Where is the exact code part of the walletnotify function? I'm new on the BTC project, so please spare me if I asked something newbies.

I downloaded the latest 0.92 BTC source code and trying to make a customize to the walletnotify function for myself while running the bitcoind. But when I search into the code, that triggered runCommand() at util.cpp and pew "End of the road".

I googled about this walletnotify function, but all I seemed are only the usage infos. So I think it's time to ask the question here but thinking the dead-end by myself.
I don't know exactly what you are asking, but

wallet.cpp (from line 591)

    // notify an external script when a wallet transaction comes in or is updated
    std::string strCmd = GetArg("-walletnotify", "");

    if ( !strCmd.empty())
    {
        boost::replace_all(strCmd, "%s", wtxIn.GetHash().GetHex());
        boost::thread t(runCommand, strCmd); // thread runs free
    }


runs a thread that executes the runCommand() function in util.cpp

wallet.cpp (from line 1335)

void runCommand(std::string strCommand)
{
    int nErr = ::system(strCommand.c_str());
    if (nErr)
        LogPrintf("runCommand error: system(%s) returned %d\n", strCommand, nErr);
}


and I think that is all code you need.