The Apache Portable Runtime (APR) is a supporting library for the Apacheweb server. It provides a set of APIs that map to the underlying operating system (OS). Where the OS doesn't support a particular function, APR will provide an emulation. Thus programmers can use the APR to make a program truly portable across platforms.
GLib provides similar functionality. It supports many more data structures and OS-independent functions, but fewer IPC-related functions. (GLib lacks local and global locking and shared-memory management.)