The ANSI/ISO C Specification Language (ACSL) is a specification language for C programs, using Hoare style pre- and postconditions and invariants, that follows the design by contract paradigm. Specifications are written as C annotation comments to the C program, which hence can be compiled with any C compiler.
The current verification tool for ACSL is Frama-C. It also implements a sister language, ANSI/ISO C++ Specification Language (ACSL++), defined for C++.