如何理解程序

Question

int mask = 0700, N_BITS = 3;
struct stat buff;
static char perm[] = {"---", "--x", "-w-", "-w x", "r--", "r-x", "rw-", "rwx"};
stat(f_d, &buff); //get the permissions info of file
printf("%3s", perm[buff.st_mode & mask >> (i - 1) * N_BITS)] //HOW TO UNDERSTAND ?`
mask >>= N_BITS; //HOW TO UNDERSTAND ?

Answer 1

>>是按位右移运算符。

X >>= N

相当于

X = X >> N

一些例子：

• （4>＆gt; 1）=（0100b＆gt;＆gt; 1）= 0010b = 2
• （12>＆gt; 2）=（1100b>＆gt; 2）= 0011b = 3

Answer 2

>>是bitwise right shift operator

mask>>=N_BITS相当于mask = mask >> N_BITS

所以最初

mask = 0b111000000;

N_BITS操作后mask>>=N_BITS等于 3

 mask = 0b000111000;