static void
mergefps (char **files, register int nfiles,
	  FILE *ofp, const char *output_file)
{
  FILE *fps[NMERGE];		/* Input streams for each file.  */
  struct buffer buffer[NMERGE];	/* Input buffers for each file. */
  struct line saved;		/* Saved line storage for unique check. */
  struct line const *savedline = NULL;
				/* &saved if there is a saved line. */
  size_t savealloc = 0;		/* Size allocated for the saved line. */
  struct line const *cur[NMERGE]; /* Current line in each line table. */
  struct line const *base[NMERGE]; /* Base of each line table.  */
  int ord[NMERGE];		/* Table representing a permutation of fps,
				   such that cur[ord[0]] is the smallest line
				   and will be next output. */
  register int i, j, t;
  struct keyfield *key = keylist;
  saved.text = NULL;

  /* Read initial lines from each input file. */
  for (i = 0; i < nfiles; )
    {
      fps[i] = xfopen (files[i], "r");
      initbuf (&buffer[i], sizeof (struct line),
	       MAX (merge_buffer_size, sort_size / nfiles));
      if (fillbuf (&buffer[i], fps[i], files[i]))
	{
	  struct line const *linelim = buffer_linelim (&buffer[i]);
	  cur[i] = linelim - 1;
	  base[i] = linelim - buffer[i].nlines;
	  i++;
	}
      else
	{
	  /* fps[i] is empty; eliminate it from future consideration.  */
	  xfclose (fps[i], files[i]);
	  zaptemp (files[i]);
	  free (buffer[i].buf);
	  --nfiles;
	  for (j = i; j < nfiles; ++j)
	    files[j] = files[j + 1];
	}
    }

  if (! ofp)
    ofp = xfopen (output_file, "w");

  /* Set up the ord table according to comparisons among input lines.
     Since this only reorders two items if one is strictly greater than
     the other, it is stable. */
  for (i = 0; i < nfiles; ++i)
    ord[i] = i;
  for (i = 1; i < nfiles; ++i)
    if (0 < compare (cur[ord[i - 1]], cur[ord[i]]))
      t = ord[i - 1], ord[i - 1] = ord[i], ord[i] = t, i = 0;

  /* Repeatedly output the smallest line until no input remains. */
  while (nfiles)
    {
      struct line const *smallest = cur[ord[0]];

      /* If uniquified output is turned on, output only the first of
	 an identical series of lines. */
      if (unique)