Should log file streams be opened/closed on each write or kept open during a desktop application's lifetime?

Question

Should log classes open/close a log file stream on each write to the log file or should it keep the log file stream open throughout the application's lifetime until all logging is complete?

I'm asking in context of a desktop application. I have seen people do it both ways and was wondering which approach yields the best all-around results for a logger.

Answer 1

If you have frequent read/writes it is more efficient to keep the file open for the lifetime with a single open/close.

You might want to flush periodically or after each write though in case your application crashes you might not have all the data written to your file. Use fflush on Unix based systems and FlushFileBuffers on Windows.

If you are running on Windows as well you can use the CreateFile API with FILE_FLAG_NO_BUFFERING to go direct to file on each write.

It is also better to keep the file open for the lifetime, because each time you open/close you might have a failure if the file is in use. For example you might have a backup application that runs and open/closes your file as it's backing it up. And this might cause your program to not be able to access your own file. Ideally you would want to keep your file open always and specify sharing flags on Windows (FILE_SHARE_READ). On Unix based systems, sharing will be default.