SIGVTALRM
The alarm clock that only counts the time the process spent on the CPU.
SIGALRM's bookkeeping sibling: the virtual clock only ticks while the process runs in user mode. Waiting on I/O, sleeping, losing the CPU to someone else — none of it counts. It is working time, not wall time.
Old runtimes used this tick to slice time among green threads; today it is a museum piece with the occasional use: capping the CPU consumption of a stretch of code without depending on the clock of the world outside.
$ ./cruncher --limite-cpu 5s
Virtual timer expired
(five seconds of CPU, not of wall clock)