Logging compatibly with logrotate

Question

I am writing a Linux daemon that writes a log. I'd like the log to be rotated by logrotate. The program is written in C.

Normally, my program would open the log file when it starts, then write entries as needed and then, finally, close the log file on exit.

What do I need to do differently in order to support log rotation using logrotate? As far as I have understood, my program should be able to reopen the log file each time logrotate has finished it's work. The sources that I googled didn't, however, specify what reopening the log file exactly means. Do I need to do something about the old file and can I just create another file with the same name? I'd prefer quite specific instructions, like some simple sample code.

I also understood that there should be a way to tell my program when it is time to do the reopening. My program already has a D-Bus interface and I thought of using that for those notifications.

Note: I don't need instructions on how to configure logrotate. This question is only about how to make my own software compatible with it.

Answer 1

There are several common ways:

1. you use logrotate and your program should be able to catch a signal (usually SIGHUP) as a request to close and reopen its log file. Then logrotate sends the signal in a postrotate script
2. you use logrotate and your program is not aware of it, but can be restarted. Then logrotate restarts your program in a postrotate script. Cons: if the start of the program is expensive, this may be suboptimal

3. you use logrotate and your program is not aware of it, but you pass the copytruncate option to logrotate. Then logrotate copies the file and then truncates it. Cons: in race conditions you can lose messages. From rotatelog.conf manpage

   ... Note that there is a very small time slice between copying the file and truncating it, so some logging data might be lost...

4. you use rotatelogs, an utility for httpd Apache. Instead of writing directly to a file, you programs pipes its logs to rotatelogs. Then rotatelogs manages the different log files. Cons: your program should be able to log to a pipe or you will need to install a named fifo.

But beware, for critical logs, it may be interesting to close the files after each message, because it ensures that everything has reached the disk in case of an application crash.