public void set_writer_func (owned LogWriterFunc func)
Set a writer function which will be called to format and write out each log message.
Each program should set a writer function, or the default writer ( writer_default) will be used.
Libraries **must not** call this function — only programs are allowed to install a writer function, as there must be a single, central point where log messages are formatted and outputted.
There can only be one writer function. It is an error to set more than one.
log writer function, which must not be null
user data to pass to
function to free