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My recent favorite is glibc's hack to implement _Static_assert under C99: https://codebrowser.dev/glibc/glibc/misc/sys/cdefs.h.html#564 https://codebrowser.dev/

by fuhsnn 2y ago

My recent favorite is glibc's hack to implement _Static_assert under C99: https://codebrowser.dev/glibc/glibc/misc/sys/cdefs.h.html#564 https://codebrowser.dev/glibc/glibc/misc/sys/cdefs.h.html#56... It uses the constant expression to create a bitfield of size -1 when failed, and leaves the compiler to error on that as the intended assertion. The actual statement is an extern pointer to a function returning a pointer to an array which has sizeof the aforementioned bitfield struct as its size. Another one encountered in Toybox is (0 || "foo") being a const expression that evaluates to 1. Apparently the string literal must have been soundly created in data section, so its pointer address is safely assumed to be non-zero.

lifthrasiir 2y ago
You have missed one important thing: every passing assertion will define a single extern function pointer with the same signature, so multiple `_Static_assert` invocations can coexist in a single scope. An extern definition doesn't have to be a function pointer by the way, I guess it helped a linker to have an easier time when removing unused symbols.
- fuhsnn 2y ago
  Oops too late to edit, that's really a function prototype. So it wouldn't take storage space or affect symbol unless the user naughtily calls the __Static_assert_function.