#include <unistd.h>
#include <time.h>
#include <stdio.h>
#include <inttypes.h>
#include <fcntl.h>
#include <stdlib.h>

static void* initial_brk;

unsigned f_return0() {
    return 0;
}

unsigned f_getpid() {
    return getpid();
}

unsigned f_getppid() {
    return getppid();
}

unsigned f_time() {
    return time(nullptr);
}

unsigned f_clock_gettime() {
    timespec ts;
    clock_gettime(CLOCK_REALTIME, &ts);
    return ts.tv_nsec;
}

unsigned f_clock_gettime_coarse() {
    timespec ts;
    clock_gettime(CLOCK_REALTIME_COARSE, &ts);
    return ts.tv_nsec;
}

unsigned f_sbrk() {
    return reinterpret_cast<uintptr_t>(sbrk(0));
}

unsigned f_brk() {
    return brk(initial_brk);
}

unsigned f_open_close() {
    int fd = open("/dev/null", O_RDONLY);
    close(fd);
    return fd;
}

unsigned f_close() {
    return close(0);
}

void usage() {
    fprintf(stderr, "Usage: ./syscall [-0pPtcCsbox]\n");
    exit(1);
}

int main(int argc, char** argv) {
    unsigned (*volatile f)(void) = f_getpid;
    initial_brk = sbrk(0);

    int opt;
    while ((opt = getopt(argc, argv, "0pPtcCsbox")) != -1) {
        switch (opt) {
        case '0':
            f = f_return0;
            break;
        case 'p':
            f = f_getpid;
            break;
        case 'P':
            f = f_getppid;
            break;
        case 't':
            f = f_time;
            break;
        case 'c':
            f = f_clock_gettime;
            break;
        case 'C':
            f = f_clock_gettime_coarse;
            break;
        case 's':
            f = f_sbrk;
            break;
        case 'b':
            f = f_brk;
            break;
        case 'o':
            f = f_open_close;
            break;
        case 'x':
            f = f_close;
            break;
        default:
            usage();
        }
    }

    if (optind != argc) {
        usage();
    }

    timespec ts0;
    clock_gettime(CLOCK_REALTIME, &ts0);
    unsigned long n = 0;

    for (unsigned i = 0; i != 1000000; ++i) {
        n += f();
    }

    timespec ts1;
    clock_gettime(CLOCK_REALTIME, &ts1);

    double t0 = ts0.tv_sec + ts0.tv_nsec / 1e9;
    double t1 = ts1.tv_sec + ts1.tv_nsec / 1e9;
    printf("result: %lu in %.06fs\n", n, t1 - t0);
}