[ori_b]

You can allocate the string with one malloc:

    RString *s = malloc(sizeof(RString) + length); /* allocate 'length' bytes extra memory past the end of 's' */
    s->data = s + 1; /* to the extra memory past the start of the string struct */

[teaspoon]

Ruby strings are mutable, so you need to be able to free s->data without freeing the RString itself.

[ori_b]

    s = realloc(s, sizeof(RString));

[charliesome]

MRI objects are not relocatable so that won't work if realloc has to move the structure in memory

[dchest]

Nitpick: sizeof(RString) + length may overflow size_t.