libftrig is a portable Unix C programming interface allowing a process (the subscriber or listener) to be instantly notified when another process (the notifier or writer) signals an event.

What is notification ?

Notification vs. polling

Process A is notified of an event E when it gets a instant notice that event E has happened; the notice may disrupt A's execution flow. Notification is the opposite of polling, where A has to periodically (every T milliseconds) check whether E happened and has no other way to be informed of it.

Polling is generally considered bad practice - and is inferior to notification in practically every case - for three reasons:

Notification, on the other hand, is generally optimal: reaction time is zero, and resource consumption is minimal - a process can sleep as soon as it's not handling an event, and only wake up when needed.

Of course, the problem of notification is that it's often more difficult to implement. Notification frameworks are generally more complex, involving lots of asynchronism; polling is widely used because it's easy.

Notifications and Unix

Unix provides several frameworks so that a process B (or the kernel) can notify process A.

What we want

We need a general framework to:

This requires a many-to-many approach that Unix does not provide natively, and that is what libftrig does.

That's what a bus is for. D-Bus already does all this.

Yes, a bus is a good many-to-many notification mechanism indeed. However, a Unix bus can only be implemented via a daemon - you need a long-running process, i.e. a service, to implement a bus. And s6 is a supervision suite, i.e. a set of programs designed to manage services; we would like to be able to use notifications in the supervision suite, to be able to wait for a service to be up or down... without relying on a particular service to be up. libftrig provides a notification mechanism that does not need a bus service to be up, that's its main advantage over a bus.

If you are not concerned with supervision and can depend on a bus service, though, then yes, by all means, use a bus for your notification needs. There is a skabus project in the making, which aims to be simpler, smaller and more maintainable than D-Bus.

How to use libftrig

libftrig is really a part of libs6: all the functions are implemented in the libs6.a archive, or the libs6.so dynamic shared object. However, the interfaces are different for notifiers and listeners: