Get boot time and uptime on macOS in C.

uptime.c

// Tested on macOS 10.12.
//
// Usage:
// $ clang uptime.c && ./a.out
// boot time (UNIX time): 1502299682.147510
// uptime (seconds): 410563.269028116

#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <sys/sysctl.h>

struct timeval get_boottime();
struct timespec get_uptime();
void print_error();

int main(int argc, char** argv)
{
    struct timeval boottime = get_boottime();
    printf("boot time (UNIX time): %li.%06i\n", boottime.tv_sec, boottime.tv_usec);

    struct timespec uptime = get_uptime();
    printf("uptime (seconds): %li.%09li\n", uptime.tv_sec, uptime.tv_nsec);
}

struct timeval get_boottime()
{
    struct timeval boottime;
    int mib[2] = { CTL_KERN, KERN_BOOTTIME };
    size_t size = sizeof(boottime);

    if (0 != sysctl(mib, 2, &boottime, &size, NULL, 0))
    {
        print_error();
    }

    return boottime;
}

struct timespec get_uptime()
{
    struct timespec uptime;
    if (0 != clock_gettime(CLOCK_MONOTONIC_RAW, &uptime))
    {
        print_error();
    }

    return uptime;
}

void print_error()
{
    printf("%s\n", strerror(errno));
    exit(errno);
}