Another way to do it, at the expense of having twice-as-many child processes, is to initially fork a child which sets an alarm and then forks the actual child.   If the alarm goes off, this process kills its child and then exits (with a return-code of 1).   It does nothing else but to wait, either for its child to exit or for the alarm to go off.

The way I saw it done was with a small command:   timed_exec -t timeout command.   That’s what the parent-process actually executed and waited-for.   But it was such a handy thing that I saw it being used in a lot of shell-scripts, too.