Random reservoir sampler in C. Lacks a good deal of proper error checking.

resrv.c

#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <stdlib.h>


int main(int argc, char **argv) {
  int size = atoi(argv[1]);
  char **resrv = calloc(size, sizeof(char*));
  int i = 0; size_t nread;
  char *line = NULL;
  while (i < size && getline(&line, &nread, stdin) > 0) {
    // printf ("Got line in 1st loop: i:%d, read: %d bytes\n", i, nread);
    resrv[i++] = line;
    line = NULL;
  }
  // printf ("After 1st loop: i:%d, read: %d bytes\n", i, nread);

  for (; line=NULL, getline(&line, &nread, stdin) > 0; ++i) {
    int off = random() % i;
    // printf ("1nd loop: i:%d, off:%d, read: %d bytes\n", i, off, nread);
    if(off < size) {
      free(resrv[off]);
      resrv[off] = line;
    } else {
      free(line);
    }

    line = NULL;
  }

  for (int i = 0; i < size && resrv[i]; ++i) {
    fputs(resrv[i], stdout);
    free(resrv[i]);
  }
}