docs: Clarify default error handler.

[libguestfs.git] / src / guestfs.pod

diff --git a/src/guestfs.pod b/src/guestfs.pod
index da03128..b21a25d 100644 (file)
--- a/src/guestfs.pod
+++ b/src/guestfs.pod
@@ -877,8 +877,8 @@ This closes the connection handle and frees up all resources used.
 
 =head1 ERROR HANDLING
 
-The convention in all functions that return C<int> is that they return
-C<-1> to indicate an error.
+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
@@ -889,7 +889,32 @@ 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>.
 
-The default error handler prints the information string to C<stderr>.
+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.
+
+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
@@ -906,10 +931,6 @@ returns C<NULL>.
 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);