Example: using the libguestfs API from C

I spent about the last 18 hours contributing a section on libguestfs to the RHEL 6 Virtualization Guide. There is precisely zero in the guide at the moment on this subject, but hopefully there will be lots when RHEL 6.1 is released.

You might have forgotten that libguestfs is really a C library that just happens to have a lot of high level wrappers around it. But you can use the C API directly and (for a C API) it’s not too hard.

Below is the example that will appear in the Virtualization Guide next year. You can compile and run the program by saving it to a file test.c and typing:

gcc -Wall test.c -o test -lguestfs

After you run it, there will be a disk image in the current directory called disk.img which you can view using guestfish:

guestfish -a disk.img -m /dev/sda1

For more information about the libguestfs C API, read the API overview section of the documentation.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <guestfs.h>

main (int argc, char *argv[])
  guestfs_h *g;
  size_t i;

  g = guestfs_create ();
  if (g == NULL) {
    perror ("failed to create libguestfs handle");
    exit (EXIT_FAILURE);

  /* Create a raw-format sparse disk image, 512 MB in size. */
  int fd = open ("disk.img", O_CREAT|O_WRONLY|O_TRUNC|O_NOCTTY, 0666);
  if (fd == -1) {
    perror ("disk.img");
    exit (EXIT_FAILURE);
  if (ftruncate (fd, 512 * 1024 * 1024) == -1) {
    perror ("disk.img: truncate");
    exit (EXIT_FAILURE);
  if (close (fd) == -1) {
    perror ("disk.img: close");
    exit (EXIT_FAILURE);

  /* Set the trace flag so that we can see each libguestfs call. */
  guestfs_set_trace (g, 1);

  /* Set the autosync flag so that the disk will be synchronized
   * automatically when the libguestfs handle is closed.
  guestfs_set_autosync (g, 1);

  /* Add the disk image to libguestfs. */
  if (guestfs_add_drive_opts (g, "disk.img",
        GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw", /* raw format */
        GUESTFS_ADD_DRIVE_OPTS_READONLY, 0, /* for write */
        -1) /* this marks end of optional arguments */
      == -1)
    exit (EXIT_FAILURE);

  /* Run the libguestfs back-end. */
  if (guestfs_launch (g) == -1)
    exit (EXIT_FAILURE);

  /* Get the list of devices.  Because we only added one drive
   * above, we expect that this list should contain a single
   * element.
  char **devices = guestfs_list_devices (g);
  if (devices == NULL)
    exit (EXIT_FAILURE);
  if (devices[0] == NULL || devices[1] != NULL) {
    fprintf (stderr, "error: expected a single device from list-devices\n");
    exit (EXIT_FAILURE);

  /* Partition the disk as one single MBR partition. */
  if (guestfs_part_disk (g, devices[0], "mbr") == -1)
    exit (EXIT_FAILURE);

  /* Get the list of partitions.  We expect a single element, which
   * is the partition we have just created.
  char **partitions = guestfs_list_partitions (g);
  if (partitions == NULL)
    exit (EXIT_FAILURE);
  if (partitions[0] == NULL || partitions[1] != NULL) {
    fprintf (stderr, "error: expected a single partition from list-partitions\n");
    exit (EXIT_FAILURE);

  /* Create a filesystem on the partition. */
  if (guestfs_mkfs (g, "ext4", partitions[0]) == -1)
    exit (EXIT_FAILURE);

  /* Now mount the filesystem so that we can add files. */
  if (guestfs_mount_options (g, "", partitions[0], "/") == -1)
    exit (EXIT_FAILURE);

  /* Create some files and directories. */
  if (guestfs_touch (g, "/empty") == -1)
    exit (EXIT_FAILURE);
  const char *message = "Hello, world\n";
  if (guestfs_write (g, "/hello", message, strlen (message)) == -1)
    exit (EXIT_FAILURE);
  if (guestfs_mkdir (g, "/foo") == -1)
    exit (EXIT_FAILURE);

  /* This one uploads the local file /etc/resolv.conf into the disk image. */
  if (guestfs_upload (g, "/etc/resolv.conf", "/foo/resolv.conf") == -1)
    exit (EXIT_FAILURE);

  /* Because 'autosync' was set (above) we can just close the handle
   * and the disk contents will be synchronized.  You can also do
   * this manually by calling guestfs_umount_all and guestfs_sync.
  guestfs_close (g);

  /* Free up the lists. */
  for (i = 0; devices[i] != NULL; ++i)
    free (devices[i]);
  free (devices);
  for (i = 0; partitions[i] != NULL; ++i)
    free (partitions[i]);
  free (partitions);

  exit (EXIT_SUCCESS);

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