HP C (formerly Compaq C) is a standard conforming implementation of the C programming language with HP extensions. The HP C compiler runs under the Open- VMS VAX, Alpha, and I64 Operating Systems and generates optimized and position-independent code.

HP C is a native-mode language product, and is integrated into the Common Language Environments. All OpenVMS system services are available to programs written in HP C. HP C programs can invoke, as functions, modules written in other languages.

HP C supports Record Management Services (RMS) for sequential file organizations and associated access methods. HP C also supports stream file-access methods common among many C implementations.

HP C provides extensive standard-conformance checking, as well as many optional code-quality and portability diagnostics, and supports the lint-like features of the HP Source Code Analyzer. The HP Source Code Analyzer allows the programmer to check for consistent function usage throughout a program environment. HP C also generates complete debug and traceback records for use with OpenVMS Debug. Debug allows the C programmer to set breakpoints, examine and modify the contents of user variables, and selectively halt or continue program execution.

HP C on OpenVMS I64 provides IEEE floating-point support in conformance with, the IEEE 754 Standard.