Question

Recently I’ve taken up learning pipes for fun. I’ve gotten stuck on a few parts but I have it mostly figured out I think, however I cannot figure out how to get input to forward to a program and output from that program at the same time.

Currently I have this Perl script that handles the piping:

#!/usr/bin/perl
use strict;
use warnings;
use threads;
use FileHandle;
use IPC::Open2;

my $cv_program = "./test"; #test is the compiled C program below
my $cv_message = "";
my $cv_currentkey = "";
my $pid = open2(*PIN, *POUT, $cv_program);
my $thread_pipeout = threads->create('PIPEOUT', \&PIN);

$thread_pipeout->detach();

while($cv_currentkey ne "\n")
{
    $cv_currentkey = getc(STDIN);
    $cv_message .= $cv_currentkey;
}
print POUT $cv_message;

sub PIPEOUT
{
    my $PIN = shift;
    while(<PIN>)
    {
        print $_;
    }
}

And then I have this C program which just outputs something, asks for a string, then prints that string:

#include <stdio.h>

int main(int argc, char const *argv[])
{
    char input[100] = {0};

    printf("This is a test.\n");
    fgets(input, 100, stdin);
    printf("You entered %s\n", input);
    return 0;
}

The output from running the Perl script is:

~/Programming/Perl Pipes$ ./pipe.pl
Hello
This is a test.
You entered Hello

Notice it blocks while taking input and then prints everything in a block after. I need it to print This is a test, then wait for input like the actual program would.

Also I’ll note is the reason I use getc instead of STDIN in the Perl script is because I couldn’t find a way to get STDIN to not block the output from test.c, however getc isn’t working too well at the moment either.

Answer 1

I think the problem is mostly on the C end rather than the Perl end. C sees that its standard-output is going to a pipe, so it isn’t very punctilious about making sure it flushes its standard-output buffer before querying standard-input.

To fix this, you should be able to just add this line:

fflush(stdout);

after the first printf and before the fgets.

(Disclaimer: Not tested.)