Next qmail 13

Trigger?

qmail-send is always running, usually firing off deliveries
But if it has nothing to do, it might go to sleep
It does this by trying to read queue/lock/trigger, which is a pipe file
Unix puts it to sleep until there is something in the pipe
If qmail-queue writes a byte into the pipe, qmail-send wakes up
- Then it examines queue/todo immediately
This a simple, reliable way to get one program to wake up another
- It's tempting to try to use signals
- This is better
- Signals can get lost
- Signals can kill the process
The byte in the pipe is safe
- And it stays in the pipe until it is read by qmail-send