synchronizing I/O to a file stream

/*
 * Copyright 2011, Red Hat, Inc.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <libgen.h>
#include <limits.h>
#include <fcntl.h>
#include <string.h>
#include "sync-samples.h"

const char *message = "This is very important data!\n";

int
main(int argc, char **argv)
{
	int ret;
	size_t message_len;
	FILE *fp;
	int fd, dir_fd;
	char *containing_dir;

	if (argc < 2) {
		fprintf(stderr, "Usage: %s <filename>\n", basename(argv[0]));
		exit(USER_ERR);
	}

	/*
	 * Note that this will truncate the file.
	 */
	fp = fopen(argv[1], "w");
	if (!fp) {
		perror("fopen");
		exit(LIB_ERR);
	}

	message_len = strlen(message);
	ret = fwrite(message, message_len, 1, fp);
	if (ret != 1) {
		fprintf(stderr, "fwrite failed: %d", ferror(fp));
		exit(LIB_ERR);
	}
	/*
	 * After the fwrite call returns, the data is in libc's stdio
	 * buffer (still in the application's address space).  So, the
	 * next thing we want to do is flush that buffer.
	 */
	if (fflush(fp) != 0) {
		perror("fflush");
		if (errno == EBADF)
			exit(LIB_ERR);
		else
			exit(SYS_ERR);
	}

	/*
	 * Now the data is in the kernel's page cache.  The next steps
	 * flush the page cache for this file to disk.
	 */
	fd = fileno(fp);
	if (fd == -1) {
		perror("fileno");
		exit(LIB_ERR);
	}
	if (fsync(fd) < 0) {
		perror("fsync");
		exit(SYS_ERR);
	}
	/*
	 * Because we just created this file, we also need to ensure that
	 * the new directory entry gets flushed to disk.
	 */
	/*
	 * Basename and dirname may modify the string passed in.  We
	 * are not reusing argv[1], though, so we won't worry about
	 * it.
	 */
	containing_dir = dirname(argv[1]);
	/*
	 * You can't write directly to a directory.  fsync, however
	 * is allowed on the directory, even when opened read-only.
	 */
	dir_fd = open(containing_dir, O_RDONLY);
	if (dir_fd < 0) {
		perror("open");
		exit(SYS_ERR);
	}
	if (fsync(dir_fd) < 0) {
		perror("fsync2");
		exit(SYS_ERR);
	}

	/*
	 * There really shouldn't be any errors returned from close,
	 * here.  However, in the case of memory corruption
	 * (overwriting the dir_fd, for example), you can get a failure.
	 * Also, the close call can be interrupted, which we don't
	 * specifically handle.  The exit will take care of finishing
	 * the job.
	 */
	if (close(dir_fd) < 0) {
		perror("close");
		exit(SYS_ERR);
	}
	if (fclose(fp) < 0) {
		perror("fclose");
		exit(SYS_ERR);
	}

	exit(0);
}