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Comment by jbooth | Hacker News Reader

This article should really have the word "stack" in it someplace.

Not knowing much C, I was confused about why malloc() wouldn't get called for the RString structure. This is elucidating:

http://www.cs.usfca.edu/~wolber/SoftwareDev/C/CStructs.htm

Particularly this part: "// automatic allocation, all fields placed on stack"

Even if you're not putting the RString struct on the stack, the embedded string optimization means calling malloc() just once instead of twice.

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 */

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

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

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

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

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