Ruby Extensions

A few weeks ago I came across François Lamontagne's "Ruby and C : Part 1" post. He talks about the VALUE type in Ruby and how that's typedef'd to an unsigned long. You can see this in ruby.h; in ruby 1.8.6 p369 it's line 86:

typedef unsigned long VALUE;

One interesting thing that François' article notes is that VALUE can contain either a pointer to the data or the data itself. You can see this in Aman Gupta's latest post to Time to Bleed. In one example he creates an Array containing a few Fixnums and a String and then dumps the heap using memprof. This yields:

  {
    "type": "array",
    "length": 4,
    "data": [
      1,
      2,
      3,
      "0x12aa0c0"
    ]
  }

So the three Fixnums are stored as immediate values, and the last value is the pointer to the String.

What other Ruby types are stored as immediate values? From ruby.h around line 697 in the rb_type function:

    if (FIXNUM_P(obj)) return T_FIXNUM;
    if (obj == Qnil) return T_NIL;
    if (obj == Qfalse) return T_FALSE;
    if (obj == Qtrue) return T_TRUE;
    if (obj == Qundef) return T_UNDEF;
    if (SYMBOL_P(obj)) return T_SYMBOL;

Looks like Fixnums, Nil, False, True, and Symbols. Everything else gets identified using the BUILTIN_TYPE macro. The Ruby 1.9.1 code looks kind of similar, although it uses different macros (like RTEST) and does the comparisons in a different order.