Re: [Libguestfs] [libnbd] More thoughts on callbacks and more

On Mon, Jul 22, 2019 at 10:08:25AM +0100, Richard W.M. Jones wrote: Here's a possibly better idea which still changes the API a bit but not as invasively. We overload the callback so that it can either be a callback function or a "free function". They are distinguished by an extra flag argument passed to the callback: extern int nbd_set_debug_callback ( struct nbd_handle *h, int (*debug_fn) (int valid_flag, // <-- note void *user_data, const char *context, const char *msg), void *user_data); The extra ‘valid_flag’ contains one or both of LIBNBD_CALLBACK_VALID and LIBNBD_CALLBACK_FREE. Callback code would look like: int my_debug_fn (int valid_flag, void *user_data, const char *context, const char *msg) { if (valid_flag & LIBNBD_CALLBACK_VALID) { // This is the normal callback as before. printf ("debug msg = %s\n", msg); } if (valid_flag & LIBNBD_CALLBACK_FREE) { // In this case only user_data is valid. The other parameters // may be unspecified (unless VALID was also set). free (user_data); } } Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html