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()`
1508 VALUE 1509 rb_cvar_get(klass, id) 1510 VALUE klass; 1511 ID id; 1512 { 1513 VALUE value; 1514 VALUE tmp; 1515 1516 tmp = klass; 1517 while (tmp) { 1518 if (RCLASS→iv_tbl) { 1519 if (st_lookup(RCLASS→iv_tbl,id,&value)) { 1520 if (RTEST) { 1521 cvar_override_check(id, tmp); 1522 } 1523 return value; 1524 } 1525 } 1526 tmp = RCLASS→super; 1527 } 1528 1529 rb_name_error(id,“uninitialized class variable %s in %s”, 1530 rb_id2name(id), rb_class2name(klass)); 1531 return Qnil; /* not reached */ 1532 }(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 `ID`s, the whole name of the variables is taken into account, including the prefix: `rb_intern()` will return different `ID`s 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()`
1377 void 1378 rb_const_set(klass, id, val) 1379 VALUE klass; 1380 ID id; 1381 VALUE val; 1382 { 1383 mod_av_set(klass, id, val, Qtrue); 1384 }(variable.c)
`mod_av_set()` does all the hard work:
▼ `mod_av_set()`
1352 static void 1353 mod_av_set(klass, id, val, isconst) 1354 VALUE klass; 1355 ID id; 1356 VALUE val; 1357 int isconst; 1358 { 1359 char *dest = isconst ? “constant” : “class variable”; 1360 1361 if (!OBJ_TAINTED(klass) && rb_safe_level() >= 4) 1362 rb_raise(rb_eSecurityError, “Insecure: can’t set %s”, dest); 1363 if (OBJ_FROZEN(klass)) rb_error_frozen(“class/module”); 1364 if (!RCLASS→iv_tbl) { 1365 RCLASS→iv_tbl = st_init_numtable(); 1366 } 1367 else if (isconst) { 1368 if (st_lookup(RCLASS→iv_tbl, id, 0) || 1369 (klass == rb_cObject && st_lookup(rb_class_tbl, id, 0))) { 1370 rb_warn(“already initialized %s %s”, dest, rb_id2name(id)); 1371 } 1372 } 1373 1374 st_insert(RCLASS→iv_tbl, id, val); 1375 }(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→iv_tbl) {
RCLASS→iv_tbl = st_init_numtable();
}
st_insert(RCLASS→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()`
1156 VALUE 1157 rb_const_get(klass, id) 1158 VALUE klass; 1159 ID id; 1160 { 1161 VALUE value, tmp; 1162 int mod_retry = 0; 1163 1164 tmp = klass; 1165 retry: 1166 while (tmp) { 1167 if (RCLASS→iv_tbl && st_lookup(RCLASS→iv_tbl,id,&value)) { 1168 return value; 1169 } 1170 if (tmp == rb_cObject && top_const_get(id, &value)) return value; 1171 tmp = RCLASS→super; 1172 } 1173 if (!mod_retry && BUILTIN_TYPE(klass) == T_MODULE) { 1174 mod_retry = 1; 1175 tmp = rb_cObject; 1176 goto retry; 1177 } 1178 1179 /* Uninitialized constant / 1180 if (klass && klass != rb_cObject) { 1181 rb_name_error(id, “uninitialized constant %s at %s”, 1182 rb_id2name(id), 1183 RSTRING)→ptr); 1184 } 1185 else { / global_uninitialized / 1186 rb_name_error(id, “uninitialized constant %s”,rb_id2name(id)); 1187 } 1188 return Qnil; / not reached */ 1189 }(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→iv_tbl && st_lookup(RCLASS→iv_tbl,id,&value)) { return value; } if (tmp == rb_cObject && top_const_get(id, &value)) return value; tmp = RCLASS→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()`
1102 static int 1103 top_const_get(id, klassp) 1104 ID id; 1105 VALUE klassp; 1106 { 1107 / pre-defined class / 1108 if (st_lookup(rb_class_tbl, id, klassp)) return Qtrue; 1109 1110 / autoload */ 1111 if (autoload_tbl && st_lookup(autoload_tbl, id, 0)) { 1112 rb_autoload_load(id); 1113 *klassp = rb_const_get(rb_cObject, id); 1114 return Qtrue; 1115 } 1116 return Qfalse; 1117 }(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
21 static st_table *rb_global_tbl;
334 struct global_entry {
335 struct global_variable *var;
336 ID id;
337 };
324 struct global_variable {
325 int counter; /* reference counter */
326 void data; / value of the variable */
327 VALUE (getter)(); / function to get the variable */
328 void (setter)(); / function to set the variable */
329 void (marker)(); / function to mark the variable */
330 int block_trace;
331 struct trace_var *trace;
332 };
(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).
The structure representing the variables is split in two to be able to create `alias`es. When an `alias` is established, two `global_entry`s 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 in 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_var`s 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()`
716 VALUE 717 rb_gv_get(name) 718 const char *name; 719 { 720 struct global_entry *entry; 721 722 entry = rb_global_entry(global_id(name)); 723 return rb_gvar_get(entry); 724 } 649 VALUE 650 rb_gvar_get(entry) 651 struct global_entry *entry; 652 { 653 struct global_variable *var = entry→var; 654 return (*var→getter)(entry→id, var→data, var); 655 }(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()`
351 struct global_entry*
352 rb_global_entry(id)
353 ID id;
354 {
355 struct global_entry *entry;
356
357 if (!st_lookup(rb_global_tbl, id, &entry)) {
358 struct global_variable *var;
359 entry = ALLOC;
360 st_add_direct(rb_global_tbl, id, entry);
361 var = ALLOC;
362 entry→id = id;
363 entry→var = var;
364 var→counter = 1;
365 var→data = 0;
366 var→getter = undef_getter;
367 var→setter = undef_setter;
368 var→marker = undef_marker;
369
370 var→block_trace = 0;
371 var→trace = 0;
372 }
373 return entry;
374 }
(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()`
385 static void 386 undef_setter(val, id, data, var) 387 VALUE val; 388 ID id; 389 void data; 390 struct global_variable *var; 391 { 392 var→getter = val_getter; 393 var→setter = val_setter; 394 var→marker = val_marker; 395 396 var→data = (void)val; 397 }(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?
