Compiled with "gcc linux_time.c && ./a.out" , getting -> undefined reference to `clock_gettime'. Fixed using "gcc test_linux.c -Xlinker -lrt"

linux_time.c

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

double get_wall_time(){
  struct timeval tv;
  gettimeofday(&tv, NULL);
  return tv.tv_sec + (tv.tv_usec/1000000.0);
}
double get_cpu_time(){
  struct timespec clock;
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID , &clock);
  return clock.tv_sec + (clock.tv_nsec/1000000000.0);
}

int main() {
  double wall_time1 = get_wall_time();
  double cpu_time1 = get_cpu_time();

  int count = 0;
  int i=0;
  for (; i < (1 << 30); ++i)
  {
    count += 1;
  }

  double wall_time2 = get_wall_time();
  double cpu_time2 = get_cpu_time();

  fprintf(stderr, "Count = %u -- wall = %f -- cpu = %f \n", count, wall_time2 - wall_time1, cpu_time2 - cpu_time1);
}