21.10. Using the API from Programming Languages
21.10. Using the API from Programming Languages The libguestfs API can be used directly from the following languages in Red Hat Enterprise Linux 7: C, C++, Perl, Python, Java, Ruby and OCaml. To install C and C++ bindings, enter the following command: To install Perl bindings: To install Python bindings: To install Java bindings: To install Ruby bindings: To install OCaml bindings: The binding for each language is essentially the same, but with minor syntactic changes. A C statement: Would appear like the following in Perl: Or like the following in OCaml: Only the API from C is detailed in this section. In the C and C++ bindings, you must manually check for errors. In the other bindings, errors are converted into exceptions; the additional error checks shown in the examples below are not necessary for other languages, but conversely you may wish to add code to catch exceptions. See the following list for some points of interest regarding the architecture of the libguestfs API: The libguestfs API is synchronous. Each call blocks until it has completed. If you want to make calls asynchronously, you have to create a thread. The libguestfs API is not thread safe: each handle should be used only from a single thread, or if you want to share a handle between threads you should implement your own mutex to ensure that two threads cannot execute commands on one handle at the same time. You should not open multiple handles on the same disk image. It is permissible if all the handles are read-only, but still not recommended. You should not add a disk image for writing if anything else could be using that disk image (a live VM, for example). Doing this will cause disk corruption. Opening a read-only handle on a disk image that is currently in use (for example by a live VM) is possible. However, the results may be unpredictable or inconsistent, particularly if the disk image is being heavily written to at the time you are reading it. 21.10.1. Interaction with the API using a C program Your C program should start by including the <guestfs.h> header file, and creating a handle: Save this program to a file ( test.c ). Compile this program and run it with the following two commands: At this stage it should print no output. The rest of this section demonstrates an example showing how to extend this program to create a new disk image, partition it, format it with an ext4 file system, and create some files in the file system. The disk image will be called disk.img and be created in the current directory. The outline of the program is: Create the handle. Add disk(s) to the handle. Launch the libguestfs back end. Create the partition, file system and files. Close the handle and exit. Here is the modified program: Compile and run this program with the following two commands: If the program runs to completion successfully, you should be left with a disk image called disk.img , which you can examine with guestfish: By default (for C and C++ bindings only), libguestfs prints errors to stderr. You can change this behavior by setting an error handler. The guestfs(3) man page discusses this in detail.
yum install libguestfs-devel yum install 'perl(Sys::Guestfs)' yum install python-libguestfs yum install libguestfs-java libguestfs-java-devel libguestfs-javadoc yum install ruby-libguestfs yum install ocaml-libguestfs ocaml-libguestfs-devel guestfs_launch (g); USDg->launch () g#launch () #include <stdio.h> #include <stdlib.h> #include <guestfs.h> int main (int argc, char *argv[]) { guestfs_h *g; g = guestfs_create (); if (g == NULL) { perror ("failed to create libguestfs handle"); exit (EXIT_FAILURE); } /* ... */ guestfs_close (g); exit (EXIT_SUCCESS); } gcc -Wall test.c -o test -lguestfs ./test #include <stdio.h> #include <stdlib.h> #include <string.h> #include <fcntl.h> #include <unistd.h> #include <guestfs.h> int 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 an ext4 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 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); } gcc -Wall test.c -o test -lguestfs ./test guestfish --ro -a disk.img -m /dev/sda1 ><fs> ll / ><fs> cat /foo/resolv.conf