dollar sign in variable name?

It's an extension of some compilers and not in the C standard

MSVC:

Microsoft Specific

Only the first 2048 characters of Microsoft C++ identifiers are significant. Names for user-defined types are "decorated" by the compiler to preserve type information. The resultant name, including the type information, cannot be longer than 2048 characters. (See Decorated Names for more information.) Factors that can influence the length of a decorated identifier are:

• Whether the identifier denotes an object of user-defined type or a type derived from a user-defined type.
• Whether the identifier denotes a function or a type derived from a function.
• The number of arguments to a function.

The dollar sign is also a valid identifier in Visual C++.

// dollar_sign_identifier.cpp
struct $Y1$ {
    void $Test$() {}
};

int main() {
    $Y1$ $x$;
    $x$.$Test$();
}

https://web.archive.org/web/20100216114436/http://msdn.microsoft.com/en-us/library/565w213d.aspx

Newest version: https://docs.microsoft.com/en-us/cpp/cpp/identifiers-cpp?redirectedfrom=MSDN&view=vs-2019

GCC:

6.42 Dollar Signs in Identifier Names

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.

http://gcc.gnu.org/onlinedocs/gcc/Dollar-Signs.html#Dollar-Signs

The only legal characters according to the standard are alphanumerics and the underscore. The standard does require that just about anything Unicode considers alphabetic is acceptable (but only as single code-point characters). In practice, implementations offer extensions (i.e. some do accept a $) and restrictions (most don't accept all of the required Unicode characters). If you want your code to be portable, restrict symbols to the 26 unaccented letters, upper or lower case, the ten digits, and the '_'.