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EDIT: this is a rewrite of the question since it was so unspecific before.

So I'm having a problem solving the issue of variables being shared across instances in C extensions. Here is an example of what I'm encountering.

>> t = SCOPE::TestClass.new #=> #<SCOPE::TestClass:0x000001011e86e0>
>> t.set = 4 #=> 4
>> t.get #=> 4
>> v = SCOPE::TestClass.new #=> #<SCOPE::TestClass:0x00000101412bf0>
>> v.set = 5 #=> 5
>> v.get #=> 5
>> t.get #=> 5

Would the best solution in the code below be to simply use ruby variables that you can set up like

void rb_define_variable(const char *name, VALUE *var)

Or is there a solution in C that I'm not seeing or understanding?

Code:

#include <stdlib.h>
#include <ruby.h>

VALUE TestClass;
VALUE SCOPE;
VALUE test_var;

VALUE set(self, val);
VALUE get();

VALUE set(VALUE self, VALUE val) {
    test_var = NUM2INT(val);
    return Qnil;
}

VALUE get() {
    return INT2NUM(test_var);
}

void Init_scope() 
{
    SCOPE = rb_define_module("SCOPE");
    TestClass = rb_define_class_under(SCOPE, "TestClass", rb_cObject);

    rb_define_method(TestClass, "set=", set, 1);
    rb_define_method(TestClass, "get", get, 0);
}
Matheus Moreira
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doubleconfess
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  • Sorry, if I create multiple instances of a C extension, I am getting a situation where they are sharing the values of external variables. – doubleconfess Jun 26 '11 at 17:59

2 Answers2

1

Global variables are according to the specification of ruby c extensions shared amongst each other (see documentation). It is the best option to limit variable scopes to the least visible which does the job. If you happen to have a shared variable it should be at least safe to synchronization issues.

fyr
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  • Hi Joerg, in the example I posted above would you just make everything a ruby variable or is there a solution in C that I'm not seeing? – doubleconfess Jun 26 '11 at 18:33
  • I think you are looking for: `VALUE rb_ivar_get(VALUE obj, ID id)` and the following: `VALUE rb_ivar_set(VALUE obj, ID id, VALUE val)` – fyr Jun 26 '11 at 19:43
1

Ok, now I think I see the problem. Your 

  VALUE test_var;

is a sort of "shared" value among every instance of the test class. This is an error of course, since it is overwritten as new instances are created or method set is called. So you can have just a single instance and a value shared among every instance.

Of course you are doing something wrong: ruby has to provide context and a way to retrieve it, likely the proto for the get function must have at least VALUE self as argument, like set. The value can't be stored into global or static local variable: it must be someway stored into the "context" of the object. In order to know how to do so, I need a fast tutorial about ruby ext. In the meantime try to read deeper here.

In particular, focus your attention on "Accessing Variables" and how you define instance variables.

I have done this, that seems to work; you could like to work on it to achieve your extension purpose(s) (I've renamed something, and fixed something else; I've also removed the INT2NUM and NUM2INT stuff, you can put it back at your need)

#include <stdlib.h>
#include <ruby.h>

VALUE TestClass;
VALUE SCOPE;

VALUE set(VALUE, VALUE);
VALUE get(VALUE);

VALUE set(VALUE self, VALUE val) {
    (void)rb_iv_set(self, "@val", val);
    return Qnil;
}

VALUE get(VALUE self) {
  return rb_iv_get(self, "@val");
}

void Init_RubyTest() 
{
    SCOPE = rb_define_module("RubyTest");
    TestClass = rb_define_class_under(SCOPE, "TestClass", rb_cObject);

    rb_define_method(TestClass, "set=", set, 1);
    rb_define_method(TestClass, "get", get, 0);
}

This question can't be answered fully if we don't know how "C extension" (I suppose, to Ruby?) works, and sincerely I don't know.

A "global" variable declared static is local to the file where it is defined and can't be accessed from outside, i.e. it is global inside that file, but it is not a global for all the linked files.

func1 can access bar, indeed; it can't just because the symbol is not known until it is declared (for the same reason func1 can't call func2, or at least compiler gives a warning for the missing prototype, then the code to func2 will be found anyway), but anyway, once the symbol is known, it can be accessed. On the contrary, those variables bar and foo can't be seen from outside (and so are not global) since the symbols foo and bar are not visible.

If this code is supposed to be compiled as a shared object or a static library, foo and bar won't be visible by the code that links the shared object / static library.

ShinTakezou
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