Getting pointer from native syscall in Java

Question

Having the following Java code

Object[] params = new Object[] {new Object(), null}
int ret = lib.getClass().getMethod("syscall", int.class, Object[].class).invoke(
    lib, 116, params
);

where 116 is the code (on MacOS) for gettimofday system function,

how should I specify params correctly so that I can extract timeval struct containing the result (as specified by https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/gettimeofday.2.html)

Answer 1

The function expects specific struct as a first parameter which it will populate:

@Structure.FieldOrder({"sec", "usec"})
public static class Timeval extends Structure {
    /**
     * Seconds.
     */
    public long sec;

    /**
     * Microseconds.
     */
    public long usec;
}

Supplying parameters as follows succeed the call:

final Timeval timeval = new Timeval();
final Object[] params = new Object[]{timeval, null};
lib.syscall(116, params)

Result can be retrieved from timeval.sec and timeval.usec.

Thanks to @Mark Rotteveel for the hints.