Ruby C extension/wrapper around C function that uses scanf

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. Launching/closing an external process 3000 times
in a row results in errors. And that’s not efficient. 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)…

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.

 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

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);
}

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