Mock time in unix process

demo_time.js

console.log("System time is:", new Date().toISOString());

mock_time.c

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>
#include <time.h>
#include <sys/time.h>
#include <stdlib.h>
#include <string.h>

time_t read_time_from_file() {
    static char path[4096];
    const char *state = getenv("NETERO_STATE");
    if (!state) return time(NULL);

    size_t len = strlen(state);
    if (len + 9 >= sizeof(path)) return time(NULL);
    strcpy(path, state);
    if (path[len-1] != '/') {
        path[len] = '/';
        path[len+1] = '\0';
        len += 1;
    }
    strcat(path, "now.txt");

    FILE *f = fopen(path, "r");
    if (!f) return time(NULL);

    char buf[64];
    if (!fgets(buf, sizeof(buf), f)) {
        fclose(f);
        return time(NULL);
    }
    fclose(f);

    struct tm tm = {0};
    if (!strptime(buf, "%Y-%m-%dT%H:%M:%SZ", &tm)) {
        return time(NULL);
    }
    time_t t = timegm(&tm);
    return t;
}

time_t time(time_t *tloc) {
    time_t t = read_time_from_file();
    if (tloc) *tloc = t;
    return t;
}

int gettimeofday(struct timeval *tv, void *tz) {
    time_t t = read_time_from_file();
    if (tv) {
        tv->tv_sec = t;
        tv->tv_usec = 0;
    }
    if (tz) {
        struct timezone *ptz = (struct timezone *)tz;
        ptz->tz_minuteswest = 0;
        ptz->tz_dsttime = 0;
    }
    return 0;
}

run.sh

nix run nixpkgs#gcc_multi -- -shared -fPIC -o libmocktime.so mock_time.c -ldl
nix shell nixpkgs#nodejs --command watch LD_PRELOAD=$PWD/libmocktime.so node demo_time.js