Pointer to middle of allocation, part 1

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 it 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).

Tagged #redis, #data-structures, #pointers, #c, #programming.

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