我应该在前面放多少个零来表示八进制数？

Question

以下描述来自linux系统中的man 2 open。有时他们会在前面放两个零，有时三个。但我认为一个零是足够好的。这背后的想法是什么？

          S_IRWXU  00700 user (file owner) has read, write, and execute permission

          S_IRUSR  00400 user has read permission

          S_IWUSR  00200 user has write permission

          S_IXUSR  00100 user has execute permission

          S_IRWXG  00070 group has read, write, and execute permission

          S_IRGRP  00040 group has read permission

          S_IWGRP  00020 group has write permission

          S_IXGRP  00010 group has execute permission

          S_IRWXO  00007 others have read, write, and execute permission

          S_IROTH  00004 others have read permission

          S_IWOTH  00002 others have write permission

          S_IXOTH  00001 others have execute permission

          According to POSIX, the effect when other bits are set in mode is unspecified.  On Linux, the following bits are also honored in mode:

          S_ISUID  0004000 set-user-ID bit

          S_ISGID  0002000 set-group-ID bit (see stat(2))

          S_ISVTX  0001000 sticky bit (see stat(2))