There is no C API; C code is tested from C++.

µTest++ is built on C++20 features, such as templates, concepts, and lambdas, which have no equivalent in C, so it has no C API.

To test C code, write the test cases in C++ and call the C functions from them. If the C headers do not declare their functions with extern "C" themselves, include them in an extern "C" block:

extern "C"
{
#include "adder.h"
}
 
ts.test ("Check the C adder", [] (auto& t) {
  t.expect (mt::eq (add (2, 3), 5));
});