Function otMessageRegisterTxCallback 

pub unsafe extern "C" fn otMessageRegisterTxCallback(
    aMessage: *mut otMessage,
    aCallback: otMessageTxCallback,
    aContext: *mut c_void,
)

Registers a callback to be notified of a message’s transmission outcome.

Calling this function again for the same message will replace any previously registered callback.

If the message is never actually sent (e.g., it’s not passed to otIp6Send() or other send APIs), the callback will still be invoked when the message is freed. In this case, OT_ERROR_DROP will be passed as the error.

@param[in] aMessage The message to register the callback with. @param[in] aCallback The TX callback. @param[in] aContext A pointer to a user-provided arbitrary context for the callback.