Use line buffered mode for stdout.

Closes #2354.
Closes #2749.

ChangeLog.txt

@@ -6,6 +6,7 @@ Broker:
 - Fix std* files not being redirected when daemonising, when built with
   assertions removed. Closes #2708.
 - Fix default settings incorrectly allowing TLS v1.1. Closes #2722.
+- Use line buffered mode for stdout. Closes #2354. Closes #2749.
 
 Client library:
 - Use CLOCK_BOOTTIME when available, to keep track of time. This solves the

src/logging.c

@@ -130,6 +130,9 @@ int log__init(struct mosquitto__config *config)
 			log__printf(NULL, MOSQ_LOG_ERR, "Error: Unable to open log file %s for writing.", config->log_file);
 		}
 	}
+	if(log_destinations & MQTT3_LOG_STDOUT){
+		setlinebuf(stdout);
+	}
 #ifdef WITH_DLT
 	dlt_fifo_check();
 	if(dlt_allowed){