A small library for changing the speed of time in games for Linux.

libtimehook.c

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dlfcn.h>

#define HOOK(ret, name, args) ret name args { \
	static ret (*old_##name) args = 0; \
	if (!old_##name) { \
		old_##name = (ret (*) args) dlsym(RTLD_NEXT, #name); \
		if (!old_##name) { printf("dlsym \"%s\" failed\n", #name); exit(1); } \
	}

#include <time.h>
#include <sys/time.h>

static double speed, rspeed;
static int64_t old_timeval;

__attribute__((constructor))
static void speed_init() {
	const char *str = getenv("HOOK_SPEED");

	{
		struct timeval tv;
		speed = rspeed = 1;
		gettimeofday(&tv, NULL);
		old_timeval = tv.tv_sec * (int64_t)1000000 + tv.tv_usec;
		rspeed = 1 / (speed = 0.5);
	}

	if (str) {
		double newspeed = atof(str);
		printf("HOOK_SPEED = %f\n", newspeed);
		if (newspeed == newspeed && newspeed >= 0.1 && newspeed <= 2.0) {
			rspeed = 1 / (speed = newspeed);
		} else {
			printf("HOOK_SPEED out of range, set to default %f\n", speed);
		}
	}
}

static void change_timeval(const struct timeval *tv, struct timeval *new, double mul) {
	int64_t n = 1000000, t = tv->tv_sec * n + tv->tv_usec;
	t -= old_timeval;
	t *= mul;
	t += old_timeval;
	new->tv_usec = t % n;
	new->tv_sec = t / n;
}

static void change_timespec(const struct timespec *tp, struct timespec *new, double mul) {
	int64_t n = 1000000000, t = tp->tv_sec * n + tp->tv_nsec;
	t *= mul;
	new->tv_nsec = t % n;
	new->tv_sec = t / n;
}

HOOK(int, gettimeofday, (struct timeval *tv, void *tz))
	int ret = old_gettimeofday(tv, tz);
	if (!ret && tv) change_timeval(tv, tv, speed);
	return ret;
}

HOOK(int, clock_gettime, (clockid_t clk_id, struct timespec *tp))
	int ret = old_clock_gettime(clk_id, tp);
	if (!ret && tp) change_timespec(tp, tp, speed);
	return ret;
}

HOOK(int, nanosleep, (const struct timespec *req, struct timespec *rem))
	struct timespec req2, rem2;
	if (req) change_timespec(req, &req2, rspeed), req = &req2;
	int ret = old_nanosleep(req, rem);
	if (!ret && rem) change_timespec(rem, &rem2, speed);
	return ret;
}

Build:

cc -Os libtimehook.c -o libtimehook.so -fPIC -shared -ldl

Usage:

export HOOK_SPEED=0.65
export LD_PRELOAD="path/to/libtimehook.so"
./game