From: Charles Mills <cmills@...>
Date: 2005-04-05T04:19:43+09:00
Subject: Re: Extension question

Joe Van Dyk wrote:
> On Apr 4, 2005 10:35 AM, Joe Van Dyk <joevandyk@gmail.com> wrote:
> > On Apr 4, 2005 8:34 AM, Charles Mills <cmills@freeshell.org> wrote:
> > >
> > > > Are there any general rules of thumb that you use when writing
> > > extensions?
> > >
> > > http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/47700
> > >
> > > You shouldn't be able to cause your extension to seg fault from
Ruby.
> > > That may sound like a joke, but I just mean using StringValue,
> > > NUM2LONG, etc before you starting using Ruby objects in C.
> > >
> > > >
> > > > My first problem was trying to figure out what should remain
Ruby
> > > code
> > > > and what should be moved out to C.  For example, if I opened
the file
> > > > that contained binary data in Ruby, how could I "get" to that
data
> > > > using a FILE pointer in C?  Or should I just pass the filename
to the
> > > > C extension?
> > > >
> > > You can do either.  IMO it depends on the layout of the file.  If
your
> > > reading line by line you can do this easily using the rb_io_*
functions
> > > found in intern.h.  If your going to be moving the file position
> > > around, using fgetc/ungetc, or reading into C structs it may be
easier
> > > to use the stdio functions directly.
> > >
> > > -Charlie
> >
> > So, if I have a IO object in Ruby, how could I use C's stdio
functions
> > on it?

You can't.

> > And how could I have figured it out without asking the mailing
> > list?  :-)  Cuz I'm sure I'll have lots more questions.
> >

Although you could get the FILE ptr from the Ruby IO Object it is
definately not a good idea.  Also the next version of Ruby will not use
stdio at all, so you code will break.

So basically you can do one or the other - use a Ruby IO Object (and
the rb_io_* methods) or use fopen and friends.

> > Thanks,
> > Joe
>
>
> And to expand the question...
>
> I'm using the GD library to create the image.  I'm creating the image
> by reading binary data from a file and looping through some
> predetermined width and height and setting each pixel of the picture
> based on the data from the file.
>
> Which parts of this should go in the C extension and which parts
> should stay Ruby?  I'm really scratching my head over this one.
>
> Thanks,
> Joe

Seems like you already decided the looping should be done in C.  Seems
like you have two options:
1) Open the file and read a buffer (String) in Ruby using IO#read and
pass it to your C extension which then loops through the buffer and
does the pixel stuff.
2) Open the file and read a buffer in C, then loop though it.
(Probably should use fread() to read the buffer since there is no
rb_io_read function, but if you wanted you could use a Ruby IO object
and rb_funcall.)

Option one seems more modular, but it sounds like the size of the
buffer and the range of the loop are coupled so maybe not.
Option two will probably be faster because if you can reuse the buffer
and / or declare it on the stack.

-Charlie