C++ timed read of lines from file into dynamic list<string>

/* C++ best of 10 reads (7200 rpm laptop drive):
 * read/stored 25481 lines in 0.00813842 sec.
 * total heap usage: 76,465 allocs, 76,464 frees, 2,151,883 bytes allocated
 * (reading, e.g. /usr/share/dict/words)
 */
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <list>
#include <time.h>

using namespace std;

/** return the number of seconds (nanosecond resolution)
 *  using clock_gettime/CLOCK_REALTIME
 */
double clkrtv (void)
{
    struct timespec ts;
    double sec;

    clock_gettime (CLOCK_REALTIME, &ts);
    sec = ts.tv_nsec;
    sec /= 1e9;
    sec += ts.tv_sec;

    return sec;
}

/* output lines read/stored and time taken */
void printlist (list<string> *l, double tm) {

    cout << "read/stored " << l->size() << " lines in "
        << tm << " sec." << endl;
}


int main (int argc, char **argv) {

    double t1, t2;
    string line;
    list<string> buf;   // doublely-linked list of string

    if (argc != 2) {
        cerr << "One argument is required." << endl;
        return 1;
    }

    ifstream f (argv[1]);       // open file

    t1 = clkrtv();
    while (getline (f, line)) { // read lines into buf
        buf.push_back(line);
    }
    t2 = clkrtv();

    f.close();

    printlist (&buf, t2-t1);

    return 0;
}