Question

S_IRUSR is a macro constant in sys/stat.h of posix. it stands for user read permission bit.

the prefix S_ may stand for ‘status of’
the RUSR maybe Read of User.
but what’s the meaning of ‘I’?

Answer 1

The naming is historic … dating back to the very earliest days of UNIX. The “S” is for STAT, the “I” for INODE (a term not really used in POSIX itself outside Rationale), the “R” for READ and the “USR” for USER.

Inodes do get a few mentions, and the stat structure also includes “st_ino”, which the standard describes as the “File Serial Number”. In many POSIX implementations, an inode is a data structure containing all the meta-data for the file (much of which is what is read by the stat() call).

(From Wikipedia:) The reason for designating these as “i” nodes is unknown. When asked, Unix pioneer Dennis Ritchie replied:

In truth, I don’t know either. It was just a term that we started to
use. “Index” is my best guess, because of the slightly unusual file
system structure that stored the access information of files as a flat
array on the disk, with all the hierarchical directory information
living aside from this. Thus the i-number is an index in this array,
the i-node is the selected element of the array.

(The “i-” notation was used in the 1st edition manual; its hyphen was gradually dropped.)