Unix与Linux文件权限的含义是什么

#include <stdio.h>#include <sys/stat.h>/*S_IRUSR: 使用者读权限, READ USER 4S_IWUSR: 使用者写权限, WRITE USER 2S_IRGRP: 组用户读权限, READ GROUP 4S_IROTH: 其他用户读权限, READ OTHER 4*/int main(int argc, char const *argv[]) {    printf(        "S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH=%o, %o, %o, %o, (S_IRUSR | S_IWUSR "        "| S_IRGRP | S_IROTH)=%o
",        S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH,        (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH));    return 0;}

S_IRUSR, S_IWUSR, S_IRGRP, S_IROTH=400, 200, 40, 4, (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)=644