Ruby Hacking Guide

Translated by Vincent ISAMBART

Chapter 6: Variables and constants

Outline of this chapter

Ruby variables

In Ruby there are quite a lot of different types of variables and constants. Let’s line them up, starting from the largest scope.

Global variables
Constants
Class variables
Instance variables
Local variables

Instance variables were already explained in chapter 2 “Objects”. In this chapter we’ll talk about:

Global variables
Class variables
Constants

We will talk about local variables in the third part of the book.

API for variables

The object of this chapter’s analysis is variable.c. Let me first introduce the APIs which would be the entry points.

VALUE rb_iv_get(VALUE obj, char *name)
VALUE rb_ivar_get(VALUE obj, ID name)
VALUE rb_iv_set(VALUE obj, char *name, VALUE val)
VALUE rb_ivar_set(VALUE obj, ID name, VALUE val)

These are the APIs to access instance variables which have already been described. They are shown here again because their definitions are in variable.c.

VALUE rb_cv_get(VALUE klass, char *name)
VALUE rb_cvar_get(VALUE klass, ID name)
VALUE rb_cv_set(VALUE klass, char *name, VALUE val)
VALUE rb_cvar_set(VALUE klass, ID name, VALUE val)

These functions are the API for accessing class variables. Class variables belong directly to classes so the functions take a class as parameter. There are in two groups, depending if their name starts with rb_Xv or rb_Xvar. The difference lies in the type of the variable “name”. The ones with a shorter name are generally easier to use because they take a char*. The ones with a longer name are more for internal use as they take a ID.

VALUE rb_const_get(VALUE klass, ID name)
VALUE rb_const_get_at(VALUE klass, ID name)
VALUE rb_const_set(VALUE klass, ID name, VALUE val)

These functions are for accessing constants. Constants also belong to classes so they take classes as parameter. rb_const_get() follows the superclass chain, whereas rb_const_get_at() does not (it just looks in klass).

struct global_entry *rb_global_entry(ID name)
VALUE rb_gv_get(char *name)
VALUE rb_gvar_get(struct global_entry *ent)
VALUE rb_gv_set(char *name, VALUE val)
VALUE rb_gvar_set(struct global_entry *ent, VALUE val)

These last functions are for accessing global variables. They are a little different from the others due to the use of struct global_entry. We’ll explain this while describing the implementation.

Points of this chapter

The most important point when talking about variables is “Where and how are variables stored?”, in other words: data structures.

The second most important matter is how we search for the values. The scopes of Ruby variables and constants are quite complicated because variables and constants are sometimes inherited, sometimes looked for outside of the local scope… To have a better understanding, you should think by comparing the implementation with the specification, like “It behaves like this in this situation so its implementation couldn’t be other then this!”

Class variables

Class variables are variables that belong to classes. In Java or C++ they are called static variables. They can be accessed from both the class or its instances. But “from an instance” or “from the class” is information only available in the evaluator, and we do not have one for the moment. So from the C level it’s like having no access range. We’ll just focus on the way these variables are stored.

Reading

The functions to get a class variable are rb_cvar_get() and rb_cv_get(). The function with the longer name takes ID as parameter and the one with the shorter one takes char*. Because the one taking an ID seems closer to the internals, we’ll look at it.

▼ rb_cvar_get()

VALUE
rb_cvar_get(klass, id)
    VALUE klass;
    ID id;
{
    VALUE value;
    VALUE tmp;
1515
    tmp = klass;
    while (tmp) {
        if (RCLASS(tmp)->iv_tbl) {
            if (st_lookup(RCLASS(tmp)->iv_tbl,id,&value)) {
                if (RTEST(ruby_verbose)) {
                    cvar_override_check(id, tmp);
                }
                return value;
            }
        }
        tmp = RCLASS(tmp)->super;
    }
1528
    rb_name_error(id,"uninitialized class variable %s in %s",
                  rb_id2name(id), rb_class2name(klass));
    return Qnil;                /* not reached */
}

(variable.c)

This function reads a class variable in klass.

Error management functions like rb_raise() can be simply ignored like I said before. The rb_name_error() that appears this time is a function for raising an exception, so it can be ignored for the same reasons. In ruby, you can assume that all functions ending with _error raise an exception.

After removing all this, we can see that it is just following the klass’s superclass chain one by one and searching in each iv_tbl. … At this point, I’d like you to say “What? iv_tbl is the instance variables table, isn’t it?” As a matter of fact, class variables are stored in the instance variable table.

We can do this because when creating IDs, the whole name of the variables is taken into account, including the prefix: rb_intern() will return different IDs for “@var” and “@@var”. At the Ruby level, the variable type is determined only by the prefix so there’s no way to access a class variable called @var from Ruby.

Constants

It’s a little abrupt but I’d like you to remember the members of struct RClass. If we exclude the basic member, struct RClass contains:

VALUE super
struct st_table *iv_tbl
struct st_table *m_tbl

Then, considering that:

constants belong to a class
we can’t see any table dedicated to constants in struct RClass
class variables and instance variables are both in iv_tbl

Could it mean that the constants are also…

Assignment

rb_const_set() is a function to set the value of constants: it sets the constant id in the class klass to the value val.

▼ rb_const_set()

void
rb_const_set(klass, id, val)
    VALUE klass;
    ID id;
    VALUE val;
{
    mod_av_set(klass, id, val, Qtrue);
}

(variable.c)

mod_av_set() does all the hard work:

▼ mod_av_set()

static void
mod_av_set(klass, id, val, isconst)
    VALUE klass;
    ID id;
    VALUE val;
    int isconst;
{
    char *dest = isconst ? "constant" : "class variable";
1360
    if (!OBJ_TAINTED(klass) && rb_safe_level() >= 4)
        rb_raise(rb_eSecurityError, "Insecure: can't set %s", dest);
    if (OBJ_FROZEN(klass)) rb_error_frozen("class/module");
    if (!RCLASS(klass)->iv_tbl) {
        RCLASS(klass)->iv_tbl = st_init_numtable();
    }
    else if (isconst) {
        if (st_lookup(RCLASS(klass)->iv_tbl, id, 0) ||
            (klass == rb_cObject && st_lookup(rb_class_tbl, id, 0))) {
            rb_warn("already initialized %s %s", dest, rb_id2name(id));
        }
    }
1373
    st_insert(RCLASS(klass)->iv_tbl, id, val);
}

(variable.c)

You can this time again ignore the warning checks (rb_raise(), rb_error_frozen() and rb_warn()). Here’s what’s left:

▼ mod_av_set() (only the important part)

    if (!RCLASS(klass)->iv_tbl) {
        RCLASS(klass)->iv_tbl = st_init_numtable();
    }
    st_insert(RCLASS(klass)->iv_tbl, id, val);

We’re now sure constants also reside in the instance table. It means in the iv_tbl of struct RClass, the following are mixed together:

the class’s own instance variables
class variables
constants

Reading

We now know how the constants are stored. We’ll now check how they really work.

`rb_const_get()`

We’ll now look at rb_const_get(), the function to read a constant. This function returns the constant referred to by id from the class klass.

▼ rb_const_get()

VALUE
rb_const_get(klass, id)
    VALUE klass;
    ID id;
{
    VALUE value, tmp;
    int mod_retry = 0;
1163
    tmp = klass;
  retry:
    while (tmp) {
        if (RCLASS(tmp)->iv_tbl &&
                  st_lookup(RCLASS(tmp)->iv_tbl,id,&value)) {
            return value;
        }
        if (tmp == rb_cObject && top_const_get(id, &value))
                  return value;
        tmp = RCLASS(tmp)->super;
    }
    if (!mod_retry && BUILTIN_TYPE(klass) == T_MODULE) {
        mod_retry = 1;
        tmp = rb_cObject;
        goto retry;
    }
1178
    /* Uninitialized constant */
    if (klass && klass != rb_cObject) {
        rb_name_error(id, "uninitialized constant %s at %s",
                      rb_id2name(id),
                      RSTRING(rb_class_path(klass))->ptr);
    }
    else { /* global_uninitialized */
        rb_name_error(id, "uninitialized constant %s",rb_id2name(id));
    }
    return Qnil;                /* not reached */
}

(variable.c)

There’s a lot of code in the way. First, we should at least remove the rb_name_error() in the second half. In the middle, what’s around mod_entry seems to be a special handling for modules. Let’s also remove that for the time being. The function gets reduced to this:

▼ rb_const_get (simplified)

VALUE
rb_const_get(klass, id)
    VALUE klass;
    ID id;
{
    VALUE value, tmp;

    tmp = klass;
    while (tmp) {
        if (RCLASS(tmp)->iv_tbl && st_lookup(RCLASS(tmp)->iv_tbl,id,&value)) {
            return value;
        }
        if (tmp == rb_cObject && top_const_get(id, &value)) return value;
        tmp = RCLASS(tmp)->super;
    }
}

Now it should be pretty easy to understand. The function searches for the constant in iv_tbl while climbing klass’s superclass chain. That means:

class A
  Const = "ok"
end
class B < A
  p(Const)    # can be accessed
end

The only problem remaining is top_const_get(). This function is only called for rb_cObject so top must mean “top-level”. If you don’t remember, at the top-level, the class is Object. This means the same as “in the class statement defining C, the class becomes C”, meaning that “the top-level’s class is Object”.

* the class of the top-level is Object
class A
  # the class is A
  class B
    # the class is B
  end
end

So top_const_get() probably does something specific to the top level.

`top_const_get()`

Let’s look at this top_const_get function. It looks up the id constant writes the value in klassp and returns.

▼ top_const_get()

static int
top_const_get(id, klassp)
    ID id;
    VALUE *klassp;
{
    /* pre-defined class */
    if (st_lookup(rb_class_tbl, id, klassp)) return Qtrue;
1109
    /* autoload */
    if (autoload_tbl && st_lookup(autoload_tbl, id, 0)) {
        rb_autoload_load(id);
        *klassp = rb_const_get(rb_cObject, id);
        return Qtrue;
    }
    return Qfalse;
}

(variable.c)

rb_class_tbl was already mentioned in chapter 4 “Classes and modules”. It’s the table for storing the classes defined at the top-level. Built-in classes like String or Array have for example an entry in it. That’s why we should not forget to search in this table when looking for top-level constants.

The next block is related to autoloading. It is designed to be able to register a library that is loaded automatically when accessing a particular top-level constant for the first time. This can be used like this:

autoload(:VeryBigClass, "verybigclass")   # VeryBigClass is defined in it

After this, when VeryBigClass is accessed for the first time, the verybigclass library is loaded (with require). As long as VeryBigClass is defined in the library, execution can continue smoothly. It’s an efficient approach, when a library is too big and a lot of time is spent on loading.

This autoload is processed by rb_autoload_xxxx(). We won’t discuss autoload further in this chapter because there will probably be a big change in how it works soon.

(translator’s note: The way autoload works did change in 1.8: autoloaded constants do not need to be defined at top-level anymore).

Other classes?

But where did the code for looking up constants in other classes end up? After all, constants are first looked up in the outside classes, then in the superclasses.

In fact, we do not yet have enough knowledge to look at that. The outside classes change depending on the location in the program. In other words it depends of the program context. So we need first to understand how the internal state of the evaluator is handled. Specifically, this search in other classes is done in the ev_const_get() function of eval.c. We’ll look at it and finish with the constants in the third part of the book.

Global variables

General remarks

Global variables can be accessed from anywhere. Or put the other way around, there is no need to restrict access to them. Because they are not attached to any context, the table only has to be at one place, and there’s no need to do any check. Therefore implementation is very simple.

But there is still quite a lot of code. The reason for this is that global variables of Ruby are equipped with some gimmicks which make it hard to regard them as mere variables. Functions like the following are only available for global variables:

you can “hook” access of global variables
you can alias them with alias

Let’s explain this simply.

Aliases of variables

alias $newname $oldname

After this, you can use $newname instead of $oldname. alias for variables is mainly a counter-measure for “symbol variables”. “symbol variables” are variables inherited from Perl like $= or $0. $= decides if during string comparison upper and lower case letters should be differentiated. $0 shows the name of the main Ruby program. There are some other symbol variables but anyway as their name is only one character long, they are difficult to remember for people who don’t know Perl. So, aliases were created to make them a little easier to understand.

That said, currently symbol variables are not recommended, and are moved one by one in singleton methods of suitable modules. The current school of thought is that $= and others will be abolished in 2.0.

Hooks

You can “hook” read and write of global variables.

Although hooks can be also be set at the Ruby level, I think the purpose of it seems rather to prepare the special variables for system use like $KCODE at C level. $KCODE is the variable containing the encoding the interpreter currently uses to handle strings. Essentially only special strings like "EUC" or "UTF8" can be assigned to it, but this is too bothersome so it is designed so that "e" or "u" can also be used.

p($KCODE)      # "NONE" (default)
$KCODE = "e"
p($KCODE)      # "EUC"
$KCODE = "u"
p($KCODE)      # "UTF8"

Knowing that you can hook assignment of global variables, you should understand easily how this can be done. By the way, $KCODE’s K comes from “kanji” (the name of Chinese characters in Japanese).

You might say that even with alias or hooks, global variables just aren’t used much, so it’s functionality that doesn’t really mater. It’s adequate not to talk much about unused functions, and I’d like to use more pages for the analysis of the parser and evaluator. That’s why I’ll proceed with the explanation below whose degree of half-hearted is 85%.

Data structure

I said that the point when looking at how variables work is the way they are stored. First, I’d like you to firmly grasp the structure used by global variables.

▼ Data structure for global variables

static st_table *rb_global_tbl;

struct global_entry {
    struct global_variable *var;
    ID id;
};

struct global_variable {
    int   counter;      /* reference counter */
    void *data;         /* value of the variable */
    VALUE (*getter)();  /* function to get the variable */
    void  (*setter)();  /* function to set the variable */
    void  (*marker)();  /* function to mark the variable */
    int block_trace;
    struct trace_var *trace;
};

(variable.c)

rb_global_tbl is the main table. All global variables are stored in this table. The keys of this table are of course variable names (ID). A value is expressed by a struct global_entry and a struct global_variable (figure 1).

figure 1: Global variables table at execution time

The structure representing the variables is split in two to be able to create aliases. When an alias is established, two global_entrys point to the same struct global_variable.

It’s at this time that the reference counter (the counter member of struct global_variable) is necessary. I explained the general idea of a reference counter in the previous section “Garbage collection”. Reviewing it briefly, when a new reference to the structure is made, the counter is incremented by 1. When the reference is not used anymore, the counter is decreased by 1. When the counter reaches 0, the structure is no longer useful so free() can be called.

When hooks are set at the Ruby level, a list of struct trace_vars is stored in the trace member of struct global_variable, but I won’t talk about it, and omit struct trace_var.

Reading

You can have a general understanding of global variables just by looking at how they are read. The functions for reading them are rb_gv_get() and rb_gvar_get().

▼ rb_gv_get() rb_gvar_get()

VALUE
rb_gv_get(name)
    const char *name;
{
    struct global_entry *entry;
 721
    entry = rb_global_entry(global_id(name));
    return rb_gvar_get(entry);
}

VALUE
rb_gvar_get(entry)
    struct global_entry *entry;
{
    struct global_variable *var = entry->var;
    return (*var->getter)(entry->id, var->data, var);
}

(variable.c)

A substantial part of the content seems to turn around the rb_global_entry() function, but that does not prevent us understanding what’s going on. global_id is a function that converts a char* to ID and checks if it’s the ID of a global variable. (*var->getter)(...) is of course a function call using the function pointer var->getter. If p is a function pointer, (*p)(arg) calls the function.

But the main part is still rb_global_entry().

▼ rb_global_entry()

struct global_entry*
rb_global_entry(id)
    ID id;
{
    struct global_entry *entry;
 356
    if (!st_lookup(rb_global_tbl, id, &entry)) {
        struct global_variable *var;
        entry = ALLOC(struct global_entry);
        st_add_direct(rb_global_tbl, id, entry);
        var = ALLOC(struct global_variable);
        entry->id = id;
        entry->var = var;
        var->counter = 1;
        var->data = 0;
        var->getter = undef_getter;
        var->setter = undef_setter;
        var->marker = undef_marker;
 369
        var->block_trace = 0;
        var->trace = 0;
    }
    return entry;
}

(variable.c)

The main treatment is only done by the st_lookup() at the beginning. What’s done afterwards is just creating (and storing) a new entry. As, when accessing a non existing global variable, an entry is automatically created, rb_global_entry() will never return NULL.

This was mainly done for speed. When the parser finds a global variable, it gets the corresponding struct global_entry. When reading the value of the variable, the value is just obtained from the entry (using rb_gv_get()).

Let’s now continue a little with the code that follows. var->getter and others are set to undef_xxxx. undef probably means that they are the setter/getter/marker for a global variable whose state is undefined.

undef_getter() just shows a warning and returns nil, as even undefined global variables can be read. undef_setter() is a little bit interesting so let’s look at it.

▼ undef_setter()

static void
undef_setter(val, id, data, var)
    VALUE val;
    ID id;
    void *data;
    struct global_variable *var;
{
    var->getter = val_getter;
    var->setter = val_setter;
    var->marker = val_marker;
 395
    var->data = (void*)val;
}

(variable.c)

val_getter() takes the value from entry->data and returns it. val_getter() just puts a value in entry->data. Setting handlers this way allows us not to need special handling for undefined variables (figure 2). Skillfully done, isn’t it?

figure 2: Setting and consultation of global variables