[xenotux]

I'm surprised this is aliased to char*, not const char*. The benefit of the aliasing is convenience, but the main risk is absent-mindedly passing it to a libc function that modifies the string without updating the SDS metadata. Const would result in a compiler warning while letting the intended use cases (e.g., the printf example) work fine.

[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.