loopback.c

/*
 * loopback - example loopback device using BUSE
 * Copyright (C) 2013 Adam Cozzette
 *
 * 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 2 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, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#define _GNU_SOURCE
#define _LARGEFILE64_SOURCE

#include <assert.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

#include "buse.h"

static int fd;

static void usage(void)
{
    fprintf(stderr, "Usage: loopback <phyical device> <virtual device>\n");
}

static int loopback_read(void *buf, u_int32_t len, u_int64_t offset, void *userdata)
{
    int bytes_read;
    (void)(userdata);

    lseek64(fd, offset, SEEK_SET);
    while (len > 0) {
        bytes_read = read(fd, buf, len);
        assert(bytes_read > 0);
        len -= bytes_read;
        buf = (char *) buf + bytes_read;
    }

    return 0;
}

static int loopback_write(const void *buf, u_int32_t len, u_int64_t offset, void *userdata)
{
    int bytes_written;
    (void)(userdata);

    lseek64(fd, offset, SEEK_SET);
    while (len > 0) {
        bytes_written = write(fd, buf, len);
        assert(bytes_written > 0);
        len -= bytes_written;
        buf = (char *) buf + bytes_written;
    }

    return 0;
}

static struct buse_operations bop = {
    .read = loopback_read,
    .write = loopback_write
};

int main(int argc, char *argv[])
{
    struct stat buf;
    int err;
    int64_t size;

    if (argc != 3) {
        usage();
        return -1;
    }

    fd = open(argv[1], O_RDWR|O_LARGEFILE);
    assert(fd != -1);

    /* Let's verify that this file is actually a block device. We could support
     * regular files, too, but we don't need to right now. */
    fstat(fd, &buf);
    assert(S_ISBLK(buf.st_mode));

    /* Figure out the size of the underlying block device. */
    err = ioctl(fd, BLKGETSIZE64, &size);
    assert(err != -1);
    (void)err;
    fprintf(stderr, "The size of this device is %ld bytes.\n", size);
    bop.size = size;

    buse_main(argv[2], &bop, NULL);

    return 0;
}