setjmp and longjmp are not normally compiler builtins, just some library functions. They are quite different also from the __builtins that you mention, sometime

setjmp and longjmp are not normally compiler builtins, just some library functions. They are quite different also from the __builtins that you mention, sometimes in quite surprising ways. Actually compilers are very buggy when you use these functions, even if you do follow their quite strict rules for what is allowed. Calling longjmp in a signal handler is technically undefined behavior in posix and trigger issues on some platforms that I have had the displeasure to run into. Recently various hardening options in the definitions of setjmp now make it detect these sorts of attempts and abort the program. So I wouldn’t call all of that very portable, but I agree it is fun!