In Linux (e.g. Ubuntu 18.04 or other), do getrlimit() and setrlimit() work by reading from and writing to /etc/security/limits.conf?
I am trying to understand the relations between getrlimit() and setrlimit() and /etc/security/limits.conf. Thanks.
In fact, it's the other way around: /etc/security/limits.conf controls calls made to getrlimit and setrlimit.
getrlimit and setrlimit read and write kernel state associated with the calling process. They do not access any file at all. Like most kernel state associated with a particular process, the resource limits are inherited (copied into) fork-ed children.
/etc/security/limits.conf is never modified automatically (except possibly by a system upgrade); it's intended for the sysadmin to edit, by hand.
The login program (technically, the pam_limits module that it may load, and does load in Ubuntu 18's default configuration) reads /etc/security/limits.conf, and uses the information in that file to make a series of calls to setrlimit, establishing resource limits for the user's initial shell. Those resource limits then inherit to all processes started by the shell, and so on transitively.