rtl.fx is the Reference file for the C base library used by the Cocoa platform. It contains many basic functions and C APIs used and needed by Cocoa apps and by the more advanced Cocoa frameworks, such as Foundation.fx, et al.

The types and functions provided by rtl.fx are exposed in the rtl Namespace and its sub-namespaces. Every Cocoa project automatically references rtl.fx, and the rtl namespace is automatically in scope in all source files, so that its members can commonly be accessed directly and without namespace prefix.

Note: rtl.fx has no exact matching static library .a file or .framework file. Instead, it represents code from a variety of base libraries that are linked into every Cocoa project and are part of the core OS X and iOS operating systems.