#if !__STDC_LIB_EXT1__ # error "This code needs bounds checking interface Annex K" #endif
This guard stops compilation when the bounds-checking interface required by the code is unavailable. section 8.1.5
A precondition is something that must hold before code can be used correctly. A platform precondition concerns the execution environment rather than a particular function call. Portable C code should assume as little about that environment as possible; when an assumption is unavoidable, identify it explicitly. TAKEAWAY: A missing platform precondition must abort compilation. section 8.1.5
Choose a check according to when its condition can be evaluated:
- Use #if and #error for preprocessor-checkable requirements. Conditions evaluate macros and integer literals; an unknown identifier evaluates to 0. Feature tests include defined, __has_include, __has_embed, and __has_c_attribute. section 8.1.5
- Use static_assert for a condition known at compile time but not during preprocessing, such as a comparison of type sizes. Before C23, the keyword was _Static_assert. section 8.1.5
static_assert(sizeof(double) == sizeof(long double), "Extra precision needed for convergence.");
This assertion checks a type-size requirement after preprocessing while still guaranteeing a compile-time check. section 8.1.5
A function can also have preconditions on its arguments. For example, gcd2 requires its first argument to be no greater than its second and makes that requirement explicit with assert(a <= b). Its recursive call preserves the requirement. This is a run-time call condition, not a substitute for rejecting an unsupported platform during compilation. section 7.3 section 8.1.5
For operations that could fail, check preconditions and take an alternative path when they do not hold. Check error returns separately for operations that may exhaust resources: a precondition check cannot establish that such an operation succeeded. section 15.5
