Daemon logging in Linux

Question

So I have a daemon running on a Linux system, and I want to have a record of its activities: a log. The question is, what is the "best" way to accomplish this?

My first idea is to simply open a file and write to it.

FILE* log = fopen("logfile.log", "w");
/* daemon works...needs to write to log */
fprintf(log, "foo%s\n", (char*)bar);
/* ...all done, close the file */
fclose(log);

Is there anything inherently wrong with logging this way? Is there a better way, such as some framework built into Linux?

Answer 1

As stated above you should look into syslog. But if you want to write your own logging code I'd advise you to use the "a" (write append) mode of fopen.

A few drawbacks of writing your own logging code are: Log rotation handling, Locking (if you have multiple threads), Synchronization (do you want to wait for the logs being written to disk ?). One of the drawbacks of syslog is that the application doesn't know if the logs have been written to disk (they might have been lost).

Answer 2

If you use threading and you use logging as a debugging tool, you will want to look for a logging library that uses some sort of thread-safe, but unlocked ring buffers. One buffer per thread, with a global lock only when strictly needed.

This avoids logging causing serious slowdowns in your software and it avoids creating heisenbugs which change when you add debug logging.

If it has a high-speed compressed binary log format that doesn't waste time with format operations during logging and some nice log parsing and display tools, that is a bonus.

I'd provide a reference to some good code for this but I don't have one myself. I just want one. :)

Answer 3

So far nobody mentioned boost log library which has nice and easy way to redirect your log messages to files or syslog sink or even Windows event log.

Answer 4

There are a lot of potential issues: for example, if the disk is full, do you want your daemon to fail? Also, you will be overwriting your file every time. Often a circular file is used so that you have space allocated on the machine for your file, but you can keep enough history to be useful without taking up too much space. There are tools like log4c that you can help you. If your code is c++, then you might consider log4cxx in the Apache project (apt-get install liblog4cxx9-dev on ubuntu/debian), but it looks like you are using C.