How can a user know which set of UART transmission is used when processing messages through printf ()?

[a_ziliu]

In the process of developing applications, user often needs to use printf () to process messages for debugging. If the UART supported by the NuMicro? series is more than one, it can be determined by defining "retarget.c" in DEBUG PORT that printf() uses which set of UART for transmission. The default is UART0.
The user can change the definition of DEBUG PORT and the printf () transmission port to comply with the system architecture.