CODEWITHSUNDEEP
cruby
Saurabh7
Saurabh7

Reputation: 720

Construct parameters in C to call a Ruby C extension with a variable number of args

I am trying to re-implement this Ruby method:

  def reshape new_shape
    t = reshape_clone_structure(new_shape)
    left_params  = [:*]*new_shape.size
    right_params = [:*]*self.shape.size
    t[*left_params] = self[*right_params]
    t
  end

As a C extension to a Ruby matrix library.

I am stuck on how to pass multiple values to []= which is implemented in an existing C extension with this definition:

static VALUE nm_mset(int argc, VALUE* argv, VALUE self)

How do I translate the Ruby code t[*left_params] = into C?

Upvotes: 0

Views: 128

Answers (2)

Neil Slater
Neil Slater

Reputation: 27207

The nm_mset method uses standard conventions for Ruby C extensions. The argc variable is a an integer, specifying number of parameters being sent. Then argv is a C array/pointer of type VALUE * which must contain all the Ruby-ish parameters (for an assignment, the last of these params is the value of the right hand side). Thirdly self is the Ruby Object receiver - but do remember that this might be either the class or an instance of it depending on how the method has been defined for Ruby.

To call it, you need to construct a VALUE * array with each parameter a VALUE, sent in the correct order.

I think you want to do something like this: 

// Assuming you have following values already processed
// Assignments are not shown here - that is work for you!
VALUE self; 
VALUE t; 
int self_size; 
int new_size; 

//////////////////////////////////////////////////////////////////
//
// Assuming you set above values correctly, the following should
// work.

int i;

// Get reference to :*
VALUE asterisk = ID2SYM( rb_intern("*") );

// NB +1, we need to add call to self[ *right_params ] to end later
VALUE *left_params = ALLOC_N( VALUE, new_size + 1 );
for(i=0;i<new_size;i++) { left_params[i] = asterisk; }

VALUE *right_params = ALLOC_N( VALUE, self_size );
for(i=0;i<self_size;i++) { right_params[i] = asterisk; }

// equivalent to self[ *right_params ], 
// but we push it straight into end of left_params ready for next step
left_params[ new_size ] = nm_mget( self_size, right_params, self );

// equivalent to t[ *left_params ] =
// Note +1 to size again, the last item is the value to assign
nm_mset( new_size + 1, left_params, t );

// Tidy up
xfree( left_params );
xfree( right_params );

return t;

Upvotes: 1

Farouq Jouti
Farouq Jouti

Reputation: 1667

it is doeable in C , first you need to include the header <stdarg.h> then set the function to be like this

type function(type argument , ...){}

after that use the variable type va_list and the three functions va_start , va_end and va_arg to "navigate" trought the arguments as shown here

Upvotes: 0

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