If I had to guess, they’re probably just expanding the Scheme code into C code during compilation. Also: it isn’t an endless C loop; do{/statements/}while(0) is a common macro idiom for a block of code that requires a trailing semicolon, i.e. one that behaves syntactically like a normal (void) function call. It only runs once because the condition is false.

I'm somewhat lost in the details, but IIUC, you are confused for the "use" of the macro in the file mkgs.ss The fist part of that file is a transpiler, that transpile it's second half from pseudo-scheme to c, and then the c version includes the definition of the macro and uses it https://github.com/cisco/ChezScheme/blob/fc577f234171af80859...