How portable is __get_cpuid?

Question

I'm using __get_cpuid() to get info on the x86 and x86_64 processors my programs run on. With GCC on Linux and Mac OS, it seems to compile and run OK without include any header, but how portable is that? Would it work with other compilers? Should I include a header for that?

Answer 1

It has two leading underscores. Any leading underscore at all is a big hint that the variable or function is not portable.