This could be fixed in the generator by specially marking parameters
and return values which take bytes or other units.
-=item Library should return errno with error messages.
-
-It would be a nice-to-have to be able to get the original value of
-'errno' from inside the appliance along error paths (where set).
-Currently L<guestmount(1)> goes through hoops to try to reverse the
-error message string into an errno, see the function error() in
-fuse/guestmount.c.
-
-In libguestfs 1.5.4, the protocol was changed so that the
-Linux errno is sent back from the daemon.
-
=item Ambiguity between devices and paths
There is a subtle ambiguity in the API between a device name
=head1 ERROR HANDLING
-The convention in all functions that return C<int> is that they return
-C<-1> to indicate an error. You can get additional information on
-errors by calling L</guestfs_last_error> and/or by setting up an error
-handler with L</guestfs_set_error_handler>.
+API functions can return errors. For example, almost all functions
+that return C<int> will return C<-1> to indicate an error.
+
+Additional information is available for errors: an error message
+string and optionally an error number (errno) if the thing that failed
+was a system call.
+
+You can get at the additional information about the last error on the
+handle by calling L</guestfs_last_error>, L</guestfs_last_errno>,
+and/or by setting up an error handler with
+L</guestfs_set_error_handler>.
+
+When the handle is created, a default error handler is installed which
+prints the error message string to C<stderr>. For small short-running
+command line programs it is sufficient to do:
+
+ if (guestfs_launch (g) == -1)
+ exit (EXIT_FAILURE);
+
+since the default error handler will ensure that an error message has
+been printed to C<stderr> before the program exits.
-The default error handler prints the information string to C<stderr>.
+For other programs the caller will almost certainly want to install an
+alternate error handler or do error handling in-line like this:
+
+ g = guestfs_create ();
+
+ /* This disables the default behaviour of printing errors
+ on stderr. */
+ guestfs_set_error_handler (g, NULL, NULL);
+
+ if (guestfs_launch (g) == -1) {
+ /* Examine the error message and print it etc. */
+ char *msg = guestfs_last_error (g);
+ int errnum = guestfs_last_errno (g);
+ fprintf (stderr, "%s\n", msg);
+ /* ... */
+ }
Out of memory errors are handled differently. The default action is
to call L<abort(3)>. If this is undesirable, then you can set a
The lifetime of the returned string is until the next error occurs, or
L</guestfs_close> is called.
-The error string is not localized (ie. is always in English), because
-this makes searching for error messages in search engines give the
-largest number of results.
+=head2 guestfs_last_errno
+
+ int guestfs_last_errno (guestfs_h *g);
+
+This returns the last error number (errno) that happened on C<g>.
+
+If successful, an errno integer not equal to zero is returned.
+
+If no error, this returns 0. This call can return 0 in three
+situations:
+
+=over 4
+
+=item 1.
+
+There has not been any error on the handle.
+
+=item 2.
+
+There has been an error but the errno was meaningless. This
+corresponds to the case where the error did not come from a
+failed system call, but for some other reason.
+
+=item 3.
+
+There was an error from a failed system call, but for some
+reason the errno was not captured and returned. This usually
+indicates a bug in libguestfs.
+
+=back
+
+Libguestfs tries to convert the errno from inside the applicance into
+a corresponding errno for the caller (not entirely trivial: the
+appliance might be running a completely different operating system
+from the library and error numbers are not standardized across
+Un*xen). If this could not be done, then the error is translated to
+C<EINVAL>. In practice this should only happen in very rare
+circumstances.
=head2 guestfs_set_error_handler
parameters passed to the callback are an opaque data pointer and the
error message string.
+C<errno> is not passed to the callback. To get that the callback must
+call L</guestfs_last_errno>.
+
Note that the message string C<msg> is freed as soon as the callback
function returns, so if you want to stash it somewhere you must make
your own copy.
git.annexia.org / libguestfs.git / blobdiff