zlog is a reliable, efficient, thread safe, flexible, pure C logging library. It was written because there were no good C logging libraries along the lines of Logback for Java or log4cxx for C++. printf can work, but can not be easily redirected or reformatted, and syslog is slow and designed for system use. It provides a syslog model, log format customization, multiple output, safe run-time reconfiguration, high performance, user-defined log levels, safe log rotation, a log4j-style key-value map, in-built debugging, and no external dependencies.
