clock_gettime_override.cpp

#include <dlfcn.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

extern "C"
int clock_gettime(clockid_t clk_id, struct timespec *tp) {
    using clock_gettime_ptr = int (*)(clockid_t, struct timespec*);
    const static clock_gettime_ptr real_clock_gettime
        = (clock_gettime_ptr) dlsym(RTLD_NEXT, "clock_gettime");
    int ret = real_clock_gettime(clk_id, tp);
    if (ret != -1) {
        const short AMOUNT_TO_SLOW = 4;
        const time_t NSEC_FRAC = 1000000000 / AMOUNT_TO_SLOW;
        const long nsec_part = NSEC_FRAC * (tp->tv_sec % AMOUNT_TO_SLOW);
        tp->tv_nsec = tp->tv_nsec / AMOUNT_TO_SLOW + nsec_part;
        tp->tv_sec = tp->tv_sec / AMOUNT_TO_SLOW;
    }
    return ret;
}

clock_gettime_override.rs

#[macro_use]
extern crate lazy_static;
extern crate libc;

use libc::{clockid_t, timespec, c_int};

#[no_mangle]
pub extern "C" fn clock_gettime(clk_id: clockid_t, tp: *mut timespec) -> c_int {
    type ClockGettimeType = fn(clk_id: clockid_t, tp: *mut timespec) -> c_int;
    unsafe {
        lazy_static! {
            static ref real_clock_gettime: ClockGettimeType = unsafe {
                let f = libc::dlsym(libc::RTLD_NEXT, "clock_gettime\0".as_ptr() as *const i8);
                std::mem::transmute::<*const libc::c_void, ClockGettimeType>(f)
            };
        }
        let ret = real_clock_gettime(clk_id, tp);
        if ret != -1 {
            let amount_to_slow = 4;
            let nsec_frac = 1000000000 / amount_to_slow;
            let nsec_part = nsec_frac * ((*tp).tv_sec % amount_to_slow);
            (*tp).tv_nsec = (*tp).tv_nsec / amount_to_slow + nsec_part;
            (*tp).tv_sec = (*tp).tv_sec / amount_to_slow;
        }
        ret
    }
}