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).^[2] 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.

The range of platform-independent functionality provided by APR includes: ^[3]

• Memory allocation and memory pool functionality
• Atomic operations
• Dynamic library handling
• File I/O
• Command-argument parsing
• Locking
• Hash tables and arrays
• Mmap functionality
• Network sockets and protocols
• Thread, process and mutex functionality
• Shared memory functionality
• Skip list functionality
• Time routines
• User and group ID services

• 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.)
• Netscape Portable Runtime (NSPR) is a cross-platform abstraction library used by the Mozilla project. It is used by another subproject of Mozilla application framework (XPFE) to provide cross-platform graphical user interface (GUI) functionality.
• Adaptive Communication Environment (ACE) is an object-oriented library written in C++ similar in functionality to APR. It is widely deployed in commercial products.^[4]
• commonc++ is a cross-platform C++ class library for systems programming, with much of the same functionality as APR.
• POCO is a modern C++ framework similar in concept but more extensive than APR.
• WxWidgets is an object-oriented cross-platform GUI library that also provides abstraction classes for database communication, IPC and networking functionality.
• KDE Frameworks – used by KDE SC