docs: Add section on using /dev/fd/* with upload and download calls.

[libguestfs.git] / src / guestfs.pod

diff --git a/src/guestfs.pod b/src/guestfs.pod
index 7cb05a6..7d2f91c 100644 (file)
--- a/src/guestfs.pod
+++ b/src/guestfs.pod
@@ -14,7 +14,6 @@ guestfs - Library for accessing and modifying virtual machine images
  guestfs_mount (g, "/dev/sda1", "/");
  guestfs_touch (g, "/hello");
  guestfs_umount (g, "/");
- guestfs_sync (g);
  guestfs_close (g);
 
  cc prog.c -o prog -lguestfs
@@ -100,11 +99,10 @@ this:
   * disk image.
   */
  guestfs_touch (g, "/hello");
- 
- /* You only need to call guestfs_sync if you have made
-  * changes to the guest image.  (But if you've made changes
-  * then you *must* sync).  See also: guestfs_umount and
-  * guestfs_umount_all calls.
+
+ /* This is only needed for libguestfs < 1.5.24.  Since then
+  * it is done automatically when you close the handle.  See
+  * discussion of autosync in this page.
   */
  guestfs_sync (g);
  
@@ -117,7 +115,8 @@ functions that return integers return C<-1> on error, and all
 functions that return pointers return C<NULL> on error.  See section
 L</ERROR HANDLING> below for how to handle errors, and consult the
 documentation for each function call below to see precisely how they
-return error indications.
+return error indications.  See L<guestfs-examples(3)> for fully worked
+examples.
 
 =head2 DISK IMAGES
 
@@ -335,6 +334,27 @@ Use L</guestfs_download>.  See L</DOWNLOADING> above.
 
 =back
 
+=head2 UPLOADING AND DOWNLOADING TO PIPES AND FILE DESCRIPTORS
+
+Calls like L</guestfs_upload>, L</guestfs_download>,
+L</guestfs_tar_in>, L</guestfs_tar_out> etc appear to only take
+filenames as arguments, so it appears you can only upload and download
+to files.  However many Un*x-like hosts let you use the special device
+files C</dev/stdin>, C</dev/stdout>, C</dev/stderr> and C</dev/fd/N>
+to read and write from stdin, stdout, stderr, and arbitrary file
+descriptor N.
+
+For example, L<virt-cat(1)> writes its output to stdout by
+doing:
+
+ guestfs_download (filename, "/dev/stdout");
+
+and you can write tar output to a pipe C<fd> by doing:
+
+ char devfd[64];
+ snprintf (devfd, sizeof devfd, "/dev/fd/%d", fd);
+ guestfs_tar_out ("/", devfd);
+
 =head2 LISTING FILES
 
 L</guestfs_ll> is just designed for humans to read (mainly when using
@@ -588,6 +608,8 @@ documentation for that function for details).
 Libguestfs can mount NTFS partitions.  It does this using the
 L<http://www.ntfs-3g.org/> driver.
 
+=head3 DRIVE LETTERS AND PATHS
+
 DOS and Windows still use drive letters, and the filesystems are
 always treated as case insensitive by Windows itself, and therefore
 you might find a Windows configuration file referring to a path like
@@ -605,6 +627,8 @@ outside the scope of libguestfs, but something that you can easily do.
 Where we can help is in resolving the case insensitivity of paths.
 For this, call L</guestfs_case_sensitive_path>.
 
+=head3 ACCESSING THE WINDOWS REGISTRY
+
 Libguestfs also provides some help for decoding Windows Registry
 "hive" files, through the library C<hivex> which is part of the
 libguestfs project although ships as a separate tarball.  You have to
@@ -613,6 +637,33 @@ C<hivex> functions.  See also the programs L<hivexml(1)>,
 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
 on this issue.
 
+=head3 SYMLINKS ON NTFS-3G FILESYSTEMS
+
+Ntfs-3g tries to rewrite "Junction Points" and NTFS "symbolic links"
+to provide something which looks like a Linux symlink.  The way it
+tries to do the rewriting is described here:
+
+L<http://www.tuxera.com/community/ntfs-3g-advanced/junction-points-and-symbolic-links/>
+
+The essential problem is that ntfs-3g simply does not have enough
+information to do a correct job.  NTFS links can contain drive letters
+and references to external device GUIDs that ntfs-3g has no way of
+resolving.  It is almost certainly the case that libguestfs callers
+should ignore what ntfs-3g does (ie. don't use L</guestfs_readlink> on
+NTFS volumes).
+
+Instead if you encounter a symbolic link on an ntfs-3g filesystem, use
+L</guestfs_lgetxattr> to read the C<system.ntfs_reparse_data> extended
+attribute, and read the raw reparse data from that (you can find the
+format documented in various places around the web).
+
+=head3 EXTENDED ATTRIBUTES ON NTFS-3G FILESYSTEMS
+
+There are other useful extended attributes that can be read from
+ntfs-3g filesystems (using L</guestfs_getxattr>).  See:
+
+L<http://www.tuxera.com/community/ntfs-3g-advanced/extended-attributes/>
+
 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
 
 Although we don't want to discourage you from using the C API, we will