Re: [ruby-ffi] attach_function a bit draconian

On Sep 5, 2011, at 12:45 PM, Colby Gutierrez-Kraybill wrote:

On Sep 5, 2011, at 12:10 AM, Matijs van Zuijlen wrote:

Hi Charlie,

On 04/09/2011 23:19, cfis wrote:
I've written a ffi wrapper for the free image library, which is here:

http://cfis.github.com/free-image-ruby/

The problem is that I've coded against the latest version of
FreeImage, and most linux distributions have older versions
installed. Thus when I try to load the gem it fails since
attach_function throws an error because the older versions don't have
all the functions.

You can just wrap the call to attach_function in a begin-rescue-end construct. Then in the rescue block, you can do whatever you want. In your case, this would probably mean issuing a warning and then defining a stub function. You can even wrap that in a method, say, attach_function_safely.

Regards,
Matijs

this++

I had a need to create convenience aliases for GSL routines and I created a new method that wrapped around attach_function as well as a side-along override of method_missing:

By using method_missing, you could capture any calls that a developer expects to be in the library (a more current one) and do exactly what you need doing.

See:

attach_gsl_function,
http://gsl4r.rubyforge.org/git?p=gsl4r.git;a=blob;f=lib/gsl4r/util.rb;h=3c2ce22ac866b5053b8fed7a34058e299950eae5;hb=HEAD

OH, and,

There's a lot of extra cruft in the attach_gsl_function that creates both ruby and C based tests so that the C code can be compiled and the results tested against the output of the FFI wrapper around GSL calls. I wanted to avoid having to write those tests by hand. Hope this helps.

The bit to pay attention to is:

     module AutoPrefix 

130

       $prefixLock = Monitor.new

132

       # This traps method calls intended to create shortened versions

       # of the GSL function calls.

#

       # This first checks if the called method matches the Module

       # function call gsl_complex_#{called_method} (where called_method

       # might be 'abs').

#

       # If it finds a match (respond_to), it will then create a new

       # method for the class as a whole (class_eval), making the method

       # available to not just this instance of the class, but all

       # existing instances and all those created after.

#

       # Finally, the creation is wrapped up in a synchronized call

       # to ensure thread safety.  It is only unsafe the first time

       # the method is invoked (and non-existent at that point).  Every

       # time the method is invoked after, it should not hit method_missing.

       # TODO: Is this true for java threads too, or is it per 'vm' per

       # thread?

       def method_missing( called_method, *args, &block )

152

         $prefixLock.synchronize do

154

           prefix = self.class::GSL_PREFIX

156

           if ( self.class::GSL_MODULE::Methods.respond_to?("#{prefix}#{called_method}") == false )

             prefix = ""

             if ( self.class::GSL_MODULE::Methods.respond_to?("#{called_method}") == false )

               super # NoMethodError

end

end

163

           # TODO: this could be smoothed out with the #args/#parameters parts of

           # Ruby 1.9.

           # This could inspect the definition of the parameter and if the

           # first argument in the definition were of the same type as self

           # then self could be inserted into the args list per below

           # rather than requiring the #{called_method.to_s.upcase}_ADD_SELF

           # boolean definition and check

           self.class.class_eval <<-end_eval

           def #{called_method}(*args, &block)

             if ::#{self.class::GSL_MODULE.to_s}::Methods::#{prefix.to_s.upcase}#{called_method.to_s.upcase}_ADD_SELF 

               args.insert(0, self)

end

             ::#{self.class::GSL_MODULE.to_s}::Methods::#{prefix}#{called_method}( *args, &block )

end

           end_eval

179

           __send__(called_method, *args, &block)

         end # prefixLock.synchronize

       end # method_missing

     end # AutoPrefix