The Apache Portable Runtime (APR) is a supporting library for the Apache web server. It provides a set of APIs that map to the underlying operating system (OS). Where the OS does not support a particular function, APR will provide an emulation. Thus programmers can use the APR to make a program truly portable across platforms. APR originally formed a part of Apache HTTP Server, but the Apache Software Foundation spun it off into a separate project. Other applications can use it to achieve platform independence.