cgroups: add simple listener of cgroup events to documentation

/*

 * cgroup_event_listener.c - Simple listener of cgroup events

 *

 * Copyright (C) Kirill A. Shutemov <kirill@shutemov.name>

 */

#include <assert.h>

#include <errno.h>

#include <fcntl.h>

#include <libgen.h>

#include <limits.h>

#include <stdio.h>

#include <string.h>

#include <unistd.h>

#include <sys/eventfd.h>

#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>\n"

int main(int argc, char **argv)

{

	int efd = -1;

	int cfd = -1;

	int event_control = -1;

	char event_control_path[PATH_MAX];

	char line[LINE_MAX];

	int ret;

	if (argc != 3) {

		fputs(USAGE_STR, stderr);

		return 1;

	}

	cfd = open(argv[1], O_RDONLY);

	if (cfd == -1) {

		fprintf(stderr, "Cannot open %s: %s\n", argv[1],

				strerror(errno));

		goto out;

	}

	ret = snprintf(event_control_path, PATH_MAX, "%s/cgroup.event_control",

			dirname(argv[1]));

	if (ret >= PATH_MAX) {

		fputs("Path to cgroup.event_control is too long\n", stderr);

		goto out;

	}

	event_control = open(event_control_path, O_WRONLY);

	if (event_control == -1) {

		fprintf(stderr, "Cannot open %s: %s\n", event_control_path,

				strerror(errno));

		goto out;

	}

	efd = eventfd(0, 0);

	if (efd == -1) {

		perror("eventfd() failed");

		goto out;

	}

	ret = snprintf(line, LINE_MAX, "%d %d %s", efd, cfd, argv[2]);

	if (ret >= LINE_MAX) {

		fputs("Arguments string is too long\n", stderr);

		goto out;

	}

	ret = write(event_control, line, strlen(line) + 1);

	if (ret == -1) {

		perror("Cannot write to cgroup.event_control");

		goto out;

	}

	while (1) {

		uint64_t result;

		ret = read(efd, &result, sizeof(result));

		if (ret == -1) {

			if (errno == EINTR)

				continue;

			perror("Cannot read from eventfd");

			break;

		}

		assert(ret == sizeof(result));

		ret = access(event_control_path, W_OK);

		if ((ret == -1) && (errno == ENOENT)) {

				puts("The cgroup seems to have removed.");

				ret = 0;

				break;

		}

		if (ret == -1) {

			perror("cgroup.event_control "

					"is not accessable any more");

			break;

		}

		printf("%s %s: crossed\n", argv[1], argv[2]);

	}

out:

	if (efd >= 0)

		close(efd);

	if (event_control >= 0)

		close(event_control);

	if (cfd >= 0)

		close(cfd);

	return (ret != 0);

}