# BNNSGraphContextSetMessageLogCallback(_:_:_:) Specifies a customized callback function that reports execution-time messages. ``` func BNNSGraphContextSetMessageLogCallback(_ context: bnns_graph_context_t, _ log_callback_fn: bnns_graph_execute_message_fn_t, _ additional_logging_arguments: UnsafeMutablePointer?) -> Int32 ``` ## Parameters `context` The graph context. `log_callback_fn` The message-logging callback function. `additional_logging_arguments` Additional data for the message logging functions that BNNS passes unaltered to the callback function. ## Return Value `0` on success, nonzero on failure. ## Discussion The following code adds a custom context execute-message callback function that prints messages to the console: ```swift BNNSGraphContextSetMessageLogCallback(context, messageLogCallback, nil) func messageLogCallback(msg_level: BNNSGraphMessageLevel, error_msg: UnsafePointer, op_info: UnsafePointer?, additional_logging_arguments: UnsafeMutablePointer?) { print(NSString(cString: error_msg, encoding: NSUTF8StringEncoding) ?? "") if let op_info = op_info { print(NSString(cString: op_info, encoding: NSUTF8StringEncoding) ?? "") } } ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)