How can I launch multiple instances of an application using launchd?

Question

My application is split into two parts. The main application and a helper tool. The helper tool performs a task with elevated permissions.

The launchd plist looks like this: (Only important settings included.)

<key>UserName</key>
<string>root</string>
<key>ProgramArguments</key>
<array>
    <string>/Library/PrivilegedHelperTools/helperTool</string>
</array>
<key>Sockets</key>
<dict>
    <key>IPC</key>
    <dict>
        <key>SockPathName</key>
        <string>/tmp/TheSocket</string>
    </dict>
</dict>

Is there a way to launch a new helper instance for every connection to the socket?

Or alternatively, is there a existing template for handling multiple requests? (I'm doing this myself at the moment, which is quite a lot of ugly code.)

Answer 1

This will probably be my first answer in Stackoverflow :)

First, Set inetdCompatibility with Wait to false. This will make launchd to accept the socket.

<key>inetdCompatibility</key>
<dict>
    <key>Instances</key>
    <integer>42</integer>
    <key>Wait</key>
    <false/>
</dict>

Once, launchd accepted the socket. The accepted to socket will be passed into your program as STDIN_FILENO. Your launchd process can access the accepted the socket as follows: (I copied the code from open source sshd)

int sock_in;
int sock_out;           
sock_in = sock_out = dup(STDIN_FILENO);
NSLog(@"socket descriptor: %d", sock_in);

The sock_in is already accepted. So your program can use it without calling accept.

I am assuming you already have a plist which will monitor a socket port for you. If not, it's possible to do that as follows. It will create a launchd socket listen for port 18411 with IPv4 TCP.

<key>Sockets</key>
<dict>
    <key>Listeners</key>
    <dict>
        <key>SockServiceName</key>
        <string>18411</string>
        <key>SockType</key>
        <string>stream</string>
        <key>SockFamily</key>
        <string>IPv4</string>
    </dict>
</dict>