close-fds.c

#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

int close_fds(int from_fd, int max_fd)
{
	struct pollfd pfds[1024];
	int i, total, nclosed = 0;
	int openmax = sysconf(_SC_OPEN_MAX);

	if (openmax != -1 && openmax < max_fd)
		max_fd = openmax;

	/* init events */
	total = max_fd - from_fd;
	for (i = 0; i < (total < 1024 ? total : 1024); i++) {
		pfds[i].events = 0;
	}

	while (from_fd < max_fd) {
		int nfds, r = 0;

		total = max_fd - from_fd;
		nfds =  total < 1024 ? total : 1024;

		for (i = 0; i < nfds; i++)
			pfds[i].fd = from_fd + i;

		do {
			r = poll(pfds, nfds, 0);
		} while (r == -1 && errno == EINTR);

		if (r < 0)
			return r;


		for (i = 0; i < nfds; i++)
			if (pfds[i].revents != POLLNVAL) {
				nclosed++;
				close(pfds[i].fd);
			}
		from_fd += nfds;
	}
	return nclosed;
}

int close_fds_slow(int fd, int max_fd)
{
	int openmax = sysconf(_SC_OPEN_MAX);

	if (openmax != -1 && openmax < max_fd)
		max_fd = openmax;

	int total = max_fd - fd;
	while (fd < max_fd)
		close(fd++);

	return total;
}

int main(int argc, char *argv[]) {
	/* open a few fds */
	int fd1 = open(".", O_RDONLY);
	int fd2 = open("/proc/self/exe", O_RDONLY);
	int n;

	printf("opened fds: %d and %d\n", fd1, fd2);
	system("ls /proc/self/fd");

	if (argc > 1) {
		n = close_fds_slow(3, INT_MAX);
	} else {
		n = close_fds(3, INT_MAX);
	}

	printf("Closed %i fds\n", n);
	system("ls /proc/self/fd");

	return 0;
}