Do a binary search for the max RLIMIT_NOFILE.

maxrlimit.c

/* Copyright (c) 2013, Ben Noordhuis <info@bnoordhuis.nl>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

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

int main(void)
{ 
  struct rlimit lim;
  rlim_t lo;
  rlim_t hi;

  if (getrlimit(RLIMIT_NOFILE, &lim)) {
    perror("getrlimit(RLIMIT_NOFILE)");
    exit(1);
  }

  lo = lim.rlim_cur;
  hi = lim.rlim_max;
                                    
  if (hi > 0x1000000)
    hi = 0x1000000;  /* 16.7m */

  /* Do a binary search for the max RLIMIT_NOFILE. */
  while (lo + 1 < hi) {
    lim.rlim_cur = lo + (hi - lo) / 2;
    if (setrlimit(RLIMIT_NOFILE, &lim))
      hi = lim.rlim_cur;
    else
      lo = lim.rlim_cur;
  }

  if (getrlimit(RLIMIT_NOFILE, &lim)) {
    perror("getrlimit(RLIMIT_NOFILE)");
    exit(1);
  }

  printf("RLIMIT_NOFILE=%lld\n", lim.rlim_cur);
  return 0;
}