Questions on SET operation

[sun]

Hi:    I was trying to implement an agent , using the  netSnmpHostsTable as an example and trying to understand the calling sequence for the table handler when performing SET operation.    I noticed that in a normal operation (without any error). The sequence is MODE_SET_RESERVE1->MODE_SET_RESERVE2->SET_ACTION->COMMIT   However, if I called the netsnmp_set_request_error() in SET_RESERVE2 phase,  It will be called with "MODE_FREE", the netSnmpHostsTable  didn't handle this mode. Furthermore, if I set the error in SET_COMMIT phase, The calling sequence didn't change, just like the normal sequence. I expected  it will be called with "SET_UNDO" mode, but it didn't.   My question is, when will my handler be called with "SET_UNDO " mode ? and when it will be called with SET_FREE? mode? Any detail documents describe the table handler?   Thank you very much for the help! chris sun