Using multiple NRF_LOG backends at once

Question

I'd like to log some debug stuff to a mobile app over BLE, when in a connection, using the NRF_LOG library and the Nordic UART service. I have this working just fine without NRF_LOG, just using printf() calls for the log statements and then re-wiring printf() as follows: 
 
 
 When in a BLE connection, take the first 20 characters of the string and write them over BLE using ble_nus_string_send(). 
 
 
 Always pass everything to SEGGER_RTT_Write() as well, whether we're in a BLE connection or not. 
 
 
 I'd now like to do the same using NRF_LOG for consistency, as well as getting the log statements in the SDK sources out the same way. At first glance the NRF_LOG library claims to support multiple backends, so I thought I'd write a new backend that just does the BLE stuff. It would send log statements over BLE when in a connection, and do nothing with them when not. 
 But there doesn't seem to be any way to configure multiple backends at once. Can there be only one nrf_log_backend_init() function linked in at once? 
 I note that he UART and Segger RTT backends were NOT written as separate backends at all - they're just one backend with the calls all mashed into one implementation. 
 The SDK docs say "The NRF_LOG_HANDLERS_SET macro can be used to change handlers on the fly." I think this still isn't what I want. I don't want to change the handler, I want to add an additional handler. Would also like to avoid wrapping the default backend handler in my handler and making a further mess of it. 
 Can this be done?

Krzysztof Chruscinski · Accepted Answer

Hi, 
 there is a macro called NRF_LOG_HANDLERS_SET() where you can provide functions for your backend. That would of course replace default handlers from nrf_log_backend_serial.c. You can try to create you backend that will wrap default backend and extend it. Something like: 
 bool my_std_handler(....)
{
nrf_log_backend_serial_std_handler(...);
//customize handling
}
 
 And use NRF_LOG_HANDLERS_SET() to replace the default one.

Using multiple NRF_LOG backends at once

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