18 Limiting process resources

The library(rlimit) library provides an interface to the POSIX getrlimit()/setrlimit() API that control the maximum resource-usage of a process or group of processes. This call is especially useful for servers such as CGI scripts and inetd-controlled servers to avoid an uncontrolled script claiming too much resources.

rlimit(+Resource, -Old, +New)

Query and/or set the limit for Resource. Time-values are in seconds and size-values are counted in bytes. The following values are supported by this library. Please note that not all resources may be available and accessible on all platforms. This predicate can throw a variety of exceptions. In portable code this should be guarded with catch/3. The defined resources are:

as	Max address space
cpu	CPU time in seconds
fsize	Maximum filesize
data	max data size
stack	max stack size
core	max core file size
rss	max resident set size
nproc	max number of processes
nofile	max number of open files
memlock	max locked-in-memory address

When the process hits a limit POSIX systems normally send the process a signal that terminates it. These signals may be caught using SWI-Prolog's on_signal/3 primitive. The code below illustrates this behaviour. Please note that asynchronous signal handling is dangerous, especially when using threads. 100% fail-safe operation cannot be guaranteed, but this procedure will inform the user properly‘most of the time'.

rlimit_demo :-
        rlimit(cpu, _, 2),
        on_signal(xcpu, _, cpu_exceeded),
        ( repeat, fail ).

cpu_exceeded(_Sig) :-
        format(user_error, 'CPU time exceeded~n', []),
        halt(1).