preC17

preC17 is an informal designation used in software development to describe code, libraries, and tooling that target language standards of the C family prior to the C17 revision (ISO/IEC 9899:2018). It generally encompasses C89/C90, C99, and C11, though the exact boundary can vary by project. The term is used to indicate compatibility with compilers and runtimes that may not implement features added in C17 or later amendments, and to convey expectations about standard conformance in documentation and build configurations.

In practice, preC17 code tends to rely on older language constructs and library interfaces. Developers may avoid newer features introduced after C11, and they typically rely on well-established behavior and portability guarantees across a range of legacy toolchains. Build systems often specify strict standard flags such as -std=c89, -std=c99, or -std=c11 to enforce the intended conformance. The distinction can matter for issues like thread support, certain diagnostics, and library availability, where newer standards provide capabilities that might not be present in older toolchains.

The label is most useful as a guide for portability and maintenance. It signals that a project seeks compatibility with older compilers and runtime environments, which can influence coding practices, dependency choices, and testing strategies. Because compiler support sometimes overlaps across standards, the boundary between preC17 and post-C17 code can be fluid, and teams may document the exact standard version and feature set they target to avoid ambiguity.