SDS '\0'

[Rushmmmc]

To multiplex some function in <string.h>, the end of SDS is '\0'. But the content of SDS may include '\0', in this case, several '\0' would make <string.h> function wrong.
And I check the <string.h> functions in file sds.c, it seems that all the <string.h> functions is using length parameter, so what role did the '\0' at the end of SDS play?