How to redirect program output as its input

Question

I've written a simple C++ program for tutorial purposes. My goal is to loop it infinitely.

#include <iostream>
#include <string>

int main()
{
  std::cout << "text";

  for(;;) {
    std::string string_object{};
    std::getline(std::cin, string_object);
    std::cout << string_object;
  }

  return 0;
}

After compilation I run it like this: ./bin 0>&1 What I expected to happen is that the "text" that is output to stdout, and it will now become also stdin for the program and it will loop forever. Why doesn't it happen?

Answer 1

First, you need to output newlines when printing to std::cout, otherwise std::getline() won't have any complete line to read.

Improved version:

#include <iostream>
#include <string>

int main()
{
  std::cout << "stars" << std::endl;

  for(;;) {
    std::string string_object;
    std::getline(std::cin, string_object);
    std::cout << string_object << std::endl;
  }

  return 0;
}

Now try this:

./bin >file <file

you don't see any output, because it's going to the file. But if you stop the program and look at the file, behold, it's full of

stars
stars
stars
stars

:-)

Also, the reason that the feedback loop cannot start when you try

./bin 0>&1

is, that you end up with both stdin and stdout connected to /dev/tty (meaning that you can see the output).

But a TTY device cannot ever close the loop, because it actually consists of two separate channels, one passing the output to the terminal, one passing the terminal input to the process.

If you use a regular file for in- and output, the loop can be closed. Every byte written to the file will be read from it as well, if the stdin of the process is connected to it. That's as long as no other process reads from the file simultaneously, because each byte in a stream can be only read once.