I know it's an old question, however have the impression it's still an issue ...
If I understood correctly C23 provides "bigint" types or similar, however not
all users are on C23, not all like it and it's still without I/O support?
I puzzled together a little library, vanilla-C, header only, which provides
I/O from / to binary, octal, decimal and hex strings and some other functions
for gcc builtin 128-bit integer types: Libint128, I/O for 128-bit intger types