文章：
https://www.ibm.com/support/knowledgecenter/zh/ssw_ibm_i_72/rzarg/typeof_operator.htm
https://www.jianshu.com/p/e3d69cd325db