CODEWITHSUNDEEP
rubyrubygemsffifiddle
ForeverZer0
ForeverZer0

Reputation: 2496

Return Ruby's Fiddle::Pointer from C function

I am currently working on a high-performance Vector/Matrix Ruby gem C extension, as I find the built-in implementation cumbersome and not ideal for most cases that I have personally encountered, as well as lacking in other areas.

My first approach was implementing in Ruby as a subclass of Fiddle::CStructEntity, as a goal is to make them optimized for interop without need for conversion (such as passing to native OpenGL functions). Implementing in C offers a great benefit for the math, but I ran into a roadblock when trying to implement a minor function.

I wished to have a method return a Fiddle::Pointer to the struct (basically a pointer to Rdata->data. I wished to return an actual Fiddle::Pointer object. Returning an integer address, packed string, etc. is trivial, and using that could easily be extended in a Ruby method to convert to a Fiddle::Pointer like this:

def ptr
  # Assume address is an integer address returned from C
  Fiddle::Pointer.new(self.address, self.size)
end

This kind of opened up a question to me, and that is it possible to to even do such from C? Fiddle is not part of the core, library, it is part of the standard lib, and as such, is really just an extension itself.

The problem is trivial, and can be easily overcome with a couple lines of Ruby code as demonstrated above, but was more curious if returning a Fiddle object was even possible from a C extension without hacks? I was unable to find any examples of this being done, and as always when it comes to the documentation involving Fiddle, it is quite basic and does not explain much.

Upvotes: 0

Views: 476

Answers (1)

ForeverZer0
ForeverZer0

Reputation: 2496

The solution for this is actually rather simple, though admittedly not as elegant or clean of a solution I was hoping to discover.

There are possibly more elaborate ways to go about this by including the headers for Fiddle, and building against it, but this was not really a viable solution, as I didn't want to restrict my C extension to only work with Ruby 2.0+, and would be perfectly acceptable to simply omit the method in the event Ruby version was less than 2.0.

First I include version.h, which gives access defines the macro RUBY_API_VERSION_MAJOR, which is all I really need to know in regards to whether or not Fiddle will be present or not.

This will be an abbreviated version to simply show how to get the Fiddle::Pointer class as a VALUE, and to create an instance.

#if RUBY_API_VERSION_MAJOR >= 2

    rb_require("fiddle");
    VALUE fiddle = rb_const_get(rb_cObject, rb_intern("Fiddle"));
    rb_cFiddlePointer = rb_const_get(fiddle, rb_intern("Pointer"));

#endif

In this example, the class is stored in rb_cFiddlePointer, which can then be used to create and return a Fiddle::Pointer object from C.

    // Get basic data about the struct
    struct RData *rdata = RDATA(self);
    VALUE *args = xmalloc(sizeof(VALUE) * 2);
    // Set the platform pointer-size address (could just use size_t here...)
    #if SIZEOF_INTPTR_T == 4
        args[0] = LONG2NUM((long) rdata->data);
    #elif SIZEOF_INTPTR_T == 8
        args[0] = LL2NUM((long long) rdata->data);
    #else 
        args[0] = INT2NUM(0);
    #endif
    // Get size of structure
    args[1] = INT2NUM(SIZE_OF_YOUR_STRUCTURE);
    VALUE ptr = rb_class_new_instance(2, args, rb_cFiddlePointer);
    xfree(args);
    return ptr;

After linking the function to an actual Ruby method, you can then call it to get a sized pointer to the internal structure in memory.

Upvotes: 0

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