Comment by eis

Disadvantage #1 can be simply solved by using a pointer to the "sds" type instead.

Then you can be sure that

  sdscat(&s, "Some more data")

updates s to always point at the right memory address and you can't introduce hard to find bugs by forgetting to assign to s which the compiler wont warn you about. If you'd pass just "s" instead of "&s" as the first parameter, the compiler would error out.

So all functions modifying the string should take a pointer to it.