Fastest most efficient way to add line to a file

Hi,
I'm building a log utility that will log my logs with some data with Error/warn/etc debug level and I want it to write the logs to a file as well (not just the system log file) - of course it will be configurable

anyone know whats the fastest way to add a line to a file, the method that will have min impact on my CPU consumption, my memory etc
of course each log is an atomic action so I need the file to be close after it, and flushed etc

what file method / object I should use ?

thanks