Having been delighted at my introduction to log4j in 2001, I was anxious to find a flexible logging and tracing framework suitable for introduction in the C program I'm updating. All the Google searches I tried produced results that pointed at Java, Python or PHP tools. I'd begun to despair and was thinking about putting something together on my own. It was Wikipedia's Log4J that finally pointed me to the Log4c logging and tracing framework.
I'm still working on getting it fully integrated, but the Log4c download was easy to compile on Sun's Solaris 10 (binaries for Linux are provided). One thing to note is that the CVS repository only has sources for automake, so the installation instructions are a couple steps removed.
It's not so easy to find documentation, though some effort seems to have gone into preparing some. The links for documentation go to a SourceForge site that yields an entry saying 1.0.0 with no content. I was eventually able to find a good guide to the use of Log4C on slideshare.
I haven't tried all the options, yet, but Log4C provides an elegant API that looks like it will be quite efficient. It will allow logging and tracing calls to be added with little impact on the application. It also promises to support generating syslog entries and rolling log files.