Comment by mid-kid

The only thing the SDS metadata holds is the string's length. Just like how you'd have to realloc() a regular string before using strcat(), you have to sdsgrowzero() an sds string before using strcat(). Basically, standard libc functions that tamper with the string have the same constraints as malloc()ed strings in terms of safety, only you might want to call sdsupdatelen() after truncating a string.