__ The difference between typeof and typeof

An identifier with double underscores is reserved for the implementation. typeof is a compiler specific extension to the language, so naming it __typeof ensures no user code has an identifier with the same name.

typeof() and __typeof() are compiler-specific extensions to the C language, because standard C does not include such an operator. Standard C requires compilers to prefix language extensions with a double-underscore (which is also why you should never do so for your own functions, variables, etc.)

typeof() is exactly the same, but throws the underscores out the window with the understanding that every modern compiler supports it. (Actually, now that I think about it, Visual C++ might not. It does support decltype() though, which generally provides the same behaviour as typeof().)

All three mean the same thing, but none are standard C so a conforming compiler may choose to make any mean something different.

-ansi and the various -std options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords asm, typeof and inline are not available in programs compiled with -ansi or -std (although inline can be used in a program compiled with -std=c99 or -std=c11). The ISO C99 keyword restrict is only available when -std=gnu99 (which will eventually be the default) or -std=c99 (or the equivalent -std=iso9899:1999), or an option for a later standard version, is used.

The way to solve these problems is to put ‘__’ at the beginning and end of each problematical keyword. For example, use asm instead of asm, and inline instead of inline.

An identifier with double underscores is reserved for the implementation. typeof is a compiler specific extension to the language, so naming it __typeof ensures no user code has an identifier with the same name.
https://stackoverflow.com/questions/14877415/difference-between-typeof-typeof-and-typeof-objective-c
http://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html#Alternate-Keywords
https://clang.llvm.org/docs/UsersManual.html#c-language-features