redis_3.0.7_sds.c_sdscatlen()

sds sdscatlen(sds s, const void *t, size_t len)
{
	struct sdshdr *sh;
	size_t curlen = sdslen(s);

	s = sdsMakeRoomFor(s, len);//字符串添加len个长度
	if (s == NULL) return NULL;
	sh = (void*)(s - (sizeof(struct sdshdr)));
	memcpy(s + curlen, t, len);/*
								将指针t指向的字符串指向
								s+curlen的位置，长度为len
							   */
	sh->len = curlen + len;
	sh->free = sh->free - len;
	s[curlen + len] = '\0';
	return s;
}