How to use standby backup to restore primary db

–Propose: Backup db from physical standby db by using RMAN, recove datafile of primary standby db.
–enviroment:
Three machines:
Machine A: 172.16.100.29 Linux ES3
Machine B: 172.16.100.21 Linux ES3
Machine C: 172.16.100.31 Linux ES3
–step1. build Machine A as primary db and Machine B as standby db
–create another db on Machine C
Machine DB_TNSNAME
A STD1.29
B STD1.21
C STD1.31
–step2. create recovery [...]