Function drmgr_register_pre_syscall_event_ex 

pub unsafe extern "C" fn drmgr_register_pre_syscall_event_ex(
    func: Option<unsafe extern "C" fn(drcontext: *mut c_void, sysnum: c_int) -> bool_>,
    priority: *mut drmgr_priority_t,
) -> bool_

Registers a callback function for the pre-syscall event, which behaves just like DR’s pre-syscall event dr_register_pre_syscall_event(), except that it is ordered according to \p priority. A default priority of 0 is used for events registered via drmgr_register_pre_syscall_event(). A filter event is still needed to ensure that a pre- or post-syscall event is actually called: use dr_register_filter_syscall_event(). \return whether successful.