Unixes make a further distinction: in reality (under the hood), the resource limit for a given type is not one number—it's two:
- A value for the hard limit
- A value for the soft limit
The hard limit is the true maximum; as a regular user, it's impossible to exceed this limit. What if a process attempts this? Simple: it gets killed by the OS.
The soft limit, on the other hand, can be breached: in the case of some resource limits, the process (that exceeds the soft limit) will be sent a signal by the kernel. Think of this as a warning: you're nearing the limit kind of thing. Again, don't worry, we take a deep dive into signaling in Chapter 11, Signaling - Part I, and, Chapter 12, Signaling - Part II. For example, if a process exceeds the soft limit for file size, the OS responds by delivering the SIGXFSZ signal—SIGnal: eXceeding...
