fd.c

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

int open_files(unsigned long *soft, unsigned long *hard, unsigned long *current);

int main(int argc, char **argv)
{
	if (argc == 2) {
		unsigned int max;
		if (strcmp(argv[1], "max") == 0)
			max = 0xffffffff;
		else
			max = strtoul(argv[1], NULL, 10);

		int i;
		for (i = 0; i < max; i++)
			if (!tmpfile())
				break;
	}


	unsigned long soft, hard, current;
	if (open_files(&soft, &hard, &current) != 0) {
		perror("open_files");
		return 1;
	}


	printf("%lu fds used (limit %lu/%lu)\n", current, soft, hard);
	return 0;
}

int open_files(unsigned long *soft, unsigned long *hard, unsigned long *current)
{
	int rc, i;
	struct rlimit lim;

	errno = EINVAL;
	if (!soft || !hard || !current)
		return -1;

	rc = getrlimit(RLIMIT_NOFILE, &lim);
	if (rc != 0)
		return rc;
	*soft = (unsigned long)lim.rlim_cur;
	*hard = (unsigned long)lim.rlim_max;

	struct pollfd *fds = calloc(*soft, sizeof(struct pollfd));
	if (!fds)
		return -1;

	for (i = 0; i < *soft; i++)
		fds[i].fd = i;

	rc = poll(fds, *soft, 0);
	if (rc < 0)
		return rc;

	*current = *soft;
	for (i = 0; i < lim.rlim_cur; i++)
		if (fds[i].revents & POLLNVAL)
			*current--;

	return 0;
}