FILE * friom C to ruby with swig

Hello list,

I have written a function which prototype is:

FILE * getContent (void * objetc, char * code);

This function a FILE * to a temporary file it fill with user data, and
copy
in “code” (with strcpy) some string

I warpedd this function with Swig, but when I call it from a Ruby
script, I
have two problems:

1- The class of the function output in Ruby is SWIG::TYPE_p_FILE,
what is
this class? I looked for it in the net by without results, Hows can I
recover the data from my temporary file with this?

2- when I call this function from a C program, the variable code will
contain a string value that the function copy to it from an other
variable
in internal. and with a C program, the content of the variable code is
updated very well. But when I call the wrapped function from a Ruby
script,
the argument code stays empty (empty string “”)? How have to give this
argument in such way that it will be modified?

thank you

Lyes A. wrote:

1- The class of the function output in Ruby is SWIG::TYPE_p_FILE, what is
this class? I looked for it in the net by without results, Hows can I
recover the data from my temporary file with this?

SWIG::TYPE_p_XXX is what SWIG returns when it doesn’t know how to deal
with the type returned by a C function. It’s an opaque and useless
pointer. You need to deal with FILE* - typically by using a
%typemap(out) - in some way.

I don’t know exactly how to deal with FILE* - there may be a standard
SWIG typemap to deal with this, or you might search for examples to
convert it to a Ruby File object.

2- when I call this function from a C program, the variable code will
contain a string value that the function copy to it from an other variable
in internal. and with a C program, the content of the variable code is
updated very well. But when I call the wrapped function from a Ruby script,
the argument code stays empty (empty string “”)? How have to give this
argument in such way that it will be modified?

Passing in a char* buffer which is modified by a function is a very
common idiom in C, but it’s un-rubyish. Again, you’ll need a
%typemap(in) or write a wrapper function. Have a look at the *INOUT
standard typemaps in SWIG.

Usually the most natural ruby way to deal with this is to use
%typemap(in,numinputs=0) and return two VALUEs to ruby, the file object
and a new string.

alex

On 21/08/2008, Lyes A. [email protected] wrote:

have two problems:

1- The class of the function output in Ruby is SWIG::TYPE_p_FILE, what is
this class? I looked for it in the net by without results, Hows can I
recover the data from my temporary file with this?

What you get is a FILE * object which ruby cannot handle. However, you
can use Swig to write read and write methods for this object, and you
could then get at the data.

Or you could rewrite your C function to return a file descriptor
rather than a FILE * object, and update the Swig wrapper to turn the
file descriptor into an IO object (using IO::for_fd) that you can use
in Ruby without problems. Or you could just use the automatic Swig
wrapper, and wrap the function once more in ruby to turn the fd into
IO.

2- when I call this function from a C program, the variable code will
contain a string value that the function copy to it from an other variable
in internal. and with a C program, the content of the variable code is
updated very well. But when I call the wrapped function from a Ruby script,
the argument code stays empty (empty string “”)? How have to give this
argument in such way that it will be modified?

I haven’t seen your Swig code (and I am not proficient with Swig
anyway but others might figure out something when they look at it) but
my guess is that autogenerated wrappers should convert String to char

  • just fine.

What might confuse the generator is the first argument which has no
apparent type, and you have to extract some C value from the argument
passed to the wrapped function. Which means that you will probably
have to write a manual wrapper or even multiple wrappers that convert
the first argument in different ways, and perhaps pass a different
second argument based on the conversion of the first argument.

Generally I have no idea what the function is supposed to do so I
cannot tell you how it could be wrapped.

HTH

Michal

OK, thank you all, I will try your suggets!

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