C - timed read of lines from file into dynamic memory

/* C best of 10 reads (7200 rpm laptop drive):
 * read/stored 25481 lines in 0.003850 sec.
 * total heap usage: 25,494 allocs, 25,494 frees, 733,979 bytes allocated
 * (reading, e.g. /usr/share/dict/words)
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

enum { MAXP = 8, MAXC = 512 };

/** reallocate block of 'psz' objects to twice 'nelem'.
 *  'nelem' is updated to reflect the new allocation size.
 *  exits on allocation failure.
 */
void *xrealloc2 (void *ptr, size_t psz, size_t *nelem)
{
    void *memptr = realloc ((char *)ptr, *nelem * 2 * psz);

    if (!memptr) {
        fprintf (stderr, "realloc() error: virtual memory exhausted.\n");
        exit (EXIT_FAILURE);
    }

    /* zero new memory (optional) */
    memset ((char *)memptr + *nelem * psz, 0, *nelem * psz);
    *nelem *= 2;

    return memptr;
}

/** 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;
}

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

    size_t n = 0,
        size = MAXP;        /* initialize 'size' to MAXP */
    double t1, t2;
    char tmp[MAXC] = "",    /* tmp buffer for read of MAXC chars */
        **buf = NULL;
    FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;

    if (!fp) {  /* validate file open for reading */
        fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
        return 1;
    }

    /* allocate/validate 'size' ptrs to char */
    if (!(buf = calloc (size, sizeof *buf))) {
        fprintf (stderr, "error; virtual memory exhausted.\n");
        return 1;
    }

    t1 = clkrtv();
    while (fgets (tmp, MAXC, fp)) {     /* read each line */
        size_t l = strlen (tmp);        /* get length */
        if (l && tmp[l - 1] == '\n')    /* trim '\n' */
            tmp[--l] = 0;
        if (!(buf[n] = malloc (l + 1))) /* allocate for line */
            goto memfull;
        strcpy (buf[n++], tmp);         /* copy to buf[n] */
        if (n == size)                  /* realloc as required */
            buf = xrealloc2 (buf, sizeof *buf, &size);
    }
    memfull:;
    t2 = clkrtv();

    printf ("read/stored %zu lines in %.6f sec.\n", n, t2-t1);

    if (fp != stdin) fclose (fp);   /* close file if not stdin */

    for (size_t i = 0; i < n; i++)  /* free allocated memory */
        free (buf[i]);
    free (buf);

    return 0;
}