Subsequent calls to evbuffer_add_cb() add new callbacks. To remove this
callback, call evbuffer_remove_cb or evbuffer_remove_cb_entry.
@param buffer the evbuffer to be monitored
@param cb the callback function to invoke when the evbuffer is modified,
or NULL to remove all callbacks.
@param cbarg an argument to be provided to the callback function
@return a handle to the callback on success, or NULL on failure.
Add a new callback to an evbuffer.
Subsequent calls to evbuffer_add_cb() add new callbacks. To remove this callback, call evbuffer_remove_cb or evbuffer_remove_cb_entry.
@param buffer the evbuffer to be monitored @param cb the callback function to invoke when the evbuffer is modified, or NULL to remove all callbacks. @param cbarg an argument to be provided to the callback function @return a handle to the callback on success, or NULL on failure.