Replacing an existing file

/*
 * 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 <sys/stat.h>
#include "sync-samples.h"

#define TEMPLATE "mynewfileXXXXXX"
char *template;
int template_len;

const char *message1 = "Version 1 of my data.\n";
const char *message2 = "Version 2 of my data.\n";

int
main(int argc, char **argv)
{
	int ret;
	size_t message_len;
	int fd, new_fd, dir_fd;
	mode_t old_mode;
	char *path, *containing_dir;

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

	/*
	 * basename and dirname may modify the string passed in
	 */
	path = strdup(argv[1]);
	if (!path) {
		perror("strdup");
		exit(LIB_ERR);
	}
	containing_dir = dirname(path);

	/*
	 * Note that this will truncate the file.
	 */
	old_mode = umask((mode_t)0);
	fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
	if (fd < 0) {
		perror("open");
		exit(SYS_ERR);
	}
	umask(old_mode);

	/*
	 * 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);
	}

	message_len = strlen(message1);
	ret = full_write(fd, message1, message_len);
	if (ret != (int)message_len) {
		if (ret < 0) {
			perror("write");
			exit(SYS_ERR);
		}
		/*
		 * Short write.  This can happen if the file system is
		 * full, for example.  In our case, we can't use the
		 * partial data, so just unlink the file.  If the
		 * unlink fails, report this to the user.
		 */
		if (unlink(argv[1]) < 0)
			perror("unlink");
		exit(SYS_ERR);
	}
	/*
	 * Now the data is in the kernel's page cache.  The next step
	 * flushes the page cache for this file to disk.
	 */
	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.
	 */
	if (fsync(dir_fd) < 0) {
		perror("fsync2");
		exit(SYS_ERR);
	}
	if (close(fd) < 0) {
		perror("close");
		exit(SYS_ERR);
	}

	/*
	 * Now we have version 1 of our data safely on disk.  Let's start
	 * working on version 2 by creating a new file to hold the updates.
	 * Note that we are creating the temp file in the same directory
	 * as the target file.  The reason for this is to keep the example
	 * as simple as possible.
	 */
	template_len = strlen(containing_dir) + strlen(TEMPLATE) + 2;
	template = malloc(template_len);
	if (!template) {
		perror("malloc");
		exit(SYS_ERR);
	}
	ret = snprintf(template, template_len, "%s/%s", containing_dir, TEMPLATE);
	if (ret >= template_len) {
		/*
		 * Coding error, there should have been enough room in
		 * the template.
		 */
		fprintf(stderr, "Internal Error\n");
		exit(INTERNAL_ERR);
	}
	new_fd = mkstemp(template);
	if (new_fd == -1) {
		perror("mkstemp");
		exit(SYS_ERR);
	}

	message_len = strlen(message2);
	ret = full_write(new_fd, message2, message_len);
	if (ret != (int)message_len) {
		if (ret < 0) {
			perror("write");
			exit(SYS_ERR);
		}
		/*
		 * Short write.  This can happen if the file system is
		 * full, for example.  In our case, we can't use the
		 * partial data, so just unlink the file.  If unlink
		 * fails, notify the user.
		 */
		if (unlink(template) < 0)
			perror("unlink");
		exit(SYS_ERR);
	}

	/* ok, now sync the new file out to disk. */
	if (fsync(new_fd) < 0) {
		perror("fsync");
		exit(SYS_ERR);
	}
	if (close(new_fd) < 0) {
		perror("close");
		exit(SYS_ERR);
	}
	/*
	 * It wasn't necessary to sync out the directory at this
	 * point, since we're not relying on this new file for any
	 * user data (at least not this file as it is--we will rely
	 * on it after the rename).
	 */

	/* now rename the new file to replace the old one */
	if (rename(template, argv[1]) < 0) {
		perror("rename");
		exit(SYS_ERR);
	}
	free(template);

	/* and sync out the directory fd */
	if (fsync(dir_fd) < 0) {
		perror("fsync dir_fd");
		exit(SYS_ERR);
	}
	if (close(dir_fd) < 0) {
		perror("close dir_fd");
		exit(SYS_ERR);
	}
	free(path);

	/* and that's it! */
	exit(0);
}