Variadic Macros, Static Assertions, Feature Detection
__VA_ARGS__, _Static_assert, and __STDC_*__ predicates — building code that refuses to compile when its assumptions break.
Variadic macros for logging
#define logf(fmt, ...) fprintf(stderr, fmt __VA_OPT__(,) __VA_ARGS__)
C23's __VA_OPT__ handles the trailing-comma problem that plagued ##__VA_ARGS__ hacks: with no variadic arguments, the comma disappears too. A fmt string and portable argument forwarding is all a logging layer needs.
Compile-time refusals
_Static_assert(sizeof(int) >= 4, "int must be at least 32 bits");
_Static_assert(__STDC_VERSION__ >= 202311L, "C23 required");
The condition is an integer constant expression — evaluated by the compiler, failing the build with your message. Everything the code assumes about the platform should appear as an assertion; assumptions that never fail loudly become wrong silently.
Feature detection without lies
#if defined(__GNUC__) tells you the compiler; __has_include(<stdatomic.h>) tells you headers; __STDC_NO_THREADS__ tells you what is missing. What they do NOT tell you is behavior — probing at compile time gates declarations, while behavior claims still need runtime verification.