The Redis “Simple Dynamic String” is a length-prefixed string, roughly like this:
struct sds {
size_t len;
char buf[];
};
If you have a pointer to an sds
object, where in the allocation does the pointer point to? You would think ot points to the beginning: this is how C normally works, and this is how malloc
and free
work. But Redis does things differently: instead, it passes around pointers to the buf
field, of type char*
:
|
v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| len | buf |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Why? So that Redis can then use its SDS strings as normal C-strings, passing them to C functions (strcpy
, strcmp
, and so on).
I wrote this because I felt like it. This post is my own, and not associated with my employer.
Jim. Public speaking. Friends. Vidrio.