From: Laza <lazax.com@...>
Date: 2006-07-11T10:55:14+09:00
Subject: Re: Ruby C extension/wrapper around C function that uses scanf and printf

Go through the source code of your external apps, and replace each
scanf() with sscanf(). (sscanf() reads from a string instead from
standard input). Then you can make the Ruby wrappers and pass a string
to each function. For example:

void old_external_app()
{
    float x;
    scanf("%f %f...",&x,...);
    ....
    printf("%f",x);
}

void new_external_app(char *input_str, char *output_str)
{
    float x;
    sscanf( input_str, "%f %f...",&x,...); // the same, except 1st arg
    ...
    sprintf( output_str, "%f", x);
}


Robert Klemme wrote:
> 1gor wrote:
> > I need  Ruby to access a set of external command line applications for
> > dataseries analysis. Each of these little apps simply reads data from
> > stdin and then outputs processed results to stdout (using scanf and
> > printf, they are all written in C).
> >
> > These functions have to be accessed repeatedly when iterating over
> > large chunks of data, so accessing them through the pipe (popen or
> > popen3) doesn't work.
>
> Do they need changing arguments?  Otherwise you could just start an
> instance of each via popen and then reuse that throughout the whole
> program.  Or do these programs detect end of input data via EOF on STDIN?
>
> > Launching/closing an external process 3000 times
> > in a row results in errors. And that's not efficient.
>
> Yeah, true.
>
> > So I have to
> > write extension for each of those little C programs, a wrapper of
> > sorts.
> >
> > I have learned how to write and compile an extension for Ruby. I can
> > import an extension, pass an array of data to it and return some data
> > from the extension. The problem is that I have no C experience and
> > dread thinking of having to replace each 'scanf' routine with my custom
> > array of imported data (memory allocation, writing iterator etc.). Each
> > input function here starts with:
> >
> >      while (scanf("%lf", &y) == 1) { ... etc.
> >
> > and then follows to process input data, handle cases with input buffer
> > is too large etc. I would very much like to leave these input()
> > functions as they are and just feed them my data from Ruby code.
> >
> > Is it possible to fill stdin buffer with data from Ruby code and then
> > simply call existing C function that uses 'scanf'? I couldn't simply
> > assign an array to $stdin variable in Ruby, gave TypeError (File
> > expected)...
>
> Hm, I'm not a seasoned C programmer either, so I cannot give you a
> definitive answer to this.  However, I doubt that you can simply inject
> stuff into stdin.
>
> Here are two more ideas:
>
> 1. Rewrite your programs to act as if they saw EOF from stdin when they
> receive a certain signal (say SIGUSR1).  That way you could use a single
>   process throughout your whole Ruby script with popen and just need to
> send the signal at certain points in time.  You could even change
> behavior to use the first line read from the pipe / stdin as command
> line arguments.
>
> 2. If it's feasible rewrite the processing in Ruby.  I guess you want to
> use a C program for certain reasons - these might be performance, not
> having to rewrite the code or whatever.  If it's not performance then
> maybe you can replace the stuff with 10% LOC in Ruby.
> 
> Kind regards
> 
> 	robert