Dollar signs in identifiers are not strictly conforming to the C standard, but are a widely implemented extension. The GCC documentation says:
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.
In my own testing, I have found GCC supports them for Linux on x86, x86-64, PPC64, z/Arch, 32-bit ARMv7, and 64-bit ARM; 32-bit and 64-bit x86 Windows (compiling with MinGW); and 32-bit and 64-bit macOS (by which I mean OS X, not Classic MacOS). Separately I tested clang, and found it supports them on all the above platforms too. I haven't actually come across any platform on which dollar signs in identifiers don't work. What are the target platforms on which dollar signs in identifiers are not supported?